Lean body mass calculator.
Find your lean body mass (LBM) - everything in your body that is not fat. Pick a method below: enter a known body fat percentage, or use the Boer formula from just your sex, height, and weight. This lean body mass calculator returns your LBM and fat mass in both kilograms and pounds.
Your lean body mass (LBM)
152 lb
about 68.9 kg · method: body fat %
Lean body mass
muscle, bone, organs, water
152 lb
Fat mass
total weight minus LBM
33 lb
Lean mass share
% of total weight
%
Estimates only. For the most personal number, use a measured body fat percentage. Track LBM over time and aim to hold it steady while you lose fat.
the short answer
Lean body mass is calculated two ways. From body fat %: LBM = weight x (1 - body fat% / 100). From the Boer formula: men LBM(kg) = 0.407 x kg + 0.267 x cm - 19.2, women LBM(kg) = 0.252 x kg + 0.473 x cm - 48.3. Fat mass is total weight minus LBM. For example, 185 lb at 18% body fat gives about 152 lb LBM and 33 lb of fat.
What lean body mass is and how it is calculated.
Lean body mass (LBM) is the weight of everything in your body except stored fat - your muscle, bone, organs, connective tissue, and the water inside them. It is a far more useful progress marker than scale weight alone, because two people at the same weight can carry very different amounts of muscle versus fat. Watching LBM tells you whether a diet is costing you muscle or a bulk is actually building it.
This calculator offers two methods. If you have a reliable body fat percentage - from a DEXA scan, calipers, or a smart scale - the most direct method simply removes the fat:
Fat mass = weight - LBM
If you do not have a body fat reading, the Boer formula estimates LBM from just your sex, weight, and height - one of the most validated predictive equations, also used for medical drug dosing:
Women: LBM(kg) = 0.252 x weight(kg) + 0.473 x height(cm) - 48.3
Both methods run on metric internally. If you enter imperial units, this tool converts pounds to kilograms and feet/inches to centimeters before the math, then shows your LBM and fat mass in both pounds and kilograms. The body fat method is the more personal of the two when you have an accurate reading; Boer is the dependable fallback when you do not.
How coaches use lean body mass.
Coaches use LBM to judge whether a phase is working. In a cut, the win condition is fat mass dropping while lean body mass holds - so a coach tracks LBM check-in to check-in instead of cheering for any drop on the scale. In a bulk, the same number confirms the gain is muscle and not just fat. LBM also anchors protein targets, since intake is often set per pound of lean mass rather than total weight.
The friction is recalculating this for every client each week as their weight and body fat shift. A coaching platform stores each client's measurements, recomputes lean and fat mass as new check-ins land, and charts the trend - so you spend your time adjusting the plan based on what the numbers show, not re-running a calculator one client at a time.
Track lean mass for every client - in one place.
Coachway lets you log each client's weight, body fat, and measurements, then watch lean and fat mass trend over time - alongside their meal plan and training, all in a branded client app. It is the operating system for online fitness and nutrition coaches, so you stop stitching together calculators, spreadsheets, and chat for your whole client list.
See how Coachway worksLean body mass calculator FAQ.
What is a lean body mass calculator?
A lean body mass calculator estimates the weight of everything in your body that is not fat - muscle, bone, organs, and water. You can calculate it two ways: from a known body fat percentage, where LBM = weight x (1 - body fat% / 100), or from the Boer formula, which uses only your sex, weight, and height. Subtracting LBM from total weight gives your fat mass.
How is lean body mass calculated?
There are two common methods. From body fat percentage: LBM = total weight x (1 - body fat% / 100). From the Boer formula - for men, LBM(kg) = 0.407 x weight(kg) + 0.267 x height(cm) - 19.2; for women, LBM(kg) = 0.252 x weight(kg) + 0.473 x height(cm) - 48.3. The body fat method is more accurate if you have a reliable body fat reading; Boer is a solid estimate when you do not.
What is the difference between lean body mass and fat-free mass?
They are very close and the terms are often used interchangeably. Strictly, fat-free mass excludes all fat, while lean body mass includes the small amount of essential fat stored in organs and bone marrow (a few percent of body weight). In everyday coaching and on this calculator the two numbers are treated as the same practical figure - the part of your weight that is not stored body fat.
Why does lean body mass matter for fat loss?
Lean body mass is the engine that burns most of your calories at rest, so protecting it during a diet keeps your metabolism higher and your body looking athletic rather than "skinny-fat". The goal of a good fat-loss phase is to lose fat mass while holding lean body mass steady - which is why coaches track LBM, not just scale weight, alongside enough protein and resistance training.
How accurate is the Boer formula for lean body mass?
The Boer formula is one of the more reliable predictive equations and is widely used in clinical and medical-dosing settings. But because it only uses sex, weight, and height, it cannot account for how muscular or lean you actually are - a heavily muscled lifter and an untrained person of the same height and weight get the same estimate. If you have a trustworthy body fat measurement, the body fat method on this page is more personal.
How do I increase my lean body mass?
Build lean body mass with progressive resistance training, enough protein (roughly 0.7-1 g per pound of body weight per day), and a small calorie surplus or at least maintenance. Re-run this calculator every few weeks: if your total weight climbs while your calculated fat mass stays flat, the gain is mostly lean mass, which is exactly what you want when bulking or recomposing.
This calculator is general information, not medical advice. Predictive formulas estimate lean body mass and cannot replace a DEXA scan or clinical assessment - if you have a medical condition or are making decisions about medication dosing, consult a qualified professional.
Keep going: get a body fat reading first with the body fat calculator, then turn your lean mass and height into a muscularity score with the FFMI calculator. For the bigger picture, read how to calculate TDEE and macros for clients.
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