Lean Body Mass Calculator
This lean body mass calculator gives you your Lean body mass and your Fat mass in kg or lb, updating as you type. Two methods sit side by side: enter your weight and your body fat percentage, or, if you do not know it, estimate lean mass from your weight, height and sex with the Boer, James or Hume formula. It is plain arithmetic on the numbers you type, and nothing leaves your browser.
How to calculate lean body mass from body fat percentage
This is the default Method, I know my body fat %. Lean body mass is your total weight minus your fat mass, and fat mass is your weight times your body fat percentage.
- Units: pick Metric (kg, cm) or Imperial (lb, in). It starts from your region.
- Total weight: type your weight in the unit you picked.
- Body fat percentage: type a number from 0 to 100. The hint under the field lists where one can come from: a scale, calipers, a DEXA scan or a tape measure estimate.
- Read the three results at the top: Lean body mass (the large one), Fat mass and Body fat.
Fat mass = 80 × 20 ÷ 100 = 16.0 kg
Lean body mass = 80 − 16 = 64.0 kg
The same person in pounds: 180 lb at 20 gives Fat mass 36.0 lb and Lean body mass 144.0 lb.
Fat mass is worked out as weight minus lean mass, so the two figures add up to your Total weight to one decimal. If the percentage is below 0 or above 100, the results are replaced by the message Body fat must be between 0 and 100 percent. Need a percentage first? The tape measure method in the Body Fat Calculator produces one you can enter here.
How to estimate lean body mass from height and weight
Choose Estimate from height under Method when you have no body fat figure. The Body fat percentage field is replaced by Height, Biological sex (Male or Female) and Formula (Boer, James or Hume). The formula gives lean body mass directly, and the calculator works backwards: Fat mass is your Total weight minus that lean mass, and Implied body fat is Fat mass divided by Total weight. Here implied means the body fat that the estimate leaves over, not something measured.
In the formulas below, W is weight in kg and H is height in cm. If you chose Imperial, the tool converts your pounds and inches first, then converts the answer back.
| Formula | Male | Female |
|---|---|---|
| Boer | 0.407 W + 0.267 H − 19.2 | 0.252 W + 0.473 H − 48.3 |
| James | 1.1 W − 128 (W ÷ H)² | 1.07 W − 148 (W ÷ H)² |
| Hume | 0.3281 W + 0.33929 H − 29.5336 | 0.29569 W + 0.41813 H − 43.2933 |
The result is kept between 0 and your Total weight. If a formula would give more lean mass than you weigh, it shows your whole weight as lean mass and 0.0% body fat. If it would go below zero, it shows 0.0 lean mass and 100.0% body fat. That only happens with unusual height and weight combinations, and it is a sign the formula does not fit those numbers.
Boer, James and Hume compared on the same person
The three formulas disagree by a few kilograms, so choose one and stick with it when you compare your own numbers from week to week. Results for the same inputs, in Metric:
| Inputs | Formula | Lean body mass | Fat mass | Implied body fat |
|---|---|---|---|---|
| Male, 80 kg, 180 cm | Boer | 61.4 kg | 18.6 kg | 23.2% |
| James | 62.7 kg | 17.3 kg | 21.6% | |
| Hume | 57.8 kg | 22.2 kg | 27.8% | |
| Female, 65 kg, 165 cm | Boer | 46.1 kg | 18.9 kg | 29.0% |
| James | 46.6 kg | 18.4 kg | 28.3% | |
| Hume | 44.9 kg | 20.1 kg | 30.9% |
These are estimates for a typical body of that height, weight and sex. They are not a medical measurement, and two people with the same inputs can have very different real lean mass.
Lean body mass, fat-free mass and muscle mass
Lean body mass is all of your weight that is not fat: muscle, bone, organs, skin and the water in them. Fat-free mass is the same idea and the two terms are usually swapped freely. Muscle mass is a smaller slice of lean body mass, so this calculator does not report it. A body fat figure from a scale, calipers or a scan is only as good as the device that produced it, and any error in it carries straight into the lean mass you see here.
What this lean body mass calculator does not do
It works out one weight split, nothing more. It does not measure body fat, give a body fat category, or set calorie or protein targets, and it gives no health advice. For a resting calorie figure use the BMR Calculator; for height-to-weight screening use the BMI Calculator. Nothing you type is saved, and switching Units converts the numbers already in the fields, rounded to one decimal.
Frequently asked questions
How do I calculate lean body mass?
Subtract your fat mass from your total weight. If you know your body fat percentage, choose I know my body fat %, enter Total weight and Body fat percentage, and Lean body mass appears at once. If you do not, choose Estimate from height and enter Total weight, Height, Biological sex and a Formula instead.
How do I calculate lean body mass from body fat percentage?
Multiply your total weight by your body fat percentage divided by 100 to get fat mass, then subtract that from your total weight. At 80 kg and 20 percent body fat, fat mass is 16.0 kg and lean body mass is 64.0 kg. The calculator does both steps as you type and rounds the results to one decimal.
How do I calculate lean body mass without knowing my body fat?
Choose Estimate from height. The Boer, James and Hume formulas estimate lean body mass from your weight, height and biological sex alone, and the calculator then shows the Implied body fat that estimate leaves. These are population formulas, not a measurement of your own body, so treat the result as a rough figure.
What is the Boer formula for lean body mass?
Boer is the default Formula. For men, lean mass in kg is 0.407 times weight plus 0.267 times height in cm, minus 19.2. For women it is 0.252 times weight plus 0.473 times height, minus 48.3. James and Hume use different constants. For one man of 80 kg and 180 cm the three give 61.4, 62.7 and 57.8 kg, so pick one formula and keep it if you want to compare your own numbers over time.
Is lean body mass the same as muscle mass?
No. Lean body mass is everything in your body except fat: muscle, bone, organs, skin and the water in them. Muscle mass is only part of that, so lean body mass is always the larger number. This calculator reports lean body mass only, not muscle mass.
What is the difference between lean body mass and fat-free mass?
In everyday use they mean the same thing: your weight minus fat mass, which is how this calculator works it out. Some researchers keep a small amount of essential fat inside lean body mass and leave it out of fat-free mass, so figures from different sources can differ slightly.
Can I calculate lean body mass in pounds or kg?
Both. Units switches between Metric (kg, cm) and Imperial (lb, in), starts from your region, and converts the numbers you have already typed, rounded to one decimal. Results are shown in the unit you picked.
Is there a lean body mass calculator for women and men?
Yes. With I know my body fat %, the sum is the same for everyone. With Estimate from height, choose Male or Female under Biological sex, because each formula uses different constants for each.
Is my weight or body fat percentage uploaded?
No. The sums run in your browser as you type, and nothing you enter is sent anywhere.
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