The Mifflin-St Jeor Equation, Explained
The equation behind almost every modern calorie calculator, written out and explained term by term.
Numeric sensitivity of the equation, not a measurement: the activity multiplier changes the answer more than any single input inside the formula.
The equation itself
For men: BMR = 10 x weight in kilograms + 6.25 x height in centimetres - 5 x age + 5. For women: the same calculation with minus 161 instead of plus 5. The result is resting energy in kilocalories per day, for a full 24 hours rather than per hour.
The structure is worth understanding because it explains the behaviour of the number. Weight contributes 10 kcal per kilogram, so 10 kg more body weight means roughly 100 kcal more resting burn. Height contributes 6.25 kcal per centimetre. Age subtracts 5 kcal per year. The sex constant of 166 kcal between men and women reflects average differences in body composition.
| Term | Contribution | What it represents |
|---|---|---|
| 10 x weight (kg) | 10 kcal per kg | metabolically active tissue, mostly lean mass |
| 6.25 x height (cm) | 6.25 kcal per cm | body surface area and organ size |
| -5 x age (years) | -5 kcal per year | gradual loss of lean mass with age |
| +5 (men) | +5 kcal | average difference in body composition |
| -161 (women) | -161 kcal | average difference in body composition |
Why it replaced Harris-Benedict
Harris-Benedict was published in 1919 from a small sample of mostly normal-weight European and American adults, and it was the standard for most of the twentieth century. Mifflin-St Jeor was published in 1990 from a larger, more diverse sample and was found to predict measured resting energy more accurately, particularly in people with obesity.
Validation studies typically find Mifflin-St Jeor within 10% of measured values for about 70% to 80% of healthy adults, with Harris-Benedict reading a few percent higher on average. Most clinical dietetic guidance now recommends Mifflin-St Jeor as the default for adults.
How accurate is it, really
Validation studies compare the equation against indirect calorimetry, the laboratory method that measures oxygen use. Across healthy adult samples the equation lands within 10% of measured resting energy for roughly 70% to 80% of people, with individual errors stretching to 20% in either direction.
The errors are not random. Mifflin-St Jeor tends to overestimate in people carrying very high body fat and to underestimate lean, muscular people, because total body weight is only a proxy for metabolically active tissue and that proxy breaks down at both extremes.
| Population | Typical accuracy | Direction of error |
|---|---|---|
| Healthy adults, mixed | within 10% for 70-80% | slight overestimate on average |
| Obesity | within 10% for most | better than Harris-Benedict |
| Lean athletes | often underestimates | muscle is metabolically active |
| Adults over 70 | within 10-15% | loss of lean mass adds error |
When to distrust the number
Four situations reliably break the equation: very high or very low body fat, because weight stops tracking active tissue; significant muscle mass, for the same reason in reverse; a recent large weight change, because the equation only sees the current weight and not the composition behind it; and any period of illness, injury or medication that shifts metabolic rate.
If you are in one of those groups, treat the output as a starting point rather than a prescription. Set it as your baseline, eat to it for two to three weeks, and adjust from the trend in your weight rather than from the formula.
Using it correctly
Three habits keep the equation honest. First, use a current weight measured in the morning after the bathroom, not a number you remember from last year. Second, keep the units consistent with the equation: kilograms and centimetres are the native inputs, and imperial entries have to be converted before the maths runs.
Third, remember what the output is. Mifflin-St Jeor predicts resting energy, not daily burn. You still need to multiply by an activity factor, and that multiplier is a bigger source of error than the equation itself - it is the step people get wrong most often.
A worked example
Take a 35-year-old woman, 165 cm and 68 kg. The equation gives 10 x 68 + 6.25 x 165 - 5 x 35 - 161, which is 680 + 1,031 - 175 - 161, or 1,375 kcal of resting energy. Multiply by 1.375 for light activity and the daily total is about 1,890 kcal.
The interesting part is the sensitivity, not the arithmetic. Ten kilograms of weight moves the result by about 100 kcal. Ten years of age moves it by 50. Choosing light activity instead of moderate moves it by nearly 300 kcal - which is why the activity multiplier, not the equation, deserves your scepticism.
| Input | Change | Effect on resting energy |
|---|---|---|
| Weight | +10 kg | +100 kcal |
| Height | +10 cm | +62 kcal |
| Age | +10 years | -50 kcal |
| Sex | male instead of female | +166 kcal |
When another equation is better
If you know your body-fat percentage, Katch-McArdle is worth the extra step: it calculates resting energy from lean mass rather than total weight, so it handles very lean and very heavy people better. Cunningham does the same job for athletes, using lean mass with a slightly higher coefficient.
If you do not know your body fat, stay with Mifflin-St Jeor. The point of swapping equations is to replace a bad assumption about body composition with a measurement, and guessing body fat to feed a formula that exists to avoid guessing defeats the purpose.
| Equation | Published | Best used for |
|---|---|---|
| Mifflin-St Jeor | 1990 | general adults, the modern default |
| Harris-Benedict | 1919 | historical comparison only |
| Cunningham | 1980 | lean, muscular people and athletes |
| Katch-McArdle | 1975 | anyone who knows their body-fat percentage |
| Owen | 1986 | a conservative cross-check |
Which equation fits you
A one-line guide, for anyone who does not want to test two formulas against each other.
- Most adults without body-fat data: Mifflin-St Jeor
- You know your body-fat percentage: Katch-McArdle
- Lean athlete: Cunningham
- A conservative cross-check: Owen
Frequently asked questions
Is Mifflin-St Jeor better than Harris-Benedict?
For most adults, yes. It was derived from a larger and more diverse sample and predicts measured resting energy more closely, especially in people with obesity. Harris-Benedict reads a few percent higher on average.
Which weight should I enter?
A current morning weight. Using an old weight or a goal weight changes the output by roughly 10 kcal per kilogram, so a stale number can shift your daily target by a few hundred calories.
Does it work for teenagers?
No. It is validated for adults. Growing adolescents have different energy needs and need age-specific equations with clinical guidance.
Why does my smart scale show a different resting rate?
Scales estimate resting energy from bioelectrical impedance, which measures body composition indirectly. When the two disagree, the gap is usually in the body-fat estimate rather than the equation.
Can I use it to set a deficit?
Yes, but apply the activity multiplier first and subtract afterwards. Cutting calories from a resting number produces a deficit far deeper than intended.
How far off can it be for one person?
Individual error reaches 15% to 20%. Two people with identical inputs can differ by several hundred calories a day, which is why the result is a starting point that observation corrects.
Is Mifflin-St Jeor the same as the revised Harris-Benedict?
No. The revised Harris-Benedict is an update to the 1919 equation, while Mifflin-St Jeor is a separate equation derived from a different sample. Both predict resting energy, and Mifflin-St Jeor is the one most modern calculators default to.
Should a bodybuilder use Mifflin-St Jeor?
Only as a fallback. For very muscular people the equation underestimates resting energy, because total weight overstates how much of the body is metabolically active tissue. Cunningham or Katch-McArdle fits better when body fat is known.
Why do online calculators give me different answers?
Usually because of the activity multiplier, not the equation. Two sites using Mifflin-St Jeor can differ by 400 kcal if one treats three gym sessions as moderate and the other as light.
Does the equation work after weight loss?
It needs re-running. A lighter body burns less, so the same inputs give a smaller number, and recalculating every 5 kg keeps the target aligned with the body you actually have.
Sources
- Mifflin MD et al., A new predictive equation for resting energy expenditure in healthy individuals
- Frankenfield D et al., Comparison of predictive equations for resting metabolic rate
- Harris JA, Benedict FG, A Biometric Study of Basal Metabolism in Man
- NIH: Body weight planner
Formulas and thresholds are taken from the sources above. They are a starting point for adults, not a diagnosis.
Last reviewed September 12, 2026. How we calculate · Editorial policy