When to use Katch-McArdle instead of Mifflin-St Jeor

The Katch-McArdle equation estimates BMR from lean body mass instead of total weight, which makes it more accurate when body composition is known. This guide explains when to use it and how the BMR calculator applies it.

The two BMR equations on this site

Mifflin-St Jeor

BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age(y) + 5 (men) / − 161 (women)

This is the default on every calorie calculator here. It uses total weight, so two people of the same weight and height get the same BMR regardless of how much is muscle and how much is fat.

Katch-McArdle

BMR = 370 + (21.6 × lean mass in kg)

Lean mass is your body weight minus fat mass. If you weigh 80 kg at 20% body fat, your lean mass is 64 kg, and your Katch-McArdle BMR is 370 + (21.6 × 64) = 1,752 kcal/day.

When Katch-McArdle is more accurate

Mifflin-St Jeor uses total weight. If you carry a high body-fat percentage, it overestimates your BMR because it treats fat mass as metabolically active. Katch-McArdle corrects for this by using lean mass, which is the tissue that actually drives resting metabolism.

This matters most when:

• You have a body-fat percentage above 25% (men) or 35% (women).
• You are very muscular — Mifflin-St Jeor will underestimate your burn.
• You want a baseline for a body-recomposition phase, where lean mass matters.

Getting your body-fat percentage

The BMR calculator takes your body-fat percentage as an optional input. Get it from the body fat calculator, which uses the US Navy circumference method — tape around your neck, waist, and hips. No scale required, and accurate enough to track direction of travel over weeks.

Comparing the two equations

For a 30-year-old, 180 cm, 80 kg male at 20% body fat:

• Mifflin-St Jeor: ~1,780 kcal/day
• Katch-McArdle: ~1,752 kcal/day

The difference is small here. At 40% body fat, Mifflin-St Jeor stays around 1,780 kcal (it does not see body composition), while Katch-McArdle drops to about 1,150 kcal — because 48 kg of that 80 kg is lean mass, and lean mass burns fewer calories than total weight implies.

Formulas are estimates, not medical advice.

Sources