How to Calculate your BMI and Body Fat Percentage

Calculating 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

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.

 

BMI Formula

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

Example

(137 X 703) divided by (62x62) = 96311 divided by 3844 = 25.05

Status Chart

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

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.

 

Body Fat Formulas

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

Female Example:

F1 (137 x 0.732) + 8.987 = 109.27

F2 6 divided by 3.140 = 1.91

F3 36 x 0.157 = 5.65

F4 40 x 0.249 = 9.96
F5 11 x 0.434 = 4.77

LBM = 109.27 + 1.91 - 5.65 – 9.96 + 4.77 = 100.34

BFW = 137 – 100.34 = 36.66

Body Fat Percentage = (36.66 x 100) divided by 137 = 26.76%

Status Chart - Women % Fat

Essential Fat 10-13%

Athletes 14-20%

Fitness 21-24%

Acceptable 25-31%

Obese 32%

Status Chart - Men % Fat

Essential Fat 2-5%

Athletes 6-13%

Fitness 14-17%

Acceptable 18-26%

Overweight 27-37%

Obese 38% +

BMI VS Body Fat Percentage

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 barely in the overweight category in the BMI chart, and 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.


Conclusion

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.

 

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Comments 9 comments

Alli 6 years ago

That's really an informative post. I really needed this information. I need to calculate my BMI and Body Fat Percentage. Thanks a lot. Cheers!!


catherine74 profile image

catherine74 6 years ago from London

Nice hub. Thanks for the comprehensive way of measuring BMI.


Rose Kolowinski profile image

Rose Kolowinski 6 years ago Author

Thank you for stopping by and commenting catherine74.


Lita C. Malicdem profile image

Lita C. Malicdem 5 years ago from Philippines

As a diabetic I constantly monitor my BMI along with my TER-Total Energy Requirement. Both require formulas. It's good you brought out this subject. It's useful to health-conscious people. I implore everybody to try it, too.


Rose Kolowinski profile image

Rose Kolowinski 5 years ago Author

Thank you for stopping by Lita. Your comments are very much appreciated.


Maurice H 4 years ago

My body fat at 62 is in a range of 19.2-20.4 which puts me in the fitness stage. But underneath that is the 18-26 range which is acceptable. I am having a hard time figuring this one out. Have I been out of college too long?


Rose Kolowinski profile image

Rose Kolowinski 4 years ago Author

So sorry, Maurice. I had the fitness range ending at 27 - should have been 17. I have corrected it and hopefully no one else will be confused! Thanks for pointing it out and for stopping by.


Kat H 4 years ago

Using the charts I have a BMI of 41%, but I am very strong, overweight(though very active). Using this calculation my BMI is 27.9% with LBM of 186.5. I feel much more confident about myself with those numbers and plan to continue my "advanced body sculpting" workouts to improve my numbers more. Thank you for posting all of this info!!


Rose Kolowinski profile image

Rose Kolowinski 4 years ago Author

Thanks for stopping by Kat and for your comments. Good luck with your workouts!

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