Lean Body Mass Calculator
Work out your fat free mass from body fat percentage, or estimate it from your weight, height and sex.
Runs entirely in your browser. Nothing you type is uploaded or stored.
How to use this tool
This lean body mass calculator gives you two ways to find your fat free mass. Pick your units first (kilograms and centimeters, or pounds and inches), then choose a mode at the top of the tool.
If you already know your body fat percentage from a scale, calipers, or a body scan, use the "From body fat percentage" mode and enter your weight and body fat. If you do not know your body fat, switch to the "Estimate (Boer formula)" mode and enter your weight, height, and sex. Press Calculate to see your lean body mass, your fat mass, and what share of your body weight is lean tissue. Use Copy result to grab the numbers.
The formula and how it works
Lean body mass (LBM), also called fat free mass, is everything in your body that is not fat: muscle, bone, organs, water, and connective tissue. The simplest and most accurate way to find it is to subtract your fat from your total weight.
Fat mass = weight x (body fat % / 100)
When you do not have a body fat measurement, the Boer formula estimates lean body mass from your height, weight, and sex. It is one of the most widely used clinical estimates.
Women: LBM = (0.252 x weight kg) + (0.473 x height cm) - 48.3
The body fat method tells you your true lean mass for the fat reading you supplied, so it is the more reliable of the two. The Boer formula is only an estimate based on population averages and works best for people of typical build.
A real example
Say you weigh 80 kg and a body scan puts your body fat at 18 percent. Your fat mass is 80 x 0.18 = 14.4 kg. Your lean body mass is 80 x (1 - 0.18) = 80 x 0.82 = 65.6 kg. That means about 82 percent of your body weight is lean tissue.
Now suppose you did not know your body fat. Using the Boer formula for a man who is 178 cm tall and 80 kg: (0.407 x 80) + (0.267 x 178) - 19.2 = 32.56 + 47.53 - 19.2 = 60.9 kg of estimated lean mass. The two methods land close, which is a good sign your numbers are reasonable.
Common questions
What is lean body mass?
Lean body mass, or fat free mass, is the weight of everything in your body that is not fat. That includes muscle, bone, organs, skin, and the water inside you. It is your total weight minus your fat mass.
How do I find muscle mass from body fat?
This tool calculates lean body mass, which is your muscle plus bone, organs, and water. Skeletal muscle alone is only part of that figure. As a rough guide, skeletal muscle is often around 40 to 50 percent of lean mass, but a body scan is needed for an exact muscle reading.
Which method is more accurate?
The body fat percentage method is more accurate because it uses your actual measured fat. The Boer formula is an estimate from height, weight, and sex, so it is best when you have no body fat reading and are of average build.
Why does the tool ask for my sex in the estimate mode?
Men and women carry different average amounts of fat and muscle, so the Boer formula uses separate equations for each. Sex is not needed in the body fat percentage mode because your own measurement already accounts for it.
Is this medical advice?
No. This calculator is for general education and fitness planning only. Lean body mass estimates are approximate and should not replace advice from a doctor, dietitian, or certified trainer.