BMI to body fat %

Doesn't seem to work too well for me - older but fit with more muscle than most old farts my age (55).

Gives me 26%
My "guesstimate" is around 15% but haven't tested my BF for a quite a while.

Formula - 39.316 BF%
Dexa scan done in Feb 2015 - 50.9 BF%

Though I've lost 10# since the scan, pretty confident I didn't lose 11% in body fat.

I'm thinking this formula is assuming that, something like, a female at a BMI of 18=18%BF and a female at a BMI of 30=40%BF and it's just applying a conversion formula to reach those assumed BF% at those BMIs. **numbers are pulled out of my head** and then adding a little cojigger to assume older people have a higher BF%

The formula is largely dependent on BMI being accurate. Which for anybody with muscle it is not.
Formula = 28.74%
Actual 15-18%

Whoa! No, that wasn't even close for me. Maybe because I'm old and have quite a bit of muscle, but it was over 10% high. And I've had my BF% measured by DEXA scan.

Not remotely accurate for me...it gives me a BF% of 23.96. I'm pretty fit and relatively lean, but not super body builder muscular by any means...my BF% is somewhere around 12-15% per my coaches measurements. I trust those measurements given I have a pretty flat stomach and a 4 pack at this point and no love handles. It's difficult to pinch anything on my body save for the little pouch in the lower abdomen that is preventing me from showing off the last two abs needed for a 6 pack.

I'm getting oldish but not old @ 40.

I'm not surprised though...by BMI standards I'm slightly overweight at 25.8

rjmudlax13 wrote: »

Well, if the formula includes BMI, it's fundamentally flawed from the start.

Precisely...any decent amount of muscle mass drastically overstates BMI, and therefore will drastically overstate the answer to this formula.

Add in that if you're of any significant age and keep yourself in shape, it 'punishes' (for lack of a better term) you even more.

As of the middle of last month, I was 388 lbs with a Bod Pod measured bodyfat percentage of 40% (also confirmed via impedance test that same day). I am 6'2", so that gives me a BMI of 49.8.

If we put this into the formula:

1.2 x 49.8 + 0.23 x 36 - 16.2

59.76 + 8.28 - 16.2

62.04 - 16.2 = 51.84% bodyfat

I'm not going to call the formula broken (there's gonna be some of us freaks out there), but I would trust direct measurement vs. calculation just based on your height, weight, and age. You can have two people the same height, weight, and age, but with vastly different bodyfat levels.

It gave me 25.1%. There is no way my bf% is that high.

This formula is just as bad as BMI!

Formula- 28.05% Latest scan- 24.2%. That's a huge difference on a 5'2 woman.

yarwell wrote: »

I have no idea how BMI can be "inaccurate", as it's weight divided by height squared.

When people say it's "inaccurate" they're not referencing the formula. They're saying that it is not an accurate indicator of health for many people who exercise, as it does not take into account body fat vs lean mass.

There's a big difference in health between a woman who is 5'5, 150 and 25% BF, and a woman of the same size who is 40% BF.

senecarr wrote: »

I like how everyone in BMI threads rips on how BMI is inaccurate for people with muscle. It's not a good criticism of BMI because
A. For most of the population, BMI is more likely to underestimate body fat. Based on sampling far more people are within an acceptable BMI but have higher body fat percentage when recommended
B. Who are these people that have enough muscle to alter their healthy BMI range, but don't already know they're carrying extra muscle?
For what it is supposed to do, BMI is fine.

You may have counter-argued yourself in part A. You state that people criticize BMI for being inaccurate, then state that "BMI is more likely to underestimate body fat" - inaccurate is inaccurate, one way or the other. The OP was talking specifically about using BMI to guess your body fat, and I just used my numbers as an example of how inaccurate that formula could be.

And to answer B, I'd like to put myself up as an example again. I am nowhere near fit levels of body fat, so I don't have much muscle definition, and I get winded easily. For years I honestly thought I was completely out of shape and that I would have to lose weight to less than 250 lbs before I'd be considered healthy. I was convinced that "I'm not fat, I'm big boned" was just an excuse. It took a doctor to "set me straight", so to speak.

I would consider BMI good as a starting point, but I keep commenting on BMI threads because so many people are focused on getting to a specific BMI and ignoring other factors. If you're going to shoot for a specific goal for your weight loss, go for body fat - that's a more accurate measure of your progress.

Man, I came up with 26%. I have no idea what my BF% truly is, but that doesn't seem right for me.