Most BMR calculators ask for weight, height, age and sex. The Katch-McArdle formula asks for something different: how much of your body is not fat. That one change makes it the better choice for some people and a worse one for others. Here is how to tell which group you are in.
The Katch-McArdle Formula
BMR = 370 + (21.6 × lean body mass in kg)
Lean body mass (LBM) is your weight minus your fat mass: LBM = weight × (1 − body fat %). There is no age or sex term; those differences are assumed to show up in your lean mass.
Worked Example
A person weighs 80 kg with 18% body fat.
- LBM = 80 × (1 − 0.18) = 65.6 kg
- BMR = 370 + 21.6 × 65.6 = about 1,787 kcal/day
If your weight is in pounds, divide by 2.2046 first. Then multiply BMR by an activity factor to get your TDEE; our TDEE multipliers guide explains which one is realistic.
Katch-McArdle vs Mifflin-St Jeor
| Mifflin-St Jeor | Katch-McArdle | |
|---|---|---|
| Inputs | Weight, height, age, sex | Lean body mass (needs body fat %) |
| Sees muscle? | No | Yes |
| Best for | Most people without a body fat measurement | Lean or muscular people with a reliable body fat figure |
| Main weakness | Can under-estimate muscular people | Only as good as your body fat estimate |
Mifflin-St Jeor is the general default and is covered in our guide to all five BMR equations.
The Catch: Body Fat Measurement Error
A body fat estimate from a smart scale or circumference method can be off by several percentage points. On an 80 kg adult, a 3-5 point error shifts lean mass by roughly 2-4 kg and BMR by about 50-90 kcal. A guessed number gives false precision, so if you only eyeball your body fat, use Mifflin-St Jeor instead. DEXA scans and well-done skinfold measurements are more trustworthy.
When Katch-McArdle Is the Better Pick
- You lift regularly and carry more muscle than average for your size
- You have a recent, reliable body fat measurement
- Two formulas disagree and you want a composition-based tiebreaker
When to Skip It
- You do not know your body fat
- You are very lean or very high in body fat, where equations are least reliable
- You want a quick estimate; the difference is often well under 100 kcal
Turn the Number Into a Plan Safely
Any formula is an estimate. Eat at the estimated TDEE for two weeks, track your weight trend, and adjust by about 100-200 kcal if it does not move as expected. Our article on eating below your BMR explains why very low targets backfire. People with medical conditions, pregnancy or a history of eating disorders should work with a clinician.
Frequently Asked Questions
Is Katch-McArdle more accurate than Mifflin-St Jeor?
It can be for lean, muscular people with a trustworthy body fat measurement. For others the two are close, and Mifflin needs fewer assumptions.
Does it work for women?
Yes. It has no sex term, because lean mass carries that difference.
Why is my result higher than other calculators?
Because it counts your muscle. A lower body fat percentage gives a higher lean mass and a higher BMR.
Is BMR the same as RMR?
Nearly. See our BMR vs RMR explainer.
Formula and accuracy figures checked against published equations and comparisons in October 2026. This is general information, not medical advice.
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