Why We Use Mifflin-St Jeor, Not Harris-Benedict

Every calorie calculator has to pick a BMR equation, and most never tell you which one. It's worth knowing, because different equations give different answers for the same person. This guide explains which equation we use, what the main alternative is, and why we made that choice.

The two equations you'll encounter

Harris-Benedict is the older of the two, originally published in 1919 and revised in 1984. For decades it was the default in textbooks and calculators, and plenty of sites still use it.

Mifflin-St Jeor was published in the American Journal of Clinical Nutrition in 1990 by Mifflin, St Jeor, Hill and colleagues. It was developed specifically to predict resting energy expenditure more accurately in a modern population, and validated against measured resting energy expenditure in healthy adults across a wide range of body sizes.

We use Mifflin-St Jeor:

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

Why we chose it

The short version: the Academy of Nutrition and Dietetics recommends Mifflin-St Jeor as the most accurate predictive equation for the general population. That's why it has largely superseded Harris-Benedict in clinical and dietetic practice, and why it's the one behind our TDEE Calculator and BMR Calculator.

This isn't a close-run aesthetic preference. Harris-Benedict was derived from a study population and lifestyle rather different from today's, and it has a known tendency to overestimate resting energy expenditure for many people. If your BMR estimate is too high, your TDEE is too high, and a "maintenance" target quietly becomes a small surplus — which is exactly the kind of error you'd never notice from the number alone.

How much do they actually differ?

Enough to matter, not enough to be alarming. The two equations typically land within a few percent of each other, but for some body types the gap widens to a couple of hundred calories a day. Over weeks, a systematic couple-hundred-calorie error is the difference between slow progress and none.

That said, don't over-index on equation choice. As how to calculate your TDEE explains, the activity multiplier is a far bigger lever than the BMR equation — picking "moderately active" when you're really lightly active shifts the result by hundreds of calories, dwarfing the Mifflin-versus-Harris difference. Equation choice is worth getting right; activity honesty is worth getting right more.

The limits of any BMR equation

Every predictive equation shares the same fundamental limitation: it estimates from height, weight, age, and sex, and BMR actually tracks lean body mass. Two people with identical inputs but very different body compositions will get the same estimate despite genuinely different resting metabolisms.

That's why any BMR figure carries real uncertainty — roughly ±10–15% individual variation — and why we label TDEE moderate confidence rather than high. It's a well-validated equation being asked to do something inherently approximate.

If you carry notably more or less muscle than average, the Lean Body Mass Calculator and Body Fat Calculator will tell you more about why your real-world results might sit above or below the estimate.

The bottom line

We use Mifflin-St Jeor because the Academy of Nutrition and Dietetics recommends it as the most accurate option for the general population, and because Harris-Benedict tends to run high. Both are estimates, both are limited by not knowing your body composition, and neither matters as much as choosing your activity level honestly.

You can see the equation, its citation, and its version history on the TDEE Calculator and its change log.