Katch-McArdle Calculator

Mifflin-St Jeor and Harris-Benedict both use age, sex, weight, and height to approximate body composition indirectly, via a fixed offset for average lean-mass differences between men and women. Katch-McArdle skips the approximation and uses your actual lean body mass instead, derived directly from weight and body fat percentage — more accurate when your composition doesn't match the population average, less accurate if the body fat number itself is a rough estimate.

It depends on how accurate your body fat percentage is and how close your build is to average. For a lean or heavily muscled person, Katch-McArdle typically outperforms Mifflin-St Jeor because it accounts for their actual lean mass rather than a sex-average estimate. For someone with an unmeasured or poorly measured body fat percentage, the reverse can be true — Mifflin-St Jeor's inputs (age, sex, weight, height) are all directly measurable with no estimation error.

Substantially, because it's the only lever that changes lean mass at a fixed weight. At 80 kg, moving from 15% to 30% body fat drops lean mass from 68 kg to 56 kg, which lowers BMR by roughly 260 kcal/day (21.6 kcal per kg of lean mass) — a bigger single-input swing than either equation that uses height and age instead.

Activity level plays no role in the BMR formula itself — it only enters afterward, as a multiplier against a BMR that's already fixed by weight and body fat. Listing all five common tiers side by side, from Sedentary through Very Active, lets you compare them directly instead of committing to a single activity guess up front and having to recalculate if it turns out wrong.