How the Katch-McArdle BMR Formula Works
The Katch-McArdle formula estimates Basal Metabolic Rate (BMR) — the calories your body burns at complete rest just to keep organs, breathing, and circulation running — directly from your lean body mass (LBM) rather than from age and sex. The formula is: BMR = 370 + 21.6 × LBM (kg). Lean body mass is calculated first as LBM = weight × (1 − body fat % ÷ 100). Because it uses actual body composition instead of population averages, Katch-McArdle tends to be more accurate than the Mifflin-St Jeor or Harris-Benedict equations for people who are notably lean, muscular, or carrying unusually high or low body fat for their age and sex — provided the body fat percentage entered is reasonably accurate.
Worked example
Take a 180 lb person at 20% body fat. Converting to kilograms: 180 lb ÷ 2.2046 ≈ 81.6 kg. Lean body mass = 81.6 × (1 − 0.20) = 65.3 kg. BMR = 370 + 21.6 × 65.3 ≈ 370 + 1,410 ≈ 1,780 calories/day at complete rest. To estimate Total Daily Energy Expenditure (TDEE), multiply BMR by an activity factor: sedentary ×1.2, lightly active ×1.375, moderately active ×1.55, very active ×1.725, or extra active ×1.9. For this example at "lightly active," TDEE ≈ 1,780 × 1.375 ≈ 2,448 calories/day.
Getting an accurate body fat percentage
Body fat percentage is the single most important input to this formula, since it is the only body-composition variable used. A DEXA scan, hydrostatic (underwater) weighing, or air-displacement plethysmography (Bod Pod) gives the most reliable reading. Skinfold calipers taken by a trained professional and multi-point bioelectrical impedance scales are reasonable second choices; a single-point smart scale or a visual guess can easily be off by 5-10 percentage points, which will meaningfully shift the BMR result in either direction. If you do not know your body fat percentage with reasonable confidence, a formula that uses age and sex instead — such as Mifflin-St Jeor — may give a more dependable estimate.
This calculator provides an educational estimate based on a standard published formula. It is not a substitute for professional medical, nutritional, or fitness advice — consult a physician, registered dietitian, or sports medicine professional before making significant changes to your diet or training based on these numbers.