How to Calculate your BMI and Body Fat Percentage

There are many sites with calculators but if you have ever wondered about the math behind them, I will discuss and detail how to calculate both measurements.

BMI or Body Mass Index was developed by Belgium statistician Adolph Quelet in the 1800’s. His intent was this tool was to be used to measure the obesity of the general population, not the fatness of an individual. That being said, since many people and even physicians still use this measurement, here is the formula for calculating your BMI and what the results mean.

(weight in pounds times 703) divided by (height in inches) squared 

(150 X 703) divided by (62x62) = 105450 divided by 3844 = 27.4

BMI                         Weight Status *

Below 18.5              Underweight
18.5 - 24.9              Normal
25 - 29.9                 Overweight
30 & above             Obese

* US Department of Health & Human Services

Body fat percentage is the percentage of total body weight comprised of fat. A formula is not the best way to measure this but it is the quickest and easiest way. Though by the formulas below, it is a great deal more complicated for women! The best time to take your measurements is right after you wake up in the morning after sleeping 7-8 hours. Body weight and waist measurements will be more accurate. Here are the formulas for women and men and what the results mean.

Formula for Women

Abbreviations:

F1, etc = Factor 1, etc
TBW = Total Body Weight
LBM = Lean Body Mass
BFW = Body Fat Weight

F1 (TBW x 0.732) + 8.987
F2 Wrist measurement (at fullest point) divided by 3.140
F3 Waist measurement (at naval) x 0.157
F4 Hip measurement (at fullest point) x 0.249
F5 Forearm measurement (at fullest point) x 0.434

LBM = F1 + F2 –F3 – F4 + F5

BFW = TBW – LBM

Body Fat Percentage = (BFW x100) divided by TBW

Formula for Men

F1 (TBW x 1.082) + 94.42
F2 Waist measurement x 4.15

LBM = F1 – F2

BFW = TBW – LBM

Body Fat Percentage = (BFW x100) divided by TBW

F1     (150 x 0.732) + 8.987 = 118.79
F2     6.25 divided by 3.140 = 1.99
F3     41 x 0.157 = 6.44
F4     44 x 0.249 = 10.96
F5     12 x 0.434 = 5.21

LBM = 118.79 + 1.99 - 6.44 – 10.96 + 5.21 = 108.59

BFW = 150 – 108.59 = 41.41

Body Fat Percentage = (41.41 x 100) divided by 150 = 27.6% 

Classification          Women (% fat)           Men (% fat)

Essential Fat                10-12%                     2-4%
Athletes                        14-20%                     6-13%
Fitness                          21-24%                    14-27%
Acceptable                    25-31%                    18-26%
Overweight                    32-41%                    26-37%
Obese                           42% +                       38% +

BMI is the most widely use indicator of weight status but it really is not a very useful tool. It does not take into account bones, muscles and fat. Bones are denser than muscles and twice as dense as fat. So it is possible to have a high BMI with good bones, strong muscle tone and low fat. Some of the best athletes are considered obese by the BMI. A more accurate measurement is body fat percentage. According to the examples above (I am being brave and using my own stats!) I am in the overweight category in the BMI chart, but I come out in the acceptable range in body fat percentage. So it all depends on where your fat is located, if you have strong bones and if your muscle tone is good.

I could work on this and get the BMI in the normal range which would probably put me in the fitness range in the body fat chart. I have been in those ranges and I actually did feel better and had more energy. So my goal is to get there again. But weight loss doesn’t always mean fat loss. Losing body fat should be the goal, not body mass. If you feel healthy, I wouldn’t place too much emphasis on these charts. They are really just a couple of measurements (not very accurate ones) of your total health picture and a starting point for discussing your health goals. On the other hand, if you are in the obese categories it might be wise to start a discussion with your doctor.