# Body fat percentage formula from body mass index

The formula to predict your body fat percentage, based on your current BMI, age, and gender:

Child Body Fat % = (1.51 x BMI) – (0.70 x Age) – (3.6 x gender) + 1.4

Adult Body Fat % = (1.20 x BMI) + (0.23 x Age) – (10.8 x gender) – 5.4

where male gender= 1, female=0.

Well, this is just one possible body fat formula. Others exist1 from different authors, and some are specific to different races. But this formula is OK. But you should be aware that there is considerable variation in body fat percentage at any constant Body Mass Index.

References

1. Jackson AS, Stanforth PR, Gagnon J, et al. The effect of sex, age and race on estimating percentage body fat from body mass index: the Heritage Family Study. Int J Obes Relat Metab Disord, Jun 2002;26(6):789-96

Thanks. The formula was exactly what I…nevermind.

What?

I realized, I’m too lazy to use it.

There is a problem with this formula, and others like it. They are linear.

The straight lines. The pink one is from the formula on this page.

But Body fat percentage has a curved distribution relative to body mass index, not linear.

If you were a slim woman with BMI under 20, the pink line formula would say you had 25 to 30% body fat, which is totally wrong.

I’d guess about 15 percent, from the curvy chart above.

That formula is even on wikipedia. The world needs something better.

That DOES make me mad, when I use online fat calculators.

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