Lean Body Mass Calculator

Without a scan, lean body mass is estimated from height and weight with equations fitted to measurements of real people. The Boer formula (1984) is the most widely used today and tends to be the most accurate for adults of normal build. The James formula (1976) is used in some medical settings, but it gives implausible results for people with a high BMI, because it was fitted on a narrower group. The Hume formula (1966) was developed for estimating body water and blood volume. Showing all three gives you a sense of the uncertainty: when they agree closely the estimate is probably good; when they differ, the real value could be anywhere in the range.

Formulas based on height and weight cannot tell muscle from fat. A muscular athlete and a sedentary person with the same height and weight get the same estimate, even though their lean mass is very different. If you have a body-fat percentage from a reasonably accurate method — a DEXA scan, a BodPod, hydrostatic weighing, or carefully taken skinfold measurements — enter it, and the calculator works out lean mass as your weight minus your fat mass. Bathroom scales with metal pads are less precise, but averaging several readings taken at the same time of day still gives a useful trend. If you don't know your body fat, the Body Fat Calculator can estimate it with a tape measure.

Lean mass is the main driver of resting energy expenditure, which is why the Katch-McArdle BMR formula uses it instead of weight. Sports nutrition advice often sets protein per kilogram of lean mass rather than total weight, which avoids overestimating needs for people with more body fat. Tracking lean mass over time shows whether a diet is removing fat while keeping muscle — the goal of most weight-loss plans. In medicine, lean body weight is used to dose some drugs, such as certain anaesthetics, that do not distribute into fat.

All three formulas were developed for adults and describe averages. For most people they are within a few kilograms of a laboratory measurement, but very muscular people, very lean people, older adults with less muscle, and people with a high BMI can differ more. They do not apply to children or during pregnancy. Use the result as a starting point for planning and as a trend over time rather than an exact measurement, and talk to a doctor or dietitian about any health decision.