Guide · Body composition
Lean Body Mass Calculator: the mass you defend in a cut
A lean body mass calculator returns one number that does a lot of work: it sets your protein target, sharpens your calorie maintenance, and tells you whether a deficit is costing you muscle.
What lean body mass actually measures
Lean body mass is everything that isn't fat: muscle, bone, organs, skin, water. The lean body mass calculator takes your weight and body fat percentage and returns the number in kilograms and as a share of total weight.
Three height-and-weight estimates (Boer, Hume, James) appear alongside the body-fat-derived figure. They typically agree within 2 kg of each other and act as a sanity band when body fat is estimated rather than measured.
Why lean mass is the number that runs your plan
Resting metabolism scales with lean tissue. Every kilogram of lean mass burns roughly 22 kcal a day at rest, so a recomp that quietly loses 3 kg of lean mass costs you about 66 kcal a day in maintenance, permanently, until you rebuild it.
Lean mass is also the right denominator for protein. Targets like 1.6 to 2.2 g/kg of lean mass produce a sharper number than 1.6 to 2.2 g/kg of total weight, especially for higher-body-fat starting points. The lean body mass calculator feeds that protein target and the lean mass input for a Katch-McArdle TDEE.
How to use the calculator step by step
- 1. Enter your weight.Morning weight, after the bathroom, before food or drink. A 7-day average is steadier than any single day if you have it.
- 2. Enter your body fat percentage.If you don't have a recent number, run the body fat calculator first. The lean mass output is only as accurate as the body fat input.
- 3. Read the lean mass output in kilograms.The primary number is your weight minus fat mass. The kilogram figure is what you carry forward, not the percentage.
- 4. Compare against the formula band.Boer, Hume, and James estimate lean mass from height and weight alone. A spread of 4 to 6 kg between them and your body-fat-derived figure is normal when body fat is estimated, not scanned.
- 5. Use lean mass downstream.Set protein at 1.6 to 2.2 g/kg of lean mass in the protein calculator. Use the same lean mass as the input for a Katch-McArdle TDEE. That is the engine.
How lean mass moves through recomposition
Lean mass rises slowly. A trained lifter in a small surplus might add 1 to 2 kg of lean mass over a year. A novice can do more in the first six months, then the rate drops sharply. Anything faster, in any phase, almost always means added water or fat, not muscle.
Lean mass falls fast in aggressive deficits. Cutting at 1 percent of body weight per week, with low protein and no resistance training, can shed 30 to 40 percent of the loss from lean tissue. With protein at 1.6 to 2.2 g/kg of lean mass, training preserved, and the deficit kept moderate, that share drops to under 10 percent. Set the floor with the protein calculator.
Recompute lean mass every 6 to 8 weeks. If it holds steady or rises while weight drops, the cut is doing its job. If it falls, pull the deficit back, raise protein, and add a lifting day before the trend compounds. Anchor calories to a fresh TDEE and translate it with the macros calculator.
Common mistakes using a lean body mass calculator
- Trusting a single formula in isolation. Boer, Hume, and James estimate from height and weight only. They miss the lean lifter and over-credit the higher-body-fat user. Cross-check with a fresh body fat estimate before acting on the number.
- Recalculating after every meal or workout. Lean mass moves in months, not hours. Daily swings in weight and body fat are mostly water and glycogen. Recompute every 6 to 8 weeks from a steady morning weigh-in, not after a heavy training session.
- Setting protein from total weight at a high body fat. At 30 percent body fat, 1.6 g/kg of total weight overshoots what lean tissue actually needs by 20 to 30 percent. Use lean mass as the denominator and feed it into the protein calculator for a target that scales correctly.
- Ignoring lean mass during a cut. Weight on the scale is a blended signal. A drop in total weight with a falling lean mass is a problem, not a win. Track lean mass alongside the trend before raising the deficit through your TDEE.
Once you have your lean mass, what to do with it
Lean mass turns into three downstream numbers: a protein floor in grams per day, a calorie maintenance from Katch-McArdle, and a benchmark to compare against in 6 to 8 weeks. None of those can be set from total weight alone.
The Recomp AI lean body mass calculator returns your lean mass in kilograms, the cross-formula band, and direct links to protein and TDEE so the same number drives the rest of the plan. That is the shortest path from a single reading to a daily target.
Frequently asked questions
How accurate is a lean body mass calculator?
The math is exact. The accuracy depends on the body fat input. A skinfold or visual body fat estimate carries 3 to 5 percentage points of error, which translates to roughly 2 to 4 kg of lean mass uncertainty for an 80 kg adult. The formula band is there to show that range.
Should I use lean mass or total weight for protein?
Lean mass, when body fat is above roughly 20 percent for men or 28 percent for women. Below that, the two values converge and either works. The standard target of 1.6 to 2.2 g/kg refers to lean mass in the original studies, not total weight.
How fast can lean mass actually grow?
Trained lifters add 1 to 2 kg of lean mass a year in a small surplus. Novices can hit 5 to 8 kg in the first year, then the rate drops sharply. Numbers faster than that, week to week, are almost always water, glycogen, or fat, not muscle.
How much lean mass do you lose in a cut?
It depends on the deficit, protein, and training. An aggressive cut with low protein and no lifting can put 30 to 40 percent of the weight loss on lean tissue. A moderate deficit with 1.6 to 2.2 g/kg of lean mass and a lifting program holds that share under 10 percent.