What is the Katch-McArdle formula?
The Katch-McArdle equation calculates BMR from lean body mass (your weight minus fat). Unlike formulas based on total body weight, it recognises that fat tissue is far less metabolically active than muscle - so two people of the same weight but different body composition get different, more accurate results.
The Katch-McArdle formula
The formula is simple once you know your lean body mass. Body fat percentage is required because it's used to work out how much of your weight is lean tissue.
- Lean body mass = weight × (1 − body fat % ÷ 100)
- BMR = 370 + (21.6 × lean body mass in kg)
- TDEE = BMR × activity multiplier
Worked example
An 80 kg person at 15% body fat has a lean body mass of 80 × 0.85 = 68 kg. Their BMR is 370 + 21.6 × 68 = 370 + 1,469 = 1,839 calories per day. If they train moderately (×1.55), their TDEE is roughly 2,850 calories.
Katch-McArdle vs Mifflin–St Jeor and Harris–Benedict
The Mifflin–St Jeor and Harris–Benedict equations use total body weight, height, age and sex, and don't require body fat. They're reliable for the general population. Katch-McArdle ignores height, age and sex and instead keys off lean mass, which makes it more accurate for people who are very lean, very muscular, or who otherwise differ from the average - provided their body fat figure is accurate.
| Formula | Based on | Best for |
|---|---|---|
| Katch-McArdle | Lean body mass | Lean/muscular people who know body fat |
| Mifflin–St Jeor | Weight, height, age, sex | General population (most accurate predictive) |
| Harris–Benedict | Weight, height, age, sex | Traditional alternative to Mifflin |
How accurate is it?
Katch-McArdle is only as accurate as your body fat measurement. A DEXA scan or skilled caliper test gives the best input; smart-scale estimates are convenient but more variable. If you don't know your body fat, the Mifflin–St Jeor based REE calculator is a better choice because it doesn't depend on it.
How to find your body fat percentage
Since Katch-McArdle depends entirely on lean mass, an accurate body fat figure matters. The most accurate accessible options are a DEXA scan (very accurate but paid) and skinfold calipers used by a trained tester. Bioelectrical impedance - the technology in smart scales and handheld devices - is convenient but more variable, and readings shift with hydration.
Whichever method you use, measure under consistent conditions: ideally in the morning, fasted and well hydrated. Consistency matters more than absolute precision for tracking change, because using the same method each time keeps your Katch-McArdle estimate comparable over weeks and months.