Muscle Mass Calculator

No tape measure can isolate skeletal muscle directly, and this calculator doesn't claim to. What it measures is your neck, waist (and hip, for women) circumference, from which it estimates body fat using the US Navy equation, and from that derives lean body mass (fat-free mass) — bone, organs, skin and water, in addition to muscle. Skeletal muscle mass is then reported as an explicit range derived from population averages, not as a precise number, because that's the honest limit of what circumference data supports.

Validated against hydrostatic weighing, its standard error is roughly 3-4 percentage points of body fat — good enough for screening yourself or tracking change over months, but not precise enough to distinguish 15% from 17% on a single reading. It performs worst at the extremes: very lean, very obese, or very muscular people, since it assumes an average relationship between girth and fat that those groups don't follow as closely.

Because it's subtracted directly from your waist measurement in the formula, so a small error there has an outsized effect — roughly a 2cm error in the neck measurement shifts your estimated body fat by about 1.5 percentage points. Measure it just below the larynx, with the tape sloping slightly downward at the front, and take it more than once to check consistency.

FFMI (fat-free mass index) is your lean body mass divided by height squared, using sex-specific bands: for men, roughly 17-19 is typical for someone untrained, 21-23 is athletic, and 25+ is at the upper edge reported in drug-free athletes. For women, typical is around 13-15, athletic is 17-19. Skeletal muscle is then estimated as a range — about 51-57% of lean body mass in men and 44-50% in women — based on whole-body MRI population data, since circumference measurements alone can't separate muscle from the rest of lean tissue.

Because a bathroom impedance scale, this equation, and a DXA scan don't measure body fat the same way, and can disagree by 6-8 percentage points on the same person on the same day. None of them is a direct measurement of fat. The practical approach is to pick one method — this calculator, a scale, or a scan — and track it consistently over time, since the difference between two different methods is usually larger than a genuine month or two of real change.