What is lean body mass, and how do I work mine out?
It’s your weight minus your fat. At 80 kg and 20% fat, that’s 64 kg. But most calculators only use your height, weight and sex. They can’t see muscle you’ve built.
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What is lean body mass?
It’s everything in you that isn’t fat. Your weight is fat plus the rest: muscle, bone, organs and body water.
You may also see “fat-free mass”. It’s almost the same thing. The gap is a small amount of fat in the walls of your cells, up to 3% of weight in men and 5% in women.
Source: Yu 2013
So the scales can’t tell these two apart. Their lean mass can.
Got a body fat reading? Take it from 100, then find that share of your weight. At 80 kg and 25% fat, 75% of 80 is 60 kg lean.
How do you calculate lean body mass?
No body fat number? Use a formula. One common formula is called Boer.
- Men: 0.407 × weight in kg, plus 26.7 × height in metres, minus 19.2.
- Women: 0.252 × weight in kg, plus 47.3 × height in metres, minus 48.3.
Say you’re a man of 80 kg and 1.78 m. That’s 32.6 plus 47.5, minus 19.2. Your lean mass is about 60.9 kg.
A second formula, Janmahasatian, uses your BMI (your weight for your height) too. It works out fat-free mass, which is close to lean mass.
| You | Boer | Janmahasatian |
|---|---|---|
| Man, 70 kg, 170 cm | 54.7 kg | 54.5 kg |
| Man, 80 kg, 178 cm | 60.9 kg | 61.1 kg |
| Man, 95 kg, 178 cm | 67.0 kg | 66.9 kg |
| Woman, 60 kg, 160 cm | 42.5 kg | 38.4 kg |
| Woman, 65 kg, 165 cm | 46.1 kg | 41.3 kg |
| Woman, 80 kg, 165 cm | 49.9 kg | 46.5 kg |
Source: Puri and Blake 2022 (formulas)
For a woman, the two can be 3 to 5 kg apart. Pick one formula and stick with it, so you’re not comparing two different formulas. A formula only changes when your weight does, so it can’t show muscle you gain or lose.
How accurate is a lean body mass calculator?
Good for a rough idea, not for small changes. A King’s College London study tested ten formulas in 1,558 UK adults aged 60 to 65.
It checked each one against a DXA scan, an X-ray scan that sorts fat from muscle and bone. Two formulas came out best: Gallagher, which adds your age, and Janmahasatian.
| Formula | Men: average gap, spread | Women: average gap, spread |
|---|---|---|
| Boer | 2.6 kg, 13 kg wide | 4.5 kg, 12.1 kg wide |
| Gallagher | 2.4 kg, 12.8 kg wide | 2.3 kg, 10.7 kg wide |
| Janmahasatian | 2.3 kg, 13.3 kg wide | 0.9 kg, 10.7 kg wide |
Source: Puri and Blake 2022
The spread is the range that held most people’s errors. So one person’s answer could be several kilos out either way.
Most of the formulas were built on white people. The authors say they may not fit other ethnic groups in the UK, and they only tested people aged 60 to 65.
So treat a calculator’s answer as a rough guide, give or take several kilos, and compare like with like over time.
Why can’t a calculator see your muscle?
Because it only knows your height, weight and sex. It guesses the rest from average people. In adults, weight gain is mostly fat, so the formula counts extra kilos as mostly fat.
Think of guessing what’s in a parcel from its size and weight. The guess is often close. But two parcels the same size can hold very different things.
Two 95 kg men of 178 cm both get 67.0 kg from Boer, even if one lifts weights. That’s the muscle a calculator can’t see. If you train, your real lean mass may be higher.
| Method | Sees your muscle? | The catch |
|---|---|---|
| Formula, such as Boer | No | Same answer for a lifter and a non-lifter of the same size |
| BMI | No | Can’t tell the difference between muscle and fat |
| Body fat scale | Roughly | Shifts with your body water, food and exercise |
| DXA scan | Yes | The reference in studies, but not a perfect one |
Sources: NHS; Yu 2013; Puri and Blake 2022
Using a body fat scale? Weigh at the same time of day, before food and exercise, then use the sum from the first section. Compare devices in body fat scales vs DEXA.
How do you keep or build lean body mass?
Lean mass tends to fall as you age, often while fat rises. The NHS advises strength work for all adults.
Adults should work all the major muscle groups on at least 2 days a week: legs, hips, back, tummy, chest, shoulders and arms. Weights, bands, push-ups, digging and heavy shopping bags all count.
Six ways to track your lean mass and stay strong
- Got a body fat reading? Take it from 100, then find that share of your weight.
- No body fat number? Pick one formula, such as Boer, and stick with it.
- Weigh at the same time of day, before food and exercise.
- Work all the major muscle groups on at least 2 days a week.
- Work each one until you need a short rest before you go again.
- Aged 65 or over? Do activities that improve strength, balance and flexibility on at least 2 days a week.
Sources: NHS; NHS (65 and over); Puri and Blake 2022
Next, read how to build muscle, how much protein you need a day and keeping muscle while you lose weight.
When to speak to a GP
The NHS says to see a GP if:
- you keep losing weight without changing your diet or exercise routine
- you’re losing weight and have other symptoms too (go as soon as you can)
- your BMI is in the overweight or obese range and you want help to lose weight, your waist is more than half your height, or you have a condition such as diabetes, high blood pressure or high cholesterol
- you haven’t exercised for some time, or have medical conditions or concerns, before you start strength work
Heavy for your height but muscly? Read what your BMI means and body fat percentage for men.