Mifflin-St Jeor vs Katch-McArdle: Which BMR Equation Should You Use?
Two people can weigh exactly the same and burn very different amounts of energy at rest. Whether your BMR equation can see that difference is the entire distinction between Mifflin-St Jeor and Katch-McArdle.
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Should I use Mifflin-St Jeor or Katch-McArdle?
Use Katch-McArdle if you have a reliable body-fat measurement. Use Mifflin-St Jeor if you do not. That is the whole decision. Katch-McArdle is the more accurate equation when its input is good, and the less accurate one when its input is a guess — and body fat is very easy to guess badly.
The difference is not academic. For a 30-year-old man at 80 kg and 180 cm, Mifflin-St Jeor returns 1780 calories no matter what he is made of. Katch-McArdle returns 1925 at 10% body fat and 1580 at 30% — a 346-calorie spread that Mifflin-St Jeor cannot see, because it never asked.
What each equation actually measures
Mifflin-St Jeor takes weight, height, age, and sex. It was published in 1990 and is the modern default, including on Calora's BMR calculator. It assumes an average body composition for someone of your size — a reasonable assumption for most people, and a poor one for the very lean and the very heavy.
Katch-McArdle ignores your weight almost entirely and works from your lean body mass: BMR = 370 + 21.6 × lean kg. Muscle, organs, and bone burn energy at rest. Fat, metabolically speaking, mostly sits there. So the equation asks the question that actually matters and skips the proxy.
Calora's body-fat calculator runs Katch-McArdle if you want to compare the two against your own numbers.
The same person, three body-fat levels
Here is the comparison that makes the trade-off concrete. Every row is the same 80 kg, 180 cm, 30-year-old man — identical on the scale, identical to Mifflin-St Jeor.
| Body fat | Lean mass | Katch-McArdle | Mifflin-St Jeor | Difference |
|---|---|---|---|---|
| 10% | 72 kg | 1925 kcal | 1780 kcal | +145 kcal |
| 20% | 64 kg | 1752 kcal | 1780 kcal | -28 kcal |
| 30% | 56 kg | 1580 kcal | 1780 kcal | -200 kcal |
Read the last column carefully. At around 20% body fat the two equations nearly agree — which is exactly what you would expect, because that is close to average, and average is what Mifflin-St Jeor was built to assume.
The equations diverge at the edges. A lean, muscular man is underestimated by Mifflin-St Jeor. A man carrying more fat at the same weight is overestimated by it. If you are close to average, the choice barely matters — and most people are close to average.
The catch: your body-fat number is probably wrong
Katch-McArdle is only as good as the body-fat percentage you feed it, and that is where it usually falls apart in practice.
A DEXA scan is accurate. Hydrostatic weighing is accurate. The handheld gripper at the gym, the bathroom scale with the bioimpedance feature, and the number you eyeballed from a comparison chart are not — bioimpedance readings in particular swing with hydration, and can be off by several percentage points from one morning to the next.
An error of five percentage points on body fat moves lean mass by 4 kg for an 80 kg person, which moves Katch-McArdle by about 86 calories. Feed the equation a bad guess and you have taken the theoretically better formula and made it worse than the one that never asked.
This is why a plain scale-and-tape equation remains the sensible default: it cannot be poisoned by a measurement you do not actually have.
The short version
Use Katch-McArdle if you have a DEXA, a hydrostatic weigh-in, or callipers used consistently by someone who knows what they are doing — and especially if you are notably lean or notably muscular, where Mifflin-St Jeor will underestimate you.
Use Mifflin-St Jeor if your body-fat figure is a guess, a smart-scale reading, or a number you would not defend. For most people this is the honest answer, and it is why it is the default.
Either way the result is a starting estimate, not a measurement. Eat at it for two to three weeks, watch the scale, and let reality correct the formula — as covered in why calorie calculators disagree.
Frequently Asked Questions
Yes, when you have an accurate body-fat measurement, because it works from lean body mass — the tissue that actually burns energy at rest. But it is only as good as that input. With a guessed or bioimpedance-derived body-fat figure, Katch-McArdle can easily be less accurate than Mifflin-St Jeor, which never needed the number in the first place.
BMR = 370 + 21.6 × lean body mass in kilograms, where lean mass is your weight minus your fat mass. For an 80 kg person at 20% body fat, lean mass is 64 kg, giving a BMR of about 1,752 calories.
Because Mifflin-St Jeor assumes an average body composition for your height, weight, age, and sex, and roughly 20% body fat is close to average for an adult man. The equations converge near the average and diverge at the extremes — which is precisely where Mifflin-St Jeor's assumption breaks down.
Mifflin-St Jeor by default, across the main calorie, TDEE, and BMR calculators, because it needs no body-fat measurement. The body-fat calculator uses Katch-McArdle for people who have a reliable body-fat percentage.
Sources
- Mifflin MD, St Jeor ST, et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990;51(2):241-7.
- Frankenfield D, Roth-Yousey L, Compher C. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults. J Am Diet Assoc. 2005;105(5):775-89.
- Katch FI, McArdle WD. Nutrition, Weight Control, and Exercise. Lea & Febiger.
These results are estimates for general educational purposes only and are not medical or nutritional advice. Individual needs vary. Consult a qualified healthcare professional or registered dietitian before making significant changes to your diet.