If a calculator quietly estimates your BMR, odds are it's running the Mifflin-St Jeor equation. Published in 1990, it has become the modern default because it predicts resting energy burn more accurately than the older Harris-Benedict formula for today's population. This guide gives you the actual equation, two fully worked examples, and a clear-eyed look at where it's reliable and where it isn't.
The Mifflin-St Jeor equation estimates your Basal Metabolic Rate (BMR): the calories your body burns at complete rest just to keep you alive. It takes four inputs you already know about yourself: weight, height, age, and sex. No lab, no body-fat measurement, no special equipment.
It replaced the older Harris-Benedict equation as the everyday standard for one simple reason: accuracy. Harris-Benedict was derived from data collected in 1919, when the average body composition and lifestyle of the studied population looked very different from today's. When researchers compared prediction formulas against measured resting energy expenditure in modern adults, Mifflin-St Jeor came out ahead, landing closest to reality across the widest range of people. That's why it is now the formula most registered dietitians reach for first.
The equation is identical for men and women except for the final constant. Weight is in kilograms, height in centimeters, and age in years:
The only difference is the tail end: men add 5, women subtract 161. That 166-calorie gap reflects the fact that, at the same height and weight, men carry more lean muscle and less fat on average, and lean tissue burns more energy at rest.
| Term | Coefficient | Why it's there |
|---|---|---|
| Weight (kg) | × 10 | More body mass means more tissue to maintain |
| Height (cm) | × 6.25 | Taller frames have more surface area and organ mass |
| Age (years) | − 5 | Resting burn declines with age, largely from muscle loss |
| Sex constant | +5 (M) / −161 (F) | Adjusts for average body-composition differences |
Take a 35-year-old man who is 5 feet 10 inches tall and weighs 185 pounds. The equation needs metric units, so convert first:
Now plug into the men's formula:
So at complete rest, this man burns about 1,780 calories every 24 hours. That's his floor, not his target.
Now a 30-year-old woman who is 5 feet 5 inches tall and weighs 145 pounds. Convert to metric:
Plug into the women's formula:
| Profile | Inputs | BMR result |
|---|---|---|
| Man, 35 | 83.9 kg, 177.8 cm | 1,780 cal/day |
| Woman, 30 | 65.8 kg, 165.1 cm | 1,379 cal/day |
The calculator runs Mifflin-St Jeor and converts to TDEE in one step, no unit conversions required.
Calculate My BMR →BMR alone isn't something you eat at. To get your Total Daily Energy Expenditure (TDEE), the number you actually build a diet around, multiply BMR by an activity factor that accounts for movement and training:
The multipliers run from 1.2 for a sedentary desk worker up to 1.9 for a competitive athlete. Our example man, training three to four times a week, would use roughly 1.55: 1,780 × 1.55 = 2,759 calories to maintain. The woman, working out two or three times a week, would use about 1.375: 1,379 × 1.375 = 1,896 calories to maintain. From there, subtract 250–500 to cut or add 250–500 to build.
Mifflin-St Jeor is accurate to roughly ±10% for the majority of adults. For a person with a true BMR of 1,600, that means the estimate is usually within about 160 calories either way, close enough to set a sensible starting target and then calibrate with two weeks of bodyweight tracking.
Its main blind spot is body composition. The equation only sees total weight; it can't tell muscle from fat. Two people of the same age, sex, height, and weight get an identical estimate even if one is a lean athlete and the other carries a lot of fat, despite the athlete's higher resting burn. This is why accuracy slips at the extremes:
If you know your body-fat percentage and fall into one of those groups, the Katch-McArdle formula often does better, because it calculates BMR from lean body mass rather than total weight. For everyone in the broad middle, though, Mifflin-St Jeor remains the most reliable formula you can run without a body-fat number.
For most adults, yes, within about ±10% of measured resting energy expenditure. When researchers compared the common prediction equations against lab measurements, Mifflin-St Jeor was the most reliable for the general population. It's not a lab test, but it's accurate enough to set a target you then refine with real-world tracking.
Mifflin-St Jeor. Harris-Benedict was built on data from 1919 and tends to overestimate BMR for modern adults, often by 5% or more. Mifflin-St Jeor was derived from a more representative contemporary population and consistently predicts closer to measured values, which is why it's the current default.
Partly. Because the formula uses total weight and can't see muscle mass, it tends to underestimate BMR for very lean, muscular athletes. If you're in that group and know your body-fat percentage, Katch-McArdle is usually the better choice since it's built around lean body mass.
The equation is written for kilograms and centimeters, so convert before plugging in: divide pounds by 2.205 for kilograms, and multiply inches by 2.54 for centimeters. Using pounds directly will produce a wildly wrong number. A calculator handles this conversion for you automatically.
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