Formula sourced to Mifflin-St Jeor, 1990
The equation this calculator runs
Basal metabolic rate is the energy your body spends doing nothing at all: breathing, circulating blood, keeping organs running. This tool computes it with the Mifflin-St Jeor equation, published by Mark Mifflin and Sachiko St Jeor in the American Journal of Clinical Nutrition in 1990. It largely replaced the older Harris-Benedict equation from 1919 (revised in 1984) because later validation work found it tracked measured resting energy more closely in a modern adult population.
| Term | Value |
|---|---|
| Weight coefficient | 10 × weight in kilograms |
| Height coefficient | 6.25 × height in centimeters |
| Age coefficient | -5 × age in years |
| Sex constant | +5 for men, -161 for women |
Add those four pieces together and the total is calories per day at complete rest. The three activity rows in the results panel just multiply that number by a fixed factor: 1.2 for a sedentary routine, 1.55 for moderate exercise a few days a week, and 1.725 for a genuinely active week. Those multipliers are the same ones dietitians and exercise physiologists have used for decades to turn a resting number into a livable daily target.
A worked example
Take a 30-year-old man, 5 feet 10 inches tall, 170 pounds. Converting to metric gives about 77.1 kilograms and 177.8 centimeters. Plugging that into the equation: 10 × 77.1 is 771, 6.25 × 177.8 is about 1,111, minus 5 × 30 which is 150, plus the male constant of 5. Add it up and you land around 1,737 calories a day at rest, which is exactly what the calculator shows for those default inputs above. Multiply that by 1.2, 1.55 or 1.725 and you get the three daily totals in the results panel.
Change any single input and the shift is proportional but not identical across people, because height and weight interact differently depending on where they start. That is the whole reason a calculator beats a rule of thumb here: the arithmetic is simple, but doing it correctly for your own numbers by hand is tedious enough that mistakes creep in.
Why this number is a floor, not a target
BMR is what your body needs just to stay alive in bed all day. It is not a calorie goal. Eating consistently below it for an extended stretch is generally something people only do under medical supervision, because the body starts protecting itself in ways that work against whatever the eating pattern is trying to achieve. The useful number for day-to-day planning is total daily energy expenditure, or TDEE: BMR scaled up by how much you actually move. The TDEE calculator runs that next step and adds cut, maintain and gain targets on top of it.
More ways to look at your numbers
- TDEE Calculator, for a full daily target by activity level
- Calorie Deficit Calculator, for a weight-loss or weight-gain plan
- Body Fat Calculator, to see composition alongside the calorie side of things
Common questions
Is BMR the same number a fitness tracker shows?
Usually not exactly. Wearables often blend heart-rate data and movement sensors into their own resting-burn estimate, which can land a bit above or below a Mifflin-St Jeor result. Both are estimates; neither replaces a lab measurement.
Does BMR change as you lose or gain weight?
Yes. Weight and, indirectly, body size feed straight into the formula, so a lower body weight produces a lower BMR. That is one reason weight loss tends to slow down over time even without a change in effort.
Why does the formula subtract a fixed amount per year of age?
The 1990 study behind this equation found resting energy expenditure declining by a roughly consistent amount per year of age across its adult sample, so the regression bakes that slope in as a flat per-year subtraction rather than a curve.
Could two people with identical stats have different real BMR?
Easily. The equation describes an average across the people it was built from. Muscle mass, genetics, thyroid function and other factors move an individual's true resting burn above or below the predicted line.