Katch-McArdle vs Harris-Benedict

Harris-Benedict is the grandparent of metabolism formulas. Published in 1919 and revised by Roza and Shizgal in 1984, it fits a BMR to your weight, height, age and sex, and for a century it was the default calorie equation. Katch-McArdle comes from a different tradition: rather than correlating metabolism with body size, it works from the fat-free-mass research of the late twentieth century, using your lean mass as the direct driver of resting energy. One asks “how big are you?”, the other asks “how much of you is metabolically active tissue?” — and on bodies that are unusually lean or unusually muscular, those two questions produce different answers.

Take a lean, muscular man: 100 kg, 182 cm, 28 years old, at 12% body fat. Harris-Benedict runs off his size: 88.362 + 13.397 × 100 + 4.799 × 182 − 5.677 × 28 ≈ 2,143 kcal. Katch-McArdle runs off his composition: 12% body fat leaves 88 kg of lean mass, so 370 + 21.6 × 88 ≈ 2,271 kcal.

The gap is about 130 calories at rest, near 200 calories a day at a moderate activity factor. For reference, Mifflin-St Jeor puts the same man near 2,003 kcal — lower still — because it too works from size alone. Katch-McArdle sits highest here precisely because it credits the extra lean mass that a 12% body fat implies.

Harris-Benedict drifts most on bodies far from the average build it was fitted to. On a very muscular body it under-reads relative to Katch-McArdle, because it has no way to credit above-average lean mass; on a higher-body-fat body of the same weight it over-reads, because it counts every kilogram as if it were equally metabolic. The honest takeaway is that a size formula treats fat and muscle the same, and that is the exact blind spot composition-based Katch-McArdle removes — when you have a reliable body fat number. If you don't, compare all three on the TDEE Calculator, then split whichever maintenance figure you trust into protein, carbs and fat with the Macro Calculator.