Original analysis
How much do the BMR formulas actually disagree?
We ran the two most-used resting-metabolism formulas — Mifflin-St Jeor and the revised Harris-Benedict — across 840 realistic body profiles. Harris-Benedict came out higher in 81% of them, by about 58 calories a day on average. But the gap is far from uniform: it more than doubles for larger bodies and is roughly 2.5× bigger for men than for women.
What we did
We generated 840 body profiles — both sexes × 6 ages (20 to 70) × 10 heights (150 to 195 cm) × 7 BMIs (20 to 35) — and calculated each one's basal metabolic rate with both the Mifflin-St Jeor equation and the revised (1984) Harris-Benedict equation, using this site's open calculator engine. This is a transparent comparison of the two published formulas applied across the population, not new clinical measurement — but because every calculator and clinician picks one of these formulas, how far apart they sit is worth knowing. The full dataset is downloadable below, so every figure here can be checked.
Harris-Benedict reads higher — but not evenly
Across all 840 profiles, Harris-Benedict produced the higher resting-burn estimate 81% of the time, by a median of 57 and a mean of 58 kcal/day (about 3.7% of the resting figure). For a lot of people that's a rounding error. But in roughly one profile in four (26%) the two formulas differed by more than 100 kcal/day — enough to matter when you're setting a deficit.
The gap widens with body size
The disagreement isn't constant — it scales with mass. For normal-weight profiles the two formulas sat about 33 kcal/day apart on average; for obese profiles, 78. Harris-Benedict's heavier weight coefficient pulls its estimate up faster as body size increases, which is exactly the bias the Mifflin-St Jeor team set out to correct in 1990.
And it's much bigger for men
The clearest split was by sex. For men the two formulas disagreed by 83 kcal/day on average; for women, just 33 — a 2.5× difference. So a man choosing between calculators is making a more consequential choice than a woman is. Age, by contrast, barely mattered: the average gap held around 58 kcal/day whether the profile was 20 or 70.
Most of the time it's small — but not always
The bulk of profiles (about 74%) sat within 100 kcal/day of each other, and some agreed almost exactly. The widest gap we found was a tall, young, heavier man (195 cm, 133 kg, age 20): 2,455 kcal/day from Mifflin-St Jeor versus 2,694 from Harris-Benedict — a 239 kcal/day difference, enough to wipe out a meaningful slice of a planned deficit on its own.
The numbers in one table
| Cut | Group | Average gap / count |
|---|---|---|
| By body size | Normal (BMI <25) | +33 kcal/day |
| Overweight (25–30) | +53 kcal/day | |
| Obese (30+) | +78 kcal/day | |
| By sex | Men | +83 kcal/day |
| Women | +33 kcal/day | |
| Gap distribution | 0–50 kcal | 347 profiles |
| 50–100 kcal | 269 profiles | |
| 100–150 kcal | 163 profiles | |
| 150–200 kcal | 47 profiles | |
| 200+ kcal | 14 profiles |
What this means for you
For most people — especially women and anyone near a normal weight — it barely matters which formula a calculator uses; the difference is tens of calories. But if you're a larger or younger man, the formula choice can swing your estimate by 150–240 kcal/day. That's the case for using the more conservative, better-validated Mifflin-St Jeor equation as the default — and for treating any single number as a starting point you adjust from real results. You can run your own figure on the calculator or see the two formulas side by side in the comparison.