Why BMR Calculators Disagree

Run your details through three BMR calculators and you will often get three different numbers. This is not a bug, and none of the calculators is necessarily broken — they are using different equations, published decades apart, fitted to different populations.

For an average build the gap is modest — around 50–100 calories. For lean, muscular or heavier people it widens to 150–250, which is enough to matter: 250 calories a day is roughly a quarter-kilo a week. So it is worth understanding which number to trust, and when the choice actually changes anything.

Same Person, Three Answers

Take one person and run all three equations. A 30-year-old man, 78 kg, 180 cm, with a measured body fat percentage of 15%:

Formula	BMR (kcal/day)	Difference vs Mifflin
Mifflin-St Jeor	1,760	—
Revised Harris-Benedict	1,827	+67
Katch-McArdle	1,802	+42

A 67-calorie spread, which is modest. Now change one thing — make him leaner, at 8% body fat, same weight:

Formula	BMR (kcal/day)	Difference vs Mifflin
Mifflin-St Jeor	1,760	—
Revised Harris-Benedict	1,827	+67
Katch-McArdle	1,920	+160

Mifflin and Harris-Benedict did not move, because neither one knows anything about body composition. Only Katch-McArdle noticed that this person now carries more muscle and less fat at the same weight — and muscle is metabolically active tissue.

That is the whole story of why these formulas disagree. Two of them see a body as height, weight, age and sex. The third sees what the body is made of.

The Three Formulas, Worked

Here is each equation in full, with the same person's numbers substituted, so you can check any calculator's arithmetic yourself.

Mifflin-St Jeor

Published 1990. The American College of Sports Medicine recommends it as the default for healthy adults.

Men:   BMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) + 5
Women: BMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) − 161

Worked (78 kg, 180 cm, 30y, male):
  = 780 + 1125 − 150 + 5
  = 1,760 kcal/day

Why it is the default: it was fitted against indirect calorimetry — actual measured oxygen consumption — in a large modern sample, and it outperforms Harris-Benedict in most head-to-head validation studies.

Revised Harris-Benedict

The original dates to 1919. This is the 1984 Roza & Shizgal revision, still common in clinical practice.

Men:   BMR = 88.362 + (13.397 × weight kg) + (4.799 × height cm) − (5.677 × age)
Women: BMR = 447.593 + (9.247 × weight kg) + (3.098 × height cm) − (4.330 × age)

Worked (78 kg, 180 cm, 30y, male):
  = 88.362 + 1044.97 + 863.82 − 170.31
  = 1,827 kcal/day

Why it reads higher: the original 1919 sample was drawn from a population that was, on average, leaner and more physically active than a modern one. The revision narrowed the gap but did not close it, so Harris-Benedict still tends to run above Mifflin-St Jeor by 50–100 calories.

Katch-McArdle

The only one of the three that accounts for body composition, working from lean body mass rather than total weight.

Lean body mass (LBM) = weight kg × (1 − body fat % ÷ 100)
BMR = 370 + (21.6 × LBM)

Worked (78 kg at 15% body fat):
  LBM = 78 × 0.85 = 66.3 kg
  BMR = 370 + (21.6 × 66.3)
      = 1,802 kcal/day

The catch, and it is a big one: this formula is only more accurate if your body fat percentage is genuinely measured. Feed it a guess and it becomes less accurate than Mifflin-St Jeor, because you have added an error term to an otherwise sound equation.

Run all three formulas on your own numbers

Switch between Mifflin-St Jeor, revised Harris-Benedict and Katch-McArdle and watch how much the answer moves for your body.

Who the Formulas Disagree About Most

The spread is not the same for everyone. It is small for people near population averages and large at the edges.

Body Type	Formula Agreement	What Happens
Average build, average weight	Close (±70 kcal)	All three were fitted on people like this
Lean and muscular	Poor (±200 kcal)	Weight-based formulas underestimate; Katch-McArdle reads higher
Higher body fat	Poor (±200 kcal)	Weight-based formulas overestimate; Katch-McArdle reads lower
Very tall or very short	Moderate	Height coefficients differ between equations
Over 70	Poor	All three lose accuracy; none was fitted primarily on older adults

The pattern: the further you sit from the average body the equations were built on, the more they diverge — and the more the composition-aware formula earns its place.

[!IMPORTANT] If you are a muscular athlete using a weight-based formula, your real BMR is probably higher than the number you are being given. That is a common reason athletes eat too little while trying to cut, and then stall.

Which Number Should You Use?

A simple decision rule:

Do you have a body fat percentage from calipers or a DEXA scan? Use Katch-McArdle.
Are you guessing your body fat, or do you not know it? Use Mifflin-St Jeor.
Is a clinician or dietitian already using Harris-Benedict with you? Stay consistent with them — switching formulas mid-plan just adds noise.

For the large majority of people the answer is Mifflin-St Jeor, which is why it is the default on most well-built calculators.

And a point that matters more than the choice: do not eat at your BMR. BMR is what your body burns at complete rest — it is a baseline, not a target. The number you eat against is TDEE, your BMR multiplied by an activity factor. Eating at your BMR puts most people in an unintended 600–900 calorie deficit.

What Beats Any Formula

All three equations are regressions fitted to populations, and all three carry roughly ±10% error for an individual — a 150–200 calorie band for most adults. No amount of choosing between them escapes that.

What does escape it is your own data:

Pick a formula and get a maintenance estimate.
Eat at that number for two to three weeks, weighing yourself at a consistent time of day.
If weight is stable, that was your real maintenance. If it moved, adjust by 100–200 calories and repeat.

After a month you have something no equation can give you: a maintenance figure measured on the actual body in question. The formulas exist to give you a sensible starting point for that process, not to replace it.

FAQs

Which BMR formula is the most accurate? Mifflin-St Jeor for most people, and it is the American College of Sports Medicine's recommended default. Katch-McArdle is more accurate for lean and athletic people, but only with a measured body fat percentage rather than an estimate.

Why does my BMR differ between two websites? Almost always because they use different equations. A difference of 100–250 calories between Mifflin-St Jeor, revised Harris-Benedict and Katch-McArdle is expected, not an error.

Is BMR the same as RMR? Not quite. True BMR is measured under strict conditions — overnight fast, eight hours of sleep, thermally neutral room. RMR is measured under looser conditions and typically reads about 10% higher. Online calculators estimate rather than measure either.

Should I recalculate my BMR as I lose weight? Yes. BMR scales with body mass, so it falls as you lose weight — which is part of why weight loss slows over time. Recalculate every 4–5 kg of change.

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Why BMR Calculators Disagree: A Guide to 3 Formulas