TMB Katch-McArdle

Katch-McArdle: BMR from lean mass

Instead of working off your total weight, the Katch-McArdle equation builds the BMR estimate on lean body mass (LBM): BMR = 370 + 21.6 · LBM, where LBM = weight × (1 − bodyfat%). Fat tissue burns almost nothing at rest, so stripping it out tends to give a tighter estimate for athletes and muscular subjects than Mifflin-St Jeor, which leans on total weight. The catch is that you need a trustworthy body-fat reading first (DEXA, BIA, skinfolds, bod pod). Take someone at 70 kg with 15% body fat: that's 59.5 kg of LBM, so BMR = 370 + 21.6·59.5 ≈ 1,655 kcal/day. There's no slot for sex or age here, since LBM already folds in what those would tell you.

Applications and context

You'll see it all over sports nutrition — the Casey Butt natural-bodybuilding model is one example — along with physique competitors and body recomposition protocols where someone is already tracking their composition. If you're untrained, sedentary, and don't have a solid body-fat number, Mifflin-St Jeor is the easier call.

FAQ

How do I measure body-fat %? For accuracy, DEXA is the gold standard, followed by bod pod or hydrostatic weighing. Bioimpedance (BIA) scales and skinfold callipers are cheaper but you can't fully trust the numbers. Eyeballing it doesn't count.

Katch-McArdle or Mifflin-St Jeor? Go with Katch-McArdle when you've got a reliable body-fat figure and the person is athletic. Without a body-fat reading, and for most people, Mifflin-St Jeor is the safer default.

Why no sex or age? Women and older adults tend to carry less LBM, and the formula already accounts for that through the LBM input. Tacking on an extra correction would count the same effect twice.