The most truly reliable measure of body composition (fat vs. lean mass) is hydrostatic, but how many people have access to a water tank equipped with a hydrostatic scale? Certainly no gym nor doctor's office that I've ever visited. So simplicity is important, and while the BMI is not exactly "simple" in mathematical terms, it has the advantage of being based on easily-obtained measures (height and weight).
But there's a respectable measure that's even easier - the ratio of waist to height. Doctors say your waist should not exceed one-half your height; applies equally to men and women. For a 6-foot-tall man, that's half of 72 inches, or 36 inches. My last pants-size bump stepped me up to men's size 36, men's U.S. pants are sized in inches, and I'm 5'9" or 5'10", so we know I was too short for my waist size. Makes sense so far.
| height | waist measure | |
|---|---|---|
| 6'0" | 72"/2 | 36" |
| 5'10" | 70"/2 | 35" |
| 5'8" | 68"/2 | 34" |
In high school my waist measure was as low as 28. So what's the lower bound? The BMI charts readily tell us what weight is underweight, waist measure does not.