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

The purpose of BMI is not to tell a specific individual how much they should weigh. BMI uses standard weight status categories that can help doctors to track weight status across populations and identify potential issues in individuals. It gives a rather large weight range at that. Many of us use it as a general guide for what weight range we want to shoot for. If you are generally fir and healthy and feel comfortable with your current weight, don’t worry about it too much.

BMI is designed to give people a range to know where health problems associated with being underweight and overweight start. It is not designed to tell people where they will look and feel their best, although many of us do find that they look and feel their best somewhere in that "healthy" range.

You hear a lot about BMI not being "accurate." Some of that stems from people not understanding what it is for and yes, there are some people who find that it puts them in the overweight or obese category when really they're just heavier due to muscle. But the majority of people in the US who are categorized as overweight or obese aren't in that category due to excess muscle. Let's be real.

(I have no idea if you're in the US or if you currently have a lot of muscle weight, I'm just saying generally).

It's possible for a woman to be so muscular that she should fall in the overweight body mass index range, but it isn't common. For example, most female Olympic champions are in the normal body mass index category, and that includes some whose sports are strength-centric. I doubt they have low muscularity, in nearly any sport these days (though some of the speed sports do pursue very light bodyweight via body fat percent that would be too low for a non-elite athlete).

https://www.runnersworld.com/health-injuries/a20793992/bmis-of-champions-womens-edition/

Also, for equal opportunity reading: https://www.runnersworld.com/races-places/a20811275/bmis-of-champions-mens-edition/

(Casual readers: Yes, those links are from Runners World. The athletes are not all track & field athletes. Read article before commenting on it, eh?)

I'm used to being around collegiate women rowers (a strength endurance sport) from a high-level program. I know what some of them weigh. They're sometimes surprisingly heavy compared to how they look, but usually in the normal range. (They look pretty ripped when they finish a race, but in their spaghetti-strap dress-up outfits at banquets and such, they just look toned & beautiful. They do leg day with plates the size of freight-train wheels, I swear: Impressive strength.)

In women, body configuration makes a difference, too. For example, woman with well above average breast size and wide pelvis (spacing between the right and left side of the bones) is more likely to find a happy weight on the heavier side (though not necessarily outside the normal range).

Conversely, a more narrowly built woman may be best in the lower part of the normal BMI range, even with some muscularity. I'm no bodybuilder, but not completely devoid of muscle, and I'm best around BMI 20, which is toward the lower end, because I have no breasts (post mastectomies) and narrow pelvic width, even though I have broad shoulders and bigger hands/wrists. (Many of the body frame "calculators" rely on wrist/elbow size, and say I have a medium or large frame. I don't. I'm built more like a 14-year-old boy just entering puberty - broad shoulders, narrow hips - than the 65-year-old woman I actually am. 😆)

I don't have a link - someone may - but there are web sites where you can look at women of different body sizes and compare to make a better guess at your body fat percent, and extrapolate from there. Beyond that, another thing you can do is talk with your doctor about what would be a healthy personalized range, for you.

Keep in mind that if you don't have the muscle mass *yet*, it's a slow build, so you can choose to lose slowly down to a good weight for your current configuration, and then add back a bit as muscle mass increases, or to facilitate increasing it at a faster pace. That might be a good choice if you're truly overweight (i.e., have some excess bodyfat) now, as exercise is a bit easier at a healthy weight (speaking as someone who was an obese athlete a few years back).

*If* you have a good sense of body confidence, you can probably trust yourself to realize when you're a good weight. That's kind of a tentative statement on my part, especially if you're a younger woman: It's somewhat common, sadly, among young women to pursue body goals that have been unrealistically calibrated based on very thin celebrities, models, and IG influencers. Keep in mind that if you do want a female bodybuilder physique, the way they look at competition time is not how they look most of the time, walking around in real life. They lean down a lot, for competitions.

Best wishes!

Mellouk89 wrote: »

If for exemple i'm 5'9 150lbs and 15% body fat, I start lifting and gain 20lbs of muscles. Suddenly I am considered overweight in term of bmi even though my body fat percentage is low.

When it comes to gender, men have less body fat and more muscle mass. For this reason, it doesn't take much for the average man to put on enough muscle mass and for bmi to be unreliable in determining obesity or a surplus of body fat.

I gave my own exemple, there are plenty of others.

Like I said, the exception is not only for elite bodydbuilders or athletes, but just regular people who go to the gym.

At 5'9 and 170 pounds, your BMI would be considered .1 into the overweight category, so you've constructed an edge case that would immediately be addressed by your health care provider combining this with a simple waist measurement to give you the proper guidance. What's a more common problem in the west right now -- someone being 25.1 with 15% body fat and receiving the inappropriate guidance to lose weight or someone genuinely overweight?

Keep in mind that the BMI is a population-level analysis and it isn't supposed to be an ultimate guide to whether or not an individual needs to lose weight. Not too many people are in the situation of being a "regular person going to the gym" and adding twenty pounds of muscle "suddenly."

The CDC already accounts for the fact that men and women will tend to have different body fat levels at the same BMI. Given that this tool is designed to be combined with other assessments, what again is the exact concern?

Mellouk89 wrote: »

I could give another exemple, if I was 5'9 160lbs at 18% bf and gained 20lbs of muscles I would be 10lbs overweight according to bmi even though my body fat percentage would have gone down due to the increased muscle mass. Many people go to the gym or do resistance training at home, it isn't unusual and we're talking about men particularly because they have greater capacity for muscle building due to hormones. Women on average gain 10lbs of muscles the first year compared to 20lbs for men.

And in that example, your doctor would still be taking your overall physical state into account and giving you guidance based on that, not the fact that you were in the overweight range on the BMI scale. So what's the exact issue you're concerned about here?

The point is that MOST men in the west who are 5'9 and 180 aren't in danger of being falsely classified as being overweight when the point is that they've just put on 20 pounds of muscle through resistance training at home. They aren't in danger of mistakenly assuming they're overweight. The more prominent issue is that men (and women) ARE overweight.

Those people who are putting on twenty pounds of muscle at home know very well what they're doing and they aren't going to panic because they think they're overweight. You're addressing a problem that doesn't really exist, while the problem of people calculating their BMI and deciding they aren't REALLY overweight because they're sure it's all just muscle and "everybody knows" that BMI is useless is a much more real concern.

If you are talking to me - in my post about elite body builders being outside the range I also said there are some non outlier people ( ie people who are not elite body builders, 9 months pregnant, amputees etc) who sit slightly outside the range.

I am also on record on this forum as saying sporty young men ( not body builders, just fit sporty young men) are often healthy just outside the range with BMIs around 27.
Thaf might be a guy who has been lifting weights for 6 months - but he still wouldn't be very far outside the range.

None of that negates BMI as a good tool for almost all of us, like I said.

Not really sure what your argument is.