Lean Body Mass Calculator
Calculate your lean body mass and fat mass from your weight, height, and sex using the clinically used Boer formula.
Example
A man weighing 80 kg at 178 cm:
LBM = 0.407 x 80 + 0.267 x 178 - 19.2
= 32.56 + 47.526 - 19.2
= 60.9 kg
Fat mass = 80 - 60.9 = 19.1 kg
Body fat = 19.1 / 80 = 23.9%
How it works
Enter your weight (kg), height (cm), and sex; the tool applies the Boer equation to estimate lean body mass, then subtracts it from total weight to give fat mass. Results update live.
Good to know
This Lean Body Mass Calculator splits your body weight into two parts: lean mass (muscle, bone, organs, and water) and fat mass. You enter your weight in kilograms, your height in centimetres, and your sex, and it returns your estimated lean body mass, fat mass, lean percentage, and body fat percentage along with a category label. It's handy for anyone tracking a recomposition goal, a clinician sanity-checking a weight-based estimate, or a lifter who wants a quick read on where their fat percentage sits.
Reach for it when a plain bathroom scale isn't telling you the full story. Two people at the same weight can have very different proportions of muscle and fat, so a falling or stable scale number during training can hide real changes in composition. Recalculating every few weeks lets you watch whether lean mass is holding while total weight drops, which is usually the outcome people actually want.
To read the result, focus on the four stats together rather than a single number. The big figure is your estimated lean mass; fat mass is simply weight minus that figure, and the percentages restate the same split. The badge translates your body fat percentage into a band, and the thresholds differ by sex because women carry more essential fat, so a percentage that reads as "athletic" for a woman may read as "fitness" or higher for a man.
One caveat worth keeping in mind: the Boer equation predicts lean mass from only weight, height, and sex, so it cannot see your actual muscularity. A very muscular or very lean person will get a less accurate estimate than someone of average build, and methods like DEXA or hydrostatic weighing remain the reference for true body composition. Treat the output as a trend-tracking estimate, and always use the same units and consistent weigh-in conditions when you compare results over time.
Frequently asked questions
What is the Boer formula?
It is a widely used clinical equation that estimates lean body mass from weight and height, with separate coefficients for men and women. It is often preferred for medical dosing calculations.
How is fat mass calculated here?
Fat mass is simply your total body weight minus your estimated lean body mass. So if you weigh 80 kg and your LBM is 60.9 kg, your fat mass is 19.1 kg.
Is my data uploaded anywhere?
No — this calculator runs entirely in your browser; nothing is uploaded.
Is this a substitute for medical advice?
No. These are educational estimates — consult a qualified health professional for medical decisions.
People also ask
What is a good lean body mass percentage?
There is no single ideal figure because lean percentage depends on age, sex, height, and how much muscle you carry. Generally a higher lean percentage means a lower body fat percentage, and typical healthy body fat ranges run roughly 14 to 24 percent for men and 21 to 31 percent for women.
How accurate is the Boer formula for lean body mass?
The Boer formula is one of the more reliable population-based equations and is commonly used clinically, but it is still an estimate built only from weight, height, and sex. Accuracy drops for people with unusually high muscle mass, very low body fat, or obesity, where direct measurement methods are more dependable.
What is the difference between lean body mass and fat-free mass?
The terms are often used interchangeably, but technically lean body mass includes the small amount of essential fat stored within organs and the nervous system, while fat-free mass excludes all fat entirely. In everyday use and in most calculators the difference is minor and both refer to everything that is not body fat.
Why does the calculator ask for my sex?
The Boer equation uses different coefficients for men and women because the sexes differ in average muscle, bone density, and essential fat. Selecting the wrong option will shift both the lean mass estimate and the body fat category.
Can I use pounds and inches instead of kg and cm?
This tool only accepts kilograms for weight and centimetres for height, so you would need to convert imperial units first. Divide pounds by 2.205 to get kilograms, and multiply inches by 2.54 to get centimetres.
How is lean body mass different from BMI?
BMI compares weight to height as a single number and does not distinguish muscle from fat, so a muscular person can register as overweight. Lean body mass instead estimates how much of your weight is non-fat tissue, giving a composition-focused view that BMI cannot provide.
How often should I recalculate my lean body mass?
Body composition changes slowly, so recalculating every two to four weeks is usually enough to see a meaningful trend. Measuring too frequently mostly captures day-to-day fluctuations in water and food intake rather than real changes in lean or fat mass.
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