The Mifflin-St Jeor equation, explained simply

Basal Metabolic Rate (BMR) is an estimate of how many calories your body burns at complete rest, just keeping your organs running: breathing, circulating blood, maintaining temperature. It’s not the calories you burn in a day; it’s the floor underneath that number.

Published in 1990, the Mifflin-St Jeor equation replaced an older formula (Harris-Benedict, from 1919) because it more accurately reflected the more sedentary lifestyles and body compositions of modern populations. It has remained the default in most clinical and consumer tools since.

In metric units, for men: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age(years) + 5.

For women, the same formula subtracts 161 instead of adding 5. The difference accounts for typical differences in body composition between sexes, a population-level adjustment, not a judgment about any individual.

Every input is something you already know: weight, height, age. That simplicity is part of why the equation has held up, since it doesn’t require a body-fat scan or a lab visit to get a reasonable starting estimate.

BMR alone doesn’t tell you how many calories to eat, it’s just the resting floor. Total Daily Energy Expenditure (TDEE) scales that number by an activity multiplier, typically ranging from about 1.2 (sedentary, little to no exercise) to 1.9 (very active, hard daily training or a physically demanding job).

TDEE is the number most calorie and macro targets are actually built from: your estimated maintenance calories, before any adjustment for a fat-loss or muscle-gain goal.

Mifflin-St Jeor is a population average, fit to data from a large group of people. It will be close for most people and noticeably off for some, particularly at the extremes of body composition (very muscular or very low body fat) or with certain metabolic conditions. That’s true of every BMR formula, not a flaw specific to this one.

The practical takeaway: treat the number as a starting point, not a verdict. Track for a couple of weeks, watch what actually happens to your weight, and adjust the target based on real data, which is exactly what a consistent log gives you the ability to do.