Lean Body Mass Calculator

The definition

Lean body mass is your body weight minus everything stored as fat. It is the total mass of your muscle, bone, organs, skin, connective tissue and body water — everything that is not adipose tissue.

It is not muscle mass

Muscle is only one component of it. Water, bone and organs all count as lean tissue, so a lean body mass figure describes total non-fat mass rather than how much muscle you have built.

Why it is worth knowing

A scale shows one number and cannot tell you how it is divided. Lean mass separates fat from everything else, which is why it is tracked when training, cutting weight, or checking whether progress is coming from the right place.

The method

Take your body weight, work out the share of it that is not fat, and multiply the two together. This calculator performs that single step for you from the two inputs below.

lean mass = weight × (1 − body fat % ÷ 100)

Worked example

A person weighing 70 kg with 15% body fat carries 10.5 kg of fat, so the remaining 59.5 kg is lean tissue. The same person has a fat mass of 10.5 kg.

70 × (1 − 15 ÷ 100) = 70 × 0.85 = 59.5 kg

Where fat mass comes from

Fat mass is simply your body weight minus your lean body mass. The two figures are mirror images of the same calculation, and together they always add back up to the weight you entered.

The two inputs

Body weight is your total mass, entered in kilograms or pounds and converted for you. Body-fat percentage is the share of that mass held as fat, written as a number between 0 and 100.

How the formula works

Dividing by 100 turns the percentage into a decimal fraction. Subtracting it from 1 gives the non-fat share. Multiplying that share by your body weight gives your lean body mass.

lean mass = weight × (1 − body fat % ÷ 100)

Why body fat % is required

Body weight on its own gives a single number with no way to know how it is split. The body-fat percentage is the extra input that makes the split possible, which is why it is a required field here rather than an optional refinement.

What each term means

Muscle mass refers to contractile tissue alone. Lean body mass is the wider category that contains it, alongside water, bone, organs, skin and connective tissue. They overlap, but they are not interchangeable.

Why it matters for a result

A lean body mass of 60 kg describes 60 kg of non-fat tissue, not 60 kg of muscle. Reading the number as a muscle gain target sets an expectation the calculation was never designed to measure.

Fat-free mass

Fat-free mass is a closely related term, often used almost interchangeably with lean body mass in consumer contexts, although some definitions draw the boundary between the two slightly differently.

Estimated, not measured directly

Lean body mass is generally estimated rather than measured outright. This calculator is one such tool: it works backwards from a body-fat percentage you supply, so its accuracy depends on that input.

Other methods

DEXA scans separate bone, fat and lean soft tissue using X-ray attenuation. Bioelectrical impedance sends a small current through the body and infers lean mass from how it is conducted. Skinfold and circumference methods use measurements and published equations.

Which is most accurate?

None is universally best for everyone. A DEXA scan is costly, assumes a bone model, and is less accurate with oedema or implants. Impedance readings shift with hydration. Compare your own figures over time from the same method rather than between methods.

Weight splits two ways

Every kilogram of body weight is either fat mass or lean body mass, so the two figures always total your weight. Body-fat percentage is simply the fat share expressed as a percentage of that total.

How the three figures relate

• Body weightlean mass + fat mass
• Fat massweight − lean mass
• Lean body massweight × (1 − body fat % ÷ 100)

Why the percentage matters

If you gained weight while your body-fat percentage held steady, both lean and fat mass rose but their ratio did not change. That ratio is exactly what the percentage describes.

It is a BMR equation

Katch-McArdle is often mentioned next to lean body mass, but it runs the other way. It estimates basal metabolic rate and takes lean body mass as an input. It does not calculate lean body mass.

BMR = 370 + (21.6 × LBM in kg)

The dependency runs one way

To use it you first need your lean body mass — for example from the calculator above — and then substitute that figure. Any error in the lean mass estimate carries straight into the BMR result.

Why it is often used

It ignores age, sex and height, which helps when those details are unknown or unreliable, and it uses lean tissue rather than total weight, which is generally preferable once body-fat percentage is known.

There is no universal number

Any tool handing you one ideal figure for everybody is oversimplifying. A lean mass that suits a 75 kg adult may be far too low for a larger person, and far too high for a smaller one.

What shapes your lean mass

Overall body size, height, sex and age all matter, as does how your weight is distributed between fat and non-fat tissue. A larger frame generally carries more lean tissue, and the lean share tends to shift with age.

Watch the trend instead

Following how your weight, fat mass and lean mass move together over time tells you far more than a single figure compared against a generic chart. For an individual target, a registered dietitian can take your full history into account.