Lean Body Mass Calculator

What It Does

This page splits your body weight into the part that is fat and the part that is not. Give it a weight and a body-fat percentage — typed from a measurement, or derived from the same neck, waist and hip tape fields the body fat calculator reads — and it returns lean body mass, fat mass, the lean share of your weight, and the fat-free mass index (FFMI) both raw and normalised to a height of 1.80 m. It runs the three published height-and-weight estimates, Boer, James and Hume, beside your own measurement, so you can see how far a regression sits from a real reading. FFMI is the index clinicians and coaches reach for instead of BMI when the question is how much lean tissue a body carries.

Real-World Usage

Dietitians and sports nutritionists track lean mass rather than scale weight, because a training block can add muscle and lose fat at the same time and the scale will not show it. Protein targets are usually set per kilogram of lean mass, so the number this page returns is the denominator of a nutrition plan, and the National Academy of Medicine's 0.8 g per kilogram of body weight is a population floor rather than an athlete's target. In the clinic, FFMI is read with unintentional weight loss to screen for low fat-free mass and to follow a patient through treatment. Gyms and body-composition scales report lean mass as a headline figure; an estimate built from a measurement you took yourself is worth more than a scale's guess, and its trend over months is worth more than any single reading.

Local Context

The formulas are the same everywhere — each was fitted to an adult population and published — but what counts as low lean mass is read against local practice. European clinical guidance (ESPEN 2015) treats an FFMI under 17 kg/m² in men and under 15 kg/m² in women as low fat-free mass when it comes with unintentional weight loss. Reference medians from Schutz et al. (2002) are 18.9 kg/m² for men and 15.4 kg/m² for women aged 18 to 34, so most untrained adults sit well below the numbers strength athletes discuss. In the United States, DXA is the clinical body-composition reference and bioimpedance scales are everywhere in gyms; both carry their own error, and neither replaces a tape and a formula.

Lean Mass and Total Mass

Lean body mass and total body weight answer different questions. Total weight is what a scale reports and what a doctor's chart plots; lean mass is the tissue that training, protein and ageing act on. A person who loses 4 kg of fat and gains 4 kg of muscle in a year has not moved their weight at all, but their lean share has risen several points and their FFMI has moved with it. That is why the lean share on this page is worth watching as a trend: read it monthly, alongside the same tape measurements, and treat a single reading as one estimate among several.

What FFMI Is Used For

FFMI divides lean mass by height squared, the same arithmetic BMI applies to total weight, so it compares people of different heights fairly. Two uses follow. In clinical nutrition an FFMI under 17 kg/m² in men or under 15 kg/m² in women, read with unintentional weight loss, is the ESPEN criterion for low fat-free mass. In sport it is the size-independent measure of muscularity: Kouri et al. (1995) found normalised values up to 25 in athletes who had not used anabolic steroids, and higher in those who had. Treat 25 as a published reference, not a rule — the same study's pre-steroid Mr America winners averaged 25.4, and a person's FFMI says nothing certain about how they got there.

What is the difference between lean body mass and total body weight?

Total body weight is the number on the scale. Lean body mass is that number minus the fat mass: muscle, bone, organs, connective tissue and the water they carry. Two people of the same weight can differ by ten kilograms of lean mass, which is why a training plan that sets protein per kilogram of lean mass starts here rather than at the scale.

What is a normal FFMI?

In 5,635 healthy adults aged 18 to 34, Schutz et al. found a median FFMI of 18.9 kg/m² for men and 15.4 kg/m² for women, so most untrained adults sit near those figures. Under 17 kg/m² for men and under 15 kg/m² for women is the ESPEN 2015 criterion for low fat-free mass, read together with unintentional weight loss. Trained athletes run higher, and the figures strength sports discuss are far above the population median.

Which body-fat percentage should I enter?

Enter a measured value if you have one: DXA or a calibrated underwater weighing is the clinical reference, a skinfold or tape measurement is the practical alternative, and a bioimpedance scale is the noisiest of the three. If you have no measurement, switch to the tape route and let the US Navy equation estimate it from neck and waist. The percentage you type propagates straight through the arithmetic, so an error of five points moves your lean mass by about 3.5 kg on a 70 kg body.

Why do the Boer, James and Hume estimates disagree?

Each was fitted to a different population: Boer (1984) to fluid-volume studies, James (1976) to a British report on obesity, Hume (1966) to a clinical sample. On the same 70 kg, 170 cm body they return roughly 51 to 55 kg of lean mass, and Hume reads lowest at the heavier end. The spread is the honest margin of estimating body composition from height and weight alone, which is why a measured body-fat percentage is worth more than any of the three.