Katch-McArdle vs Mifflin-St Jeor

Both formulas are trying to estimate the same thing — the calories you burn at complete rest — but they look at different inputs. Mifflin-St Jeor is a size model: feed it weight, height, age and sex and it returns a BMR, which means two people of identical size get an identical answer even if one is a lean athlete and the other carries far more fat. Katch-McArdle is a composition model: it asks for your body fat percentage, converts it to lean mass, and builds the BMR from the tissue that actually spends the energy. The trade-off is symmetrical — Mifflin needs no body fat figure but cannot see composition, while Katch-McArdle sees composition but is only as good as your body fat measurement.

Take one worked example: a muscular man, 90 kg, 178 cm, 30 years old, at 15% body fat. Mifflin-St Jeor runs off his size: 10 × 90 + 6.25 × 178 − 5 × 30 + 5 ≈ 1,868 kcal. Katch-McArdle runs off his composition: 15% body fat leaves 76.5 kg of lean mass, so 370 + 21.6 × 76.5 ≈ 2,022 kcal.

The gap is about 150 calories at rest, widening to roughly 240 calories a day at a moderate activity factor. On a plan that is a meaningful amount: follow Mifflin here and this lifter would set his maintenance ~240 kcal too low, which reads as an unexpectedly aggressive deficit. The leaner and more muscular the body, the wider that gap grows.

The rule is short. If you train seriously, are lean or muscular, and know your body fat percentage, pick Katch-McArdle — it captures the muscle a size formula misses. If you have an average build, or you do not know your body fat, pick Mifflin-St Jeor, since it needs no body fat input and holds up well for the general population. You do not have to choose blind: run all three formulas side by side on the TDEE Calculator, then turn whichever maintenance figure fits into a week-by-week plan with the Calorie Deficit Calculator.