The Mifflin–St Jeor BMR formula has a known bias for muscular people. Here's the fix.

If you've used any calorie calculator on the internet — including ours — there's a roughly 95% chance the BMR underneath it is the Mifflin–St Jeor equation. The other 5% are using Harris–Benedict, which was the standard before 1990 and is slightly worse. Mifflin–St Jeor is, on the population it was derived from, the best general-purpose BMR prediction we have. The mean error against indirect calorimetry is about 5%, and it generalizes reasonably well across age, sex, and body size.

For one specific group, though, it systematically gets the answer wrong: muscular athletes. The formula can underestimate the actual BMR of a lean, muscular individual by 200–400 kcal/day. If your "maintenance" calorie target feels relentlessly insufficient and you've ruled out the usual culprits, this is probably why.

This piece walks through why the bias exists, the alternative formula that doesn't have it, and how to know which one applies to you.

What Mifflin–St Jeor actually fits

Mifflin, St Jeor, and colleagues published their formula in 1990 (American Journal of Clinical Nutrition, 51(2):241–247). They measured resting energy expenditure via indirect calorimetry in 498 adults — 251 male, 247 female — covering ages 19 to 78 and a BMI range from 17 to 42. They fit a multi-variable regression predicting BMR (kcal/day) from four inputs: weight (kg), height (cm), age (years), and sex.

The resulting equations:

• Men: BMR = 10 × weight + 6.25 × height − 5 × age + 5
• Women: BMR = 10 × weight + 6.25 × height − 5 × age − 161

The regression explained about 71% of the variance in measured BMR. That number is doing important work — it means roughly 29% of the variance in BMR within their study population is unaccounted for by the inputs the formula uses.

The hidden variable: lean body mass

The single largest source of that unexplained 29% is body composition. Muscle tissue burns roughly 13 kcal per kg per day at rest. Fat tissue burns roughly 4 kcal per kg per day. The two are almost a 3:1 ratio.

Mifflin–St Jeor uses total body weight as its mass input, which means a 90-kg sedentary man and a 90-kg muscular athlete are predicted to have the same BMR. The total mass is the same. The body composition is not — and the BMR is therefore not the same either.

Let's be specific. The sedentary man at 90 kg might be ~25% body fat, meaning about 67.5 kg of lean mass and 22.5 kg of fat mass. His resting metabolic contribution is roughly:

67.5 × 13 + 22.5 × 4 = 877.5 + 90 = 967 kcal/day from these two tissues.

The muscular athlete at 90 kg at 10% body fat has 81 kg of lean mass and 9 kg of fat mass:

81 × 13 + 9 × 4 = 1,053 + 36 = 1,089 kcal/day.

(Both numbers are missing the BMR contribution of brain, liver, heart, kidneys, etc. — which is large but roughly proportional to lean mass. The arithmetic above is illustrative, not exhaustive.)

That ~120 kcal/day gap on this rough calculation is what Mifflin–St Jeor cannot see, because the input it uses (total weight) is the same for both bodies. In practice, accounting for the full BMR contribution of the higher lean mass, the gap is closer to 200–400 kcal/day for very lean, very muscular individuals. The formula systematically under-predicts BMR for them.

The athletes-aren't-in-the-sample problem

There's a second, smaller issue worth naming. The 498 adults Mifflin et al. measured were a "healthy, normal-weight" cohort by 1990 American standards. The mean BMI was around 24. They explicitly recruited normal-weight and obese subjects, not athletes. The formula's regression coefficients are fit to a population that includes basically nobody at the muscular-and-lean end of the body-composition spectrum.

This is a fundamental limitation of any prediction equation: it can only generalize to populations similar to the one it was fit on. Mifflin–St Jeor is excellent for the middle 80% of adults. It's least reliable at the edges — very obese, very lean, very muscular, very elderly with sarcopenia. The athlete case is one of those edges.

The fix: Katch–McArdle

Frank Katch and William McArdle published their formula in 1996. It takes a different input: lean body mass instead of total weight.

BMR = 370 + 21.6 × lean body mass (kg)

That's the whole equation. No separate male/female version, no age adjustment, no height term — just resting expenditure as a function of lean mass.

The trade-off is obvious: you need to know your lean body mass, which requires a body-composition measurement. The options, in rough order of accuracy:

• DEXA scan: the gold standard. Costs $50–$150 in most US metros, takes 15 minutes, gives you bone mass, lean mass, and fat mass with a ~1% margin of error. Worth it if you're going to use the number to set a calorie target you'll follow for months.
• BodPod (air-displacement plethysmography): ~$50, also very accurate, harder to find.
• Hydrostatic weighing: classic gold-standard, now rare because DEXA is easier.
• Bioelectrical impedance (the scales at gyms or that you buy at Target): noisy. Margin of error can be ±5% body fat, which on a 90-kg body is ±4.5 kg of lean mass — a 100 kcal/day swing in your BMR estimate. Better than nothing but not by much.
• U.S. Navy circumference method (our body-fat calculator uses this): tape-measure-based. Margin of error ±3% body fat. Cheap and surprisingly decent for casual use.

The worked example

Take the muscular athlete from above: 90 kg, 10% body fat, so 81 kg of lean mass. Suppose he's 30, 180 cm tall, male.

Mifflin–St Jeor: 10 × 90 + 6.25 × 180 − 5 × 30 + 5 = 900 + 1,125 − 150 + 5 = 1,880 kcal/day.

Katch–McArdle: 370 + 21.6 × 81 = 370 + 1,750 = 2,120 kcal/day.

A 240 kcal/day gap, on the BMR alone. After applying the activity multiplier (1.55 for moderate, say), the TDEE gap becomes about 370 kcal/day — between two equally reasonable formulas, using the same body. That's the difference between a maintenance plan and a slow-cut plan, or between a slow-cut plan and an aggressive cut.

For our athlete, Katch–McArdle is closer to the right answer. Real BMR for that body composition, measured via indirect calorimetry, would typically come in around 2,050–2,150 kcal/day. Katch–McArdle is within 50 kcal; Mifflin–St Jeor is off by 170–270.

Who should use which

Some practical rules of thumb:

• BMI under 22 + visibly muscular (or you know your body fat is under ~12% for men / under ~20% for women): use Katch–McArdle. Mifflin–St Jeor will under-predict.
• BMI between 22 and 30 + average body composition: use Mifflin–St Jeor. It's what it was designed for and it's reliably good.
• BMI over 35: both formulas are less reliable. Mifflin–St Jeor outperforms Katch–McArdle here (high BMI usually means more fat, which Katch–McArdle's lean-mass-only formula handles less well). For clinical-grade accuracy, indirect calorimetry is the answer; for everyday use, Mifflin–St Jeor is the better default.
• Very elderly (70+): both formulas drift. Sarcopenia matters; lean mass declines and the metabolic profile shifts.

The honest summary

Mifflin–St Jeor is the right default because most people aren't athletes. If you are an athlete — or specifically, if you have markedly more lean mass than the average person at your weight — get a body-composition measurement and use Katch–McArdle. The 200–400 kcal/day gap is large enough to make the difference between progress and stagnation on a real training program.

And if you're not sure where you fall: the U.S. Navy body-fat calculator is a free way to estimate lean mass without leaving the house. Plug the result into Katch–McArdle, compare it to what Mifflin–St Jeor says, and if the gap is more than 100 kcal/day, the lean-mass-aware number is the one to trust.