Lean Body Mass Calculator
Result
Lean body mass
- Fat mass
- 17.5 kg
- Resting calories from lean mass (Katch-McArdle)
- 1,504 kcal
Lean body mass is what you would still weigh with the fat taken out — muscle, bone, organs and the water that goes with them — and a lean body mass calculator turns a body fat percentage into that figure and its opposite, fat mass. This page asks for two things: your weight, and a body fat percentage you already have from calipers, a tape measure or a scan. It reports both masses in kilograms, and then puts the lean mass into a second formula, Katch-McArdle, which estimates resting metabolic rate from lean tissue alone instead of from age, height and sex. Fat free mass means the same thing as lean body mass; the two names are used interchangeably, and the figure is the one that moves when you gain or lose muscle.
Lean mass, fat mass and resting calories at a fixed weight
| Body fat (%) | Lean mass (kg) | Fat mass (kg) | Resting calories (kcal) |
|---|---|---|---|
| 10 | 63 | 7 | 1731 |
| 15 | 59.5 | 10.5 | 1655 |
| 20 | 56 | 14 | 1580 |
| 25 | 52.5 | 17.5 | 1504 |
| 30 | 49 | 21 | 1428 |
| 35 | 45.5 | 24.5 | 1353 |
Every row is the same 70 kg person at a different body fat percentage, so the table isolates what composition alone does. Two things are worth noticing: fat mass rises faster than body fat percentage suggests, because the weight is held fixed, and the resting figure follows the lean column rather than the fat one — the whole reason the equation here does not ask for age or sex.
Formula
Lean mass = weight × (1 − body fat % ÷ 100) · Resting calories = 370 + 21.6 × lean mass
- weight
- Body weight, as it is today (kg)
- body fat %
- Body fat percentage you measured — 25 means 25 per cent
- lean mass
- The answer to the first line, and the input to the second (kg)
Use it when you have a body fat percentage and want the two masses it implies, or when you want a resting figure that does not depend on age and sex. The second line is Katch-McArdle, and it is the reason the lean mass matters beyond bookkeeping: two people of the same weight but different body composition burn different amounts at rest, and only the lean-mass equation can see the difference. If you do not have a body fat percentage yet, the skinfold page produces one; if you want the resting figure that the age-and-sex equations give, that is the BMR calculator, and the two will not agree.
Worked examples
70 kg with 25 per cent body fat
- Lean mass: 70 × (1 − 0.25) = 52.5 kg
- Fat mass: 70 − 52.5 = 17.5 kg
- Resting calories: 370 + 21.6 × 52.5 = 370 + 1134 = 1504 kcal
1504 kcal is the resting figure for this body composition, and it is the number most people are surprised by — the lean tissue is what costs energy, and it is a bit over half the body here.
The same person at 90 kg with 30 per cent body fat
- Lean mass: 90 × (1 − 0.30) = 63.0 kg
- Fat mass: 90 − 63 = 27.0 kg
- Resting calories: 370 + 21.6 × 63 = 370 + 1360.8 = 1730.8, shown as 1731 kcal
Twenty kilograms heavier and 10.5 kg of that is lean tissue, which is why the resting figure is 227 kcal higher rather than the modest rise people expect from fat alone. Compare the two rows of the reference table below: holding the weight fixed and raising body fat lowers both lean mass and resting calories.
Limitations
Everything here inherits the error of the body fat percentage you typed in: calipers are usually quoted as within three to four percentage points and a scale can swing more than that with hydration, and three points at 70 kg is 2.1 kg of lean mass. Katch-McArdle treats every kilogram of lean tissue as costing the same 21.6 kcal a day, which is an average rather than a law — organ tissue, muscle and bone are not equally expensive at rest, and the equation carries no age or sex term, so it cannot reflect the differences those bring. Lean mass also includes the body's water, which means a hard training session, a salty meal or a day of dehydration moves the reading without anything changing. This is an estimate to follow over months, not a measurement to compare against someone else's.
Frequently asked questions
- What is lean body mass?
- Everything in the body that is not fat: muscle, bone, organs, connective tissue and the water carried in all of them. Fat free mass is the same quantity, and the two terms are used interchangeably in the literature. The useful property is that it is the part of your weight you want to keep — when weight comes off, the question worth asking is how much of it was fat and how much was lean tissue, and this page gives both halves.
- How do I find my body fat percentage?
- Three routes, in rising order of cost and accuracy: skinfold calipers, a tape measure with a circumference equation, and a scan such as DEXA or underwater weighing. Whichever you use, keep using the same one — a percentage from calipers and a percentage from a smart scale are not the same measurement, and mixing them makes the lean mass number jump for reasons that have nothing to do with your body. The skinfold page on this site produces the figure this page asks for.
- Is lean mass the same as muscle mass?
- No, and the difference matters when you are training. Muscle is part of lean mass, but so are bone, organs and water, and those do not respond to training. Skeletal muscle is roughly half of lean mass in a healthy adult, so a lean mass figure of 52.5 kg is not 52.5 kg of muscle. A body composition scan can separate them; a percentage cannot.
- Why is the resting figure different from the BMR calculator?
- Because the equations use different inputs. The BMR calculator uses Mifflin-St Jeor, which takes weight, height, age and sex; Katch-McArdle, used here, takes lean mass alone. They agree closely for lean people and drift apart as body fat rises, since Mifflin-St Jeor reads a heavier body as a hungrier one while Katch-McArdle reads only the tissue that actually burns. This page names the equation in the output label rather than calling it BMR, so the two numbers are never confused for the same measurement.
- How accurate is Katch-McArdle?
- Its accuracy is the accuracy of your body fat percentage, because lean mass is a subtraction away from it. When the body fat figure is good — a DEXA scan, or calipers used by the same trained tester over time — it is a strong predictor, and it is the preferred one for lean and athletic people precisely because it ignores age and sex. When the body fat figure comes from a bathroom scale, the resting number inherits that noise.
- Can I use lean mass to track muscle gain?
- Yes, with one caveat: lean mass moves with water and glycogen as well as muscle, so a single reading taken after a hard session or a low-carb week will mislead you. Measure at the same time of day, under the same conditions, and read the direction over two to three months rather than the number on any one day. Gaining lean mass while losing fat is the one change that improves body composition without moving the scale.
References
- Nutrition and Athletic Performance — joint position statement — Academy of Nutrition and Dietetics, Dietitians of Canada and American College of Sports Medicine, 2016
- Human energy requirements: report of a joint FAO/WHO/UNU expert consultation — Food and Agriculture Organization of the United Nations