Body Positive Math: Fat Percent, Total Fat And FFMI

One thing that leads to a lot of confusion ( and a lot of non-body-positive bigotry) about BMI is that people don’t understand at a deep level how Body Fat Percentage works over a varying range of muscle masses or body types.

Maybe you, like most people, didn’t do so well in math. Let me lead you through the numbers to show how two very similar, “healthy” fat percent women can look totally different!!  They also can get completely opposite “diagnosis” of their health based solely on BMI, which was never intended to be used by individuals.

Maybe this will help you be more body positive.  I put the word healthy in quotes because if the weight loss industry catches on they will just lower what is “healthy” like they did with BMI.

body positive math

In the following word problem, I will be showing you how two totally different sized women can have the same body fat percentage and completely different weights and total body fat.

Lanky Woman And Fat Free Mass

OK, so lets imagine two women who are both tall, say about 5′ 8″. They are on opposite sides of the spectrum for muscle size. The first one we will look at is a lanky woman. The kind that are in “fashion” right now ( women should not come in fashions, but don’t get me started on that one!)

She has a lean weight of 75 pounds. She would be on the extreme end of a low fat free mass index of about 11 FFMI.  Fat free mass index (FFMI) is just your lean weight to height calculated similarly to BMI.  Here is a simple calculator in pounds. They have others online that use the metric system too.

low fat free mass index woman

She would look like a fashion model.

If she had a 25% body fat, about average and “healthy” for a young woman, she would have a total weight of 100#.  That would be a very thin woman. She has genes that code for a low amount of muscle.  She would only have to spread 25# of fat all over the edges of her body.  She would not have to do anything special to look this way (as SO MANY people would have you believe), she just has the genes that code for small muscles.

Some ethnicities with low FFMI are Indian, Scandinavian, French, Italian, Greek, and generally anywhere with a hot climate or with people who migrated from a hot climate 10,000 years ago or less.( see Allens rule for why and check out this cool human migration chart from National Geographic)

Stocky Woman And Fat Free Mass

On the other extreme, lets look at a very stocky woman or “Fat” woman, as is the politically correct term.  Lets say there is a woman who has a lean weight that is about 2.5x as large as the lanky woman but is still 25% body fat percent; a “doctor approved” healthy fat percentage for her young 20s age.

She would be in fashion in the 1880s, and who knows, just may come in fashion again in our life time. I know my 80 something year old father said his Grandfather married a “fashionably large” woman at the turn of the century.  So much for women’s lib back in the day either!  Weight gain products were sold till the mid 70s and if you don’t believe me Google “Wate on ad pics“.

She would be on the high range of the fat free mass index, around 30. She would have a lean weight on par with competitive body builders.  She would not need to do anything special to get this amount of lean (as SO MANY people would have you believe) other than have the right genes that code for it.

Some ethnicities with high FFMI are Polynesian, Native American (north and south), German, and Scottish or anywhere with a cold climate or with people who migrated from a cold climate 10,000 years ago or less.

She would have a lean weight of 10# and a total weight of 260#.  This woman has to not only spread another 120# of muscle on the same amount of bones as that lanky (low FFMI) woman, she has to spread another 40# of fat on it as well!

Where does she put all that? By the time you add all that weight up she looks like she has a lot of fat, a lot more than the other woman. But from a body fat percentage point of view they are exactly the same!!

high fat free mass index woman

Summary of Body Positive math

Lanky Woman (low ffmi)    Stocky Woman (high ffmi)

Height                         5’8″                                       5’8″

Total weight:              100#                                       260#

Total Lean:                  75#                                        195#

Total Fat:                    25#                                         65#

Total body fat %        25%                                         25%

BMI                             15.2 (“underweight”)            39.5 (“Obese”)

Same fat percent, same height, totally different look and BMI label.

Hope this helps you in understanding how we can be very different and also very much the same.

Check out fat percentage monitors at (but make sure you calibrate them at a hydrostatic weighing facility!  Mine was WAY off by 15%!)