Katch-McArdle Calculator

The Katch-McArdle equation stands apart from other BMR formulas because it uses lean body mass (LBM) rather than total body weight as its primary variable. Developed by Frank Katch and William McArdle, this equation is based on the well-established physiological principle that metabolically active tissue (primarily muscle) drives basal energy expenditure, while adipose tissue contributes relatively little to resting metabolism. By focusing on lean mass, the Katch-McArdle formula provides potentially more accurate BMR estimates for individuals at the extremes of body composition — whether very lean athletes or obese individuals.

The equation is elegantly simple: BMR = 370 + 21.6 x LBM (kg), where LBM = body weight x (1 - body fat percentage / 100). The 370 kcal constant represents the baseline energy needed for essential organ function independent of body size, and the 21.6 coefficient means each kilogram of lean body mass requires approximately 21.6 kcal per day at rest. This coefficient aligns well with measured values in metabolic ward studies and reflects the combined metabolic demands of muscle, bone, organs, and other non-fat tissues.

The key requirement for using the Katch-McArdle equation is knowing your body fat percentage. This can be measured through various methods: DEXA scan (considered the gold standard, accurate to within 1-2%), bioelectrical impedance analysis (BIA, found in many body composition scales, accurate to within 3-5%), skinfold calipers (accurate to within 3-5% with a skilled technician), hydrostatic weighing (accurate to within 2-3%), or visual estimation using reference photos (least accurate, within 5-8%). The accuracy of the Katch-McArdle BMR estimate depends directly on the accuracy of your body fat measurement.

For bodybuilders, athletes, and fitness enthusiasts who regularly track body composition, the Katch-McArdle formula is often the preferred choice. A 100 kg bodybuilder at 10% body fat has 90 kg of lean mass and a Katch-McArdle BMR of 2,314 kcal, compared to a 100 kg sedentary person at 35% body fat with 65 kg of lean mass and a BMR of only 1,774 kcal. The Mifflin-St Jeor equation, which uses total weight, would give both individuals the same BMR, clearly demonstrating Katch-McArdle's superior handling of body composition variation.

Importantly, the Katch-McArdle equation is gender-neutral. Since it uses lean body mass directly, and the gender differences in BMR are primarily due to differences in lean mass, the single equation works for both men and women. This is a theoretical advantage, though it requires the additional step of measuring or estimating body fat percentage. Use this calculator if you know your body fat percentage for the most composition-aware BMR estimate available.