How much of your weight is not fat — by the Boer, James and Hume formulas side by side, with a measured comparison if you know your body fat percentage.
Your numbers
%
cm
kg
Lean body mass — average of three formulas
59.2kg
76% of your body weight
Fat mass remainder
18.8kg
≈ 24% body fat
FFMI
18.7kg/m²
fat-free mass index
Three formulas, side by sideHover a bar for its value. Enter a body fat percentage above to add a measured bar for comparison.
The formulas were fitted to typical adult builds. They read low for very muscular people and drift at extreme heights or weights — a measured body fat percentage always beats the estimate.
What lean body mass actually includes
Lean body mass (LBM) is your total weight minus fat mass — but it is not a synonym for muscle. It bundles skeletal muscle (roughly half of it), bone, organs, skin, blood and a great deal of water. Strictly, LBM also includes the small amount of essential fat inside organs and nerves, which is why physiologists sometimes distinguish it from fat-free mass; for practical purposes the two are used interchangeably, and the formulas here estimate the same quantity.
It is worth knowing because it is the metabolically expensive part of you. Lean tissue drives most of your resting energy burn, determines your strength ceiling, and is the thing worth protecting in any diet — successful fat loss is defined as weight leaving while LBM stays.
The three formulas
All three estimate LBM from height, weight and sex alone, each fitted by regression against a reference method on a different population:
The modern clinical default; holds up best against DEXA across typical and higher body weights
James
1976
Accurate mid-range, but its squared term makes it bend unrealistically at high body weights
Hume
1966
The oldest; derived from a small sample, tends to read slightly low for tall or heavy adults
Why they disagree — and why the spread is informative
Height and weight simply do not contain enough information to pin down composition, so each formula encodes the average build of its study population. For a typical adult the three land within a few kilograms of each other, and their average is a fair estimate. The further your build sits from average — very muscular, very lean, very heavy — the wider they spread, and the more the honest answer becomes “measure it”. A tape-based estimate from our body fat calculator turns directly into a measured LBM: enter the percentage above and the chart shows how far the formulas miss you.
What LBM is used for
Metabolic rate. The Katch-McArdle equation estimates resting metabolism as 370 + 21.6 × LBM — no age or sex terms, because lean mass is what actually burns the calories. If you know your LBM, it is the most personal of the BMR formulas, and the best basis for calorie targets in very muscular or very lean people.
Drug dosing. Several anesthetics and chemotherapy agents are dosed on lean rather than total weight, because fat tissue absorbs and releases them differently. This is the context the Boer and James formulas were actually built for — anesthesiology references still cite them.
Tracking a cut or a bulk. Scale weight alone cannot tell fat loss from muscle loss. Recomputing LBM monthly from a consistent body fat measurement shows which one is happening — lean mass holding steady while weight falls is the signature of a well-run diet.
Raising it
Lean body mass responds to exactly two levers: progressive resistance training and adequate protein (about 1.6–2.2 g per kg of body weight daily), supported by a modest calorie surplus and sleep. The realistic rate is humbling — roughly 0.25–0.5 kg of actual muscle per month for men and about half that for women, slowing as training age increases. Anything faster on the scale is water and glycogen. That slowness cuts both ways: muscle is hard to build but also hard to lose quickly, which is why a sensible deficit with high protein preserves nearly all of it.
Frequently asked questions
What is a good lean body mass?+
For most adult men, lean body mass runs about 75–85% of total body weight; for women, about 65–75%. The absolute number depends heavily on height, so the fat-free mass index (FFMI = LBM ÷ height²) is a better comparison: around 16–20 kg/m² is typical for untrained men and 13–17 for women, with well-trained lifters reaching 22–25 and 18–21 respectively.
Why do the Boer, James and Hume formulas give different numbers?+
Each was fitted by regression to a different population in a different decade — Hume in 1966, James in 1976, Boer in 1984 — using different reference methods. They typically agree within 2–4 kg for average builds and diverge at the extremes: the James formula in particular bends downward for heavy people because of its squared weight-to-height term. Boer is the usual default in clinical use.
Is lean body mass the same as muscle mass?+
No. Lean body mass is everything except fat: skeletal muscle, but also bone, organs, skin, blood and body water. Skeletal muscle is typically only about half of it. So a 60 kg LBM does not mean 60 kg of muscle — and day-to-day LBM changes on a scale or impedance device are usually water, not muscle.
How do I increase my lean body mass?+
Progressive resistance training three or more times per week, roughly 1.6–2.2 g of protein per kg of body weight per day, a small calorie surplus (150–300 kcal), and sleep. Realistic muscle gain is about 0.25–0.5 kg per month for men and roughly half that for women, slowing with training age. Everything faster on the scale is water, glycogen or fat.
What is lean body mass used for medically?+
Several drugs — some anesthetics and chemotherapy agents among them — are dosed on lean body mass rather than total weight, because fat tissue handles them differently. LBM also feeds the Katch-McArdle formula for resting metabolic rate, and tracking it during weight loss shows whether you are losing fat or losing muscle.
Which is more accurate — a formula or a body fat measurement?+
A measurement, almost always. The formulas only see height, weight and sex, so they assume you have an average build; any tape, caliper or DEXA-based body fat estimate carries real information about your body. If you enter a body fat percentage in the calculator, treat the measured bar as the answer and the formulas as context.