Is BMI an accurate way to know how much I should weigh?

mtaratoot · December 2020

AnnPT77 wrote: »

So, since I didn't see an answer to this, I'll extend the question to others: Did *anyone* else try the RFM calculator?

https://www.cedars-sinai.org/blog/relative-fat-mass.html

If so, did it produce a number "roughly equal to your body fat percentage", or what you believe that percentage to be?

I'm curious, that's all.

I tried it. I was quite surprised it gave pretty much exactly the same number than my BIA scale gave that particular day. I don't have much faith in the actual number from my BIA scale. It varies up to a percent from day to day, and that's not really likely to be correct. I've been logging what it tells me for about three years, though, and it does seem to track ~reasonably~ well, trend wise, to my fluctuations in mass over longer time periods (weeks to months versus days to weeks).

I have no idea what my actual BF% is. I am very curious. One day perhaps I will get to utilize an accurate measurement. Using those web pages with pictures makes me think my BIA scale is close. At my goal weight, I am just under a number that's the bottom of the top third of healthy BMI. I'm closer to the top 20% at my current weight which is at the top of my maintenance range.

Confession: I usually wouldn't miss this, but I had forgot that BMI is the same scale for men and women. That made me feel better about having my goal weight closer to the top than the middle of healthy BMI. Thanks for pointing that out a few days ago.

frankwbrown · December 2020

sijomial wrote: »

frankwbrown wrote: »

What I've learned here today is that BMI applies to me, unless I am an obvious outlier, in which case it doesn't.
... because human body.

tautology... because logic.

Just curious, do you actually think you are an outlier at age 70+ or is this just a general dislike of BMI?
If you do think you are an outlier - why?

This is far from where this thread started out. But I'll answer (you asked for it).

Believe me when I say I have no illusions that I am not obese. That's why I'm here. I weighed 330 lb on July 10th and set as my goal to lose 100 lb by next July 10th. I have lost 46 lb thus far after 21 weeks. I have no idea what my body fat % will be at 230 lb. I'll find out then. It's simply my first goal, because it's a nice round number. But how exactly do I set my goal? In order to have a BMI of 25.1, I would need to weigh 190 lb. I do not believe that is achievable for me and I'll tell you why.

1. As a freshman in high school, I weighed 190 lb. I was not some super athlete, although I did lift weights some, and I felt like I was a little on the chubby side (compared to what I wanted to be).
2. I was discharged from the Air Force in Europe at the age of ~24. Prior to discharge, I had tried to lose weight and hit a wall at 212-218 lb. I backpacked around Europe for several months, at the end of which I weighed 191 lb. I think that was the leanest I've ever been, since my freshman days.
3. I graduated from college at the ripe old age of 35. I weighed 218 lb then, and had been running 2 miles a day and working out for a year. Not like a body builder either. I do have the ability to add muscle fairly easily, but I've never wanted to be an Arnold Schwarzenegger (or a Charles Atlas, for those of you as old as I am).
4. I had a DEXA scan on Sept 30th.
DEXA scan: 305 lb (total weight) = 135.6 (fat tissue) + 159.9 (lean tissue) + 9.5 (bone mineral content) -> body fat = 44.5%
BIA scale: 304.9 lb (total weight) = 150.8 (fat mass) + 146.5 (muscle mass) + 7.6 (bone mass) -> body fat = 49.5%
(please don't tell me that DEXA is not accurate enough, while defending BMI. I doubt it's less accurate.)
5. I have a BIA scale which tells me my body fat is above what the DEXA scan said. Although it fluctuates wildly (i.e. it's registered my bone mass as anywhere from 7.2 to 7.9 lb), it nevertheless shows a steady trend of decreasing fat mass and increasing lean mass over the last 21 weeks.
6. I'm sure some will say this isn't significant, but I've always been called "big boned"

, and I have a noggin which is 62 cm in circumference. ("ah, weel now that makes sense, he's a fat head."

)
7. I believe it's quite possible I will have a lean mass (including bone mass) of ~170 lb. At 190 lb total, that would be 10.5% body fat. I don't expect I'll ever fall into the "athlete" category of body fat %. And incidentally, I read (on a govt website) that for a male > 60 years old, body fat under 14% is "dangerously low" and 14%-23.2% is "excellent" (their words, not mine). I bristle at that suggestion - I don't want to hear it, but it gives me pause.
8. FInally, someone asked about RFM and Navy body fat:
Today: weight = 284.7 lb, BMI = 37.6, BIA body fat = 46%, Navy body fat = 34.7%, RFMI = 34.2.

paperpudding · December 2020

BMI Ranges are a range for a reason- people who are 'big boned' are going to be their healthiest weight somewhere in the bmi range or at most slightly out of it- just like everyone else.

Of course some individuals will be at their own ideal weight at a lower or higher point in, or slightly out of, the range.
Depending on age, gender, body type, muscle mass, ethnicity etc - that is what we mean by context

But, barring obvious outliers as mentioned earlier, yes, that will apply to everyone.

heybales · December 2020

AnnPT77 wrote: »

AnnPT77 wrote: »

frankwbrown wrote: »

Theoldguy1 wrote: »

Have you compared a DEXA scan to the Withings scale? We have an older scale and it isn't worth the powder to blow it to hell regarding getting a reading similar to DEXA scan.

Yes. Here is a comparison:

A = Withings Body+ (9/30 at 6:10 am);
B = DEXA scan (9/30 at 11:30 am)
body fat: A = 49% , B = 44.5%
weight (lb): A = 304.9, B = 305.0
fat mass (lb): A = 150.8, B = 135.6
lean mass (lb): A = 154.1, B = 169.4 (159.9 excluding bone mass)
water mass (lb): A = 125.8, B = n/a
bone mass (lb): A = 7.6, B = 9.5
BMI: A = 40.4

Bioelectrical impedance isn't that accurate, especially when only feet are involved (vs feet and hands).
I use it primarily to confirm that I'm losing fat mass and not lean mass.

Some doctors at Cedars-Sinai developed an alternative to BMI. It's called Body Fat Mass Index.
Relative Fat Mass Index:
Men: 64 – (20 x height/waist circumference) = RFM
Women: 76 – (20 x height/waist circumference) = RFM
https://www.cedars-sinai.org/blog/relative-fat-mass.html

So - since we've digressed the bejeepers out of this thread already - why not: What result does the RFM calculation yield, for you?

I'm curious, because the result it gives for me - especially if I measure my "waist" at just above the top of my pelvic bones as directed in the article, rather than at natural waist - is . . . improbable, IMO. I understand that the article says RFM is "roughly equal to your body fat percentage." Maybe they define "roughly" differently than I do?

I'm as capable as the next person of having pleasant delusions about my body composition, but the "Navy calculator" says 23%, BIA scale says 23.3%, visual (my own biased eyes, photos like those on an earlier page in the thread) would be mid-twenties % (high teens upper body look, 25-30 lower body look, roughly), BMI 20.6.

RFM, using natural waist = 29. (OK, maybe that's roughly roughly. Very roughly.)
RFM, using top of hip bone as instructed in article = 34. Huh?

I'm not truly bizarre in body geometry, either, I swear. Definitely within the range of normal human females.

So, since I didn't see an answer to this, I'll extend the question to others: Did *anyone* else try the RFM calculator?

https://www.cedars-sinai.org/blog/relative-fat-mass.html

If so, did it produce a number "roughly equal to your body fat percentage", or what you believe that percentage to be?

I'm curious, that's all.

About 5% high.
I also went back to a bodpod (knowing not as good as dexa, not as bad as bia) measurement set- 8% high.

J72FIT · December 2020

paperpudding wrote: »

BMI Ranges are a range for a reason- people who are 'big boned' are going to be their healthiest weight somewhere in the bmi range or at most slightly out of it- just like everyone else.

Of course some individuals will be at their own ideal weight at a lower or higher point in, or slightly out of, the range.
Depending on age, gender, body type, muscle mass, ethnicity etc - that is what we mean by context

But, barring obvious outliers as mentioned earlier, yes, that will apply to everyone.

The bolded is my situation. I tend to settle at 175-180lbs. At 5'10" this puts me just outside the healthy weight range into overweight. I have weighed 172lbs at my lightest and looked a tad on the gaunt side. I can't see weighing any less...

J72FIT · December 2020

AnnPT77 wrote: »

AnnPT77 wrote: »

frankwbrown wrote: »

Theoldguy1 wrote: »

Have you compared a DEXA scan to the Withings scale? We have an older scale and it isn't worth the powder to blow it to hell regarding getting a reading similar to DEXA scan.

Yes. Here is a comparison:

A = Withings Body+ (9/30 at 6:10 am);
B = DEXA scan (9/30 at 11:30 am)
body fat: A = 49% , B = 44.5%
weight (lb): A = 304.9, B = 305.0
fat mass (lb): A = 150.8, B = 135.6
lean mass (lb): A = 154.1, B = 169.4 (159.9 excluding bone mass)
water mass (lb): A = 125.8, B = n/a
bone mass (lb): A = 7.6, B = 9.5
BMI: A = 40.4

Bioelectrical impedance isn't that accurate, especially when only feet are involved (vs feet and hands).
I use it primarily to confirm that I'm losing fat mass and not lean mass.

Some doctors at Cedars-Sinai developed an alternative to BMI. It's called Body Fat Mass Index.
Relative Fat Mass Index:
Men: 64 – (20 x height/waist circumference) = RFM
Women: 76 – (20 x height/waist circumference) = RFM
https://www.cedars-sinai.org/blog/relative-fat-mass.html

So - since we've digressed the bejeepers out of this thread already - why not: What result does the RFM calculation yield, for you?

I'm curious, because the result it gives for me - especially if I measure my "waist" at just above the top of my pelvic bones as directed in the article, rather than at natural waist - is . . . improbable, IMO. I understand that the article says RFM is "roughly equal to your body fat percentage." Maybe they define "roughly" differently than I do?

I'm as capable as the next person of having pleasant delusions about my body composition, but the "Navy calculator" says 23%, BIA scale says 23.3%, visual (my own biased eyes, photos like those on an earlier page in the thread) would be mid-twenties % (high teens upper body look, 25-30 lower body look, roughly), BMI 20.6.

RFM, using natural waist = 29. (OK, maybe that's roughly roughly. Very roughly.)
RFM, using top of hip bone as instructed in article = 34. Huh?

I'm not truly bizarre in body geometry, either, I swear. Definitely within the range of normal human females.

So, since I didn't see an answer to this, I'll extend the question to others: Did *anyone* else try the RFM calculator?

https://www.cedars-sinai.org/blog/relative-fat-mass.html

If so, did it produce a number "roughly equal to your body fat percentage", or what you believe that percentage to be?

I'm curious, that's all.

I just did and got 25. So I am obese according to RFM and overweight according to BMI...

Mouse_Potato · December 2020

[/quote]

So, since I didn't see an answer to this, I'll extend the question to others: Did *anyone* else try the RFM calculator?

https://www.cedars-sinai.org/blog/relative-fat-mass.html

If so, did it produce a number "roughly equal to your body fat percentage", or what you believe that percentage to be?

I'm curious, that's all. [/quote]

Using my natural waist, it put me at about 30%.
Using the hip bone measurement, it put me at 38%.
My BMI is 20.4. I visually estimate my body fat to be 20-22%.

ETA: Bah. Messed up the quotes trying not to include everything.

Theoldguy1 · December 2020

AnnPT77 wrote: »

AnnPT77 wrote: »

frankwbrown wrote: »

Theoldguy1 wrote: »

Have you compared a DEXA scan to the Withings scale? We have an older scale and it isn't worth the powder to blow it to hell regarding getting a reading similar to DEXA scan.

Yes. Here is a comparison:

A = Withings Body+ (9/30 at 6:10 am);
B = DEXA scan (9/30 at 11:30 am)
body fat: A = 49% , B = 44.5%
weight (lb): A = 304.9, B = 305.0
fat mass (lb): A = 150.8, B = 135.6
lean mass (lb): A = 154.1, B = 169.4 (159.9 excluding bone mass)
water mass (lb): A = 125.8, B = n/a
bone mass (lb): A = 7.6, B = 9.5
BMI: A = 40.4

Bioelectrical impedance isn't that accurate, especially when only feet are involved (vs feet and hands).
I use it primarily to confirm that I'm losing fat mass and not lean mass.

Some doctors at Cedars-Sinai developed an alternative to BMI. It's called Body Fat Mass Index.
Relative Fat Mass Index:
Men: 64 – (20 x height/waist circumference) = RFM
Women: 76 – (20 x height/waist circumference) = RFM
https://www.cedars-sinai.org/blog/relative-fat-mass.html

So - since we've digressed the bejeepers out of this thread already - why not: What result does the RFM calculation yield, for you?

I'm curious, because the result it gives for me - especially if I measure my "waist" at just above the top of my pelvic bones as directed in the article, rather than at natural waist - is . . . improbable, IMO. I understand that the article says RFM is "roughly equal to your body fat percentage." Maybe they define "roughly" differently than I do?

I'm as capable as the next person of having pleasant delusions about my body composition, but the "Navy calculator" says 23%, BIA scale says 23.3%, visual (my own biased eyes, photos like those on an earlier page in the thread) would be mid-twenties % (high teens upper body look, 25-30 lower body look, roughly), BMI 20.6.

RFM, using natural waist = 29. (OK, maybe that's roughly roughly. Very roughly.)
RFM, using top of hip bone as instructed in article = 34. Huh?

I'm not truly bizarre in body geometry, either, I swear. Definitely within the range of normal human females.

So, since I didn't see an answer to this, I'll extend the question to others: Did *anyone* else try the RFM calculator?

https://www.cedars-sinai.org/blog/relative-fat-mass.html

If so, did it produce a number "roughly equal to your body fat percentage", or what you believe that percentage to be?

I'm curious, that's all.

I just tried it. It gave me an answer 1 percentage point away from a recent DEXA scan. It was 2 percentage points away from a high end BIA device at the gym that uses feet and hands as 4 points of contact. I was also close to a Bod Pod analysis I did a few year ago but I don't think I had any significant body comp changes.

cwolfman13 · December 2020

Mellouk89 wrote: »

17% is in the healthy range for a male last I checked. I would've been very lean at the higher end of bmi but we aren't talking about being ripped here. We are talking about healthy body fat levels. An average man can be 17% bf at a normal bmi. And the average man is not shredded at a bmi of 25. Dumb argument.

I didnt bring up the topic of football players, read the damn thread. If you don't want the topic to derail stop quoting and then blame me on the same occasion.

paperpudding · December 2020

J72FIT wrote: »

paperpudding wrote: »

BMI Ranges are a range for a reason- people who are 'big boned' are going to be their healthiest weight somewhere in the bmi range or at most slightly out of it- just like everyone else.

Of course some individuals will be at their own ideal weight at a lower or higher point in, or slightly out of, the range.
Depending on age, gender, body type, muscle mass, ethnicity etc - that is what we mean by context

But, barring obvious outliers as mentioned earlier, yes, that will apply to everyone.

The bolded is my situation. I tend to settle at 175-180lbs. At 5'10" this puts me just outside the healthy weight range into overweight. I have weighed 172lbs at my lightest and looked a tad on the gaunt side. I can't see weighing any less...

Yes that fits into what I said - young fit men being healthy just outside the standard BMI range.

charmmeth · December 2020

AnnPT77 wrote: »

So, since I didn't see an answer to this, I'll extend the question to others: Did *anyone* else try the RFM calculator?

https://www.cedars-sinai.org/blog/relative-fat-mass.html

If so, did it produce a number "roughly equal to your body fat percentage", or what you believe that percentage to be?

I'm curious, that's all.

Using natural waist, the RFM calculator gives me 35%. Using the "waist" measurement taken as they suggest it gives me 38%. (I'm pretty straight up and down.) I have nothing to compare this with except the photos someone posted of female body fat percentages, but I would say 35% is probably the right order of magnitude; perhaps a little high. I am normal bmi (arount 23.5) and my waist circumference is less than half my height.

kshama2001 · December 2020

paperpudding wrote: »

and even most muscular fit young men are not far out of BMI range - a point I made earlier.

This 'but young muscly men disprove BMI charts' seems nonsense to me.

Yes they may be slightly out of range - ie have BMI of 27 ish, but unlikely to be well over the range unless elite bodybuilder or some obvious outlier.

rather than vague unsubstantiated claims of "many men I know" - test this theory against known men of known statistics.

For example Kyle Hartigan - I realise most of you will never have heard of him but I chose him because he has been the full back for AFL football team I follow and his stats are public record.
Full back is not the most running position so fair to assume he will be less lean runner type and more strength body type than other players.
Also fair to assume professional sports players are fit and do gym work as well as general training.

He is 194 cm tall and weighs 98 kg - gives a BMI of 26

Anyone can do this excercise with any sports player whose stats are easily found on google

My OH has almost the same stats as Rob Gronkowski but a very different build

kshama2001 · December 2020

ccrdragon wrote: »

Mellouk89 wrote: »

Is a bmi of 29 very far out of the healthy range? We're talking about a difference of 25lbs, about what a man can gain in muscle mass with a year of training.

Lifting weights a few times a week does not mean that you will be adding muscle mass.

For a man to gain 20 to 25 lbs of muscle in a year, they would have to be dedicated to the gym (2-3 hours of lifting a day, 6 days a week, with a very regimented lifting schedule) and be very dedicated to nutrition/calories/sleep schedules/etc. That kind of growth (outside of some test fueled teens) takes a monumental amount of effort and does not just 'happen' for the guys who show up 3-4 times a week and do some curls/bench/deads/squats.

The average Joe that goes to the gym 2-3 times a week and does some curls/bench/deads/squats will NOT be putting on an appreciable amount of muscle over a year or even a more extended timeframe.

Oh, and don't confuse getting stronger (i.e. lifting more) with adding muscle. A lot of the gains that lifters see are not from adding muscle mass - it comes from training the muscles and nervous system to react to the loads that you are moving. I personally have tripled my bench/curl/tricep extention maxes and working loads in a year and only added 1/4 of an inch to my arms and chest.

TL//DR - it takes a lot of work and dedication to add muscle mass and casual lifters don't add an appreciable amount of mass over extended time frames. So the strawman arguments about the mythical guy who adds 20 lbs of muscle in a year are moot.

Yes, I've been lifting off and on since the late 80s and this has been far too casual to affect BMI. However, I was in the military with a few men who between their large frames and dedication to lifting did pass the weight standards only when using the tape test or dunk tank.

Speaking of frame size, this was an interesting thread about how a longer torso than average can affect BMI:

https://community.myfitnesspal.com/en/discussion/10814814/weight-target-based-on-sitting-height

autumnblade75 · January 2021

AnnPT77 wrote: »

So, since I didn't see an answer to this, I'll extend the question to others: Did *anyone* else try the RFM calculator?

https://www.cedars-sinai.org/blog/relative-fat-mass.html

If so, did it produce a number "roughly equal to your body fat percentage", or what you believe that percentage to be?

I'm curious, that's all.

I went and gave it a go. It gave me 40% at my hip bone, and 39% if I measured just above, where I would consider my waist.

Height: 5'3"
Weight: 145
BMI: 25.7

Not a recent picture, but I wore the same size pants then, and I can't be bothered to take a new photo. Actually, I'm wearing the exact same belt at one notch tighter these days.

I was feeling pretty good about getting close to a healthy weight range, but 40% bodyfat sounds pretty depressing.

After comparing in the mirror with this page: https://www.ruled.me/visually-estimate-body-fat-percentage/ I'd guess the RFM calculator is likely a little high, but probably within 5%.

frankwbrown · January 2021

@AnnPT77
Back on December 20th, I had a DEXA body scan. These were my measurements/calculations:

BMI = 36.7
Dexa Scan = 43%
BIA scale = 43.1%
Navy method = 33.4%
RFMI = 33.9%

AnnPT77 · January 2021

frankwbrown wrote: »

@AnnPT77
Back on December 20th, I had a DEXA body scan. These were my measurements/calculations:
BMI = 36.7

Dexa Scan = 43%

BIA scale = 43.1%

Navy method = 33.4%

RFMI = 33.9%

I see that you're making good progress: Congratulations!

By coincidence, I was looking back through the later posts on this thread earlier today. Without doing a formal tally of the responses, FWIW it looked to me as if the RFMI thing is somewhat more likely to be close for men than women. The "measure at top of hip bones" in the article is IMO particularly absurd for women. The other metrics that use a waist measurement don't use that spot as the waist measurement.

I haven't Dexa-ed (don't care that much to spend the $$) except for bone density (which has been Dexa-ed, report doesn't include full body comp), but in the interest of openness:

BMI this morning = 21.1
BIA scale this morning = 23.7% (consistently in 23s lately).
Navy scale, at last check (December) = 23%
RFMI, using natural waist = 29
RFMI, using top of hip bone as instructed in article = 34
Waist to height ratio = about 0.43 (lower end of "healthy" for all-ages adult women, on published scales; I'm 65; older ages are allowed a higher maximum ratio - I've seen 0.6 for 60+ - to still be considered healthy in some analyses)
Waist size, 28" = Benchmarks vary, but haven't seen any suggesting this is unhealthy for women

I'd cheerfully admit that 23% may be a low estimate for me, but 29% is improbable, and 34% is ridiculous, so the idea that RFMI is a good predictor of body fat is IMO not true for me.

I'm not saying RFMI is valueless, BTW. I'm in camp "use multiple metrics". Abstractly, I don't prefer the ones that give me numbers I agree with for me, and reject the ones that don't. 🤷‍♀️ I'll say that RFMI appears to predict poorly for me, as you say BMI does for you.

frankwbrown · January 2021

I agree. I have to take any metric that is based only on some combination of height, weight, waist size, neck size, etc. with a grain (or shaker) of salt. Having said that, I'm warming to the idea that BMI may not be that bad an indicator for me personally.

Having had two DEXA scans, I've done some calculations that suggest that my goal of 230 pounds may be just a milestone and not a destination. Not that I thought otherwise--I chose that goal half because it was an even 100 pounds below where I started. But now, I'm thinking even 200 pounds might be achievable.

My lean body mass currently hovers a little below 160 pounds. If I were to weigh 200 pounds, with a lean body mass of 160, I would be at 20% body fat, with a BMI of 26.4.

Since I'm a data nerd...

I created four charts that show various perspectives on my body composition.
This chart plots all the various metrics, including BMI and RFMI: Body Composition: Body Fat Metrics.

I found this scatter plot of the correlation between BMI and body fat percentage interesting:
Correlation_between_BMI_and_Percent_Body_Fat_for_Men_in_NCHS%27_NHANES_1994_Data.PNG

Correlation_between_BMI_and_Percent_Body_Fat_for_Men_in_NCHS%27_NHANES_1994_Data.PNG

JessiBelleW · January 2021

I jumped on the scale at the gym that measures you, and your body fat and all that. According to BMI I would need to weigh in under 66kgs to be at a 'healthy' weight, but according to the scale my muscle, bone skin and water already weigh close to that. So who knows

zamphir66 · January 2021

I often see folks arguing against the concept of BMI, but in their argument they seem to have an erroneous impression of what BMI actually is. Therefore, I think it's useful to go straight to a reliable source and get a refresher. This, from the CDC, with my emphasis:

Body Mass Index (BMI) is a person’s weight in kilograms divided by the square of height in meters. A high BMI can be an indicator of high body fatness. BMI can be used to screen for weight categories that may lead to health problems but it is not diagnostic of the body fatness or health of an individual.

...BMI is an inexpensive and easy screening method for weight category—underweight, healthy weight, overweight, and obesity.

BMI does not measure body fat directly, but BMI is moderately correlated with more direct measures of body fat (1,2,3). Furthermore, BMI appears to be as strongly correlated with various metabolic and disease outcome as are these more direct measures of body fatness (4,5,6,7,8,9).

To determine if BMI is a health risk, a healthcare provider performs further assessments. Such assessments include skinfold thickness measurements, evaluations of diet, physical activity, and family history (10).

In other words, the tool isn't really designed to tell any one person who much they should weigh. It only tells you where, statistically speaking, your weight begins to be correlated with certain disease risks. It doesn't mean you're absolutely going to get [insert disease] or that you have that disease now. It doesn't mean you can't be healthy, fit, and look good if you're a point or so into the overweight category. Nor does it mean that square in the middle of normal weight is where every person is going to look their best.

Similarly, an eye chart can't diagnose retinopathy, but it's where an eye doctor is going to start the process of determining your eye health.

frankwbrown · January 2021

zamphir66 wrote: »

I often see folks arguing against the concept of BMI, but in their argument they seem to have an erroneous impression of what BMI actually is. Therefore, I think it's useful to go straight to a reliable source and get a refresher. This, from the CDC, with my emphasis:

Body Mass Index (BMI) is a person’s weight in kilograms divided by the square of height in meters. A high BMI can be an indicator of high body fatness. BMI can be used to screen for weight categories that may lead to health problems but it is not diagnostic of the body fatness or health of an individual.

...BMI is an inexpensive and easy screening method for weight category—underweight, healthy weight, overweight, and obesity.

BMI does not measure body fat directly, but BMI is moderately correlated with more direct measures of body fat (1,2,3). Furthermore, BMI appears to be as strongly correlated with various metabolic and disease outcome as are these more direct measures of body fatness (4,5,6,7,8,9).

To determine if BMI is a health risk, a healthcare provider performs further assessments. Such assessments include skinfold thickness measurements, evaluations of diet, physical activity, and family history (10).

In other words, the tool isn't really designed to tell any one person who much they should weigh. It only tells you where, statistically speaking, your weight begins to be correlated with certain disease risks. It doesn't mean you're absolutely going to get [insert disease] or that you have that disease now. It doesn't mean you can't be healthy, fit, and look good if you're a point or so into the overweight category. Nor does it mean that square in the middle of normal weight is where every person is going to look their best.

Similarly, an eye chart can't diagnose retinopathy, but it's where an eye doctor is going to start the process of determining your eye health.

You said, "In other words, the tool isn't really designed to tell any one person [how] much they should weigh."
That is precisely my point. The OP titled this thread "Is BMI an accurate way to know how much I should weigh?".
To that question, I would answer, "No."
Sure, it's a good screening tool. And one can argue it's applicability for the general population. But when it comes down to the individual, it is an approximation whose accuracy will depend heavily on that individuals specific body type.

Is BMI an accurate way to know how much I should weigh?

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