Contents
The formula for men and women
The Mifflin-St Jeor equation calculates your basal metabolic rate (BMR), meaning how many calories your body burns at complete rest over 24 hours on breathing, running the heart, brain and kidneys and keeping warm. These calories get used even if you spend the whole day in bed.
For women: BMR = 10 × weight + 6.25 × height - 5 × age - 161
For men: BMR = 10 × weight + 6.25 × height - 5 × age + 5
Weight is in kilograms, height in centimetres and age in years, and the 166 kcal gap between the men's and women's versions reflects the fact that at the same weight men on average carry more muscle and less fat.
The equation was published in 1990 by Mark Mifflin, Sachiko St Jeor and colleagues at the University of Nevada. They measured the metabolism of 498 healthy people aged 19 to 78, slim and heavy, and derived an equation that matched the results better than the old formulas, and dietitians around the world have treated it as the first choice for adults ever since. BMR isn't your target yet, though, because nobody lies still all day: we walk, work and exercise, and to get real energy use the BMR is multiplied by an activity factor.
Activity factors
| Level | How the person lives | Factor |
|---|---|---|
| Sedentary | Office or remote work, car, no workouts | 1.2 |
| Light | 1-3 workouts a week or lots of walking | 1.375 |
| Moderate | 3-5 workouts a week | 1.55 |
| High | 6-7 workouts a week or physical work | 1.725 |
| Very high | Athletes, heavy manual labour plus training | 1.9 |
People usually go wrong by choosing too high a factor. Three one-hour gym sessions a week leave 165 hours of sitting, yet plenty of people with that routine tick "moderate" when "light" is closer to the truth, and studies keep finding that we overrate how much we move. From the maintenance figure you then take off 15-20% to lose weight or add 10-15% to gain, and our calorie calculator works with a 15% deficit.
Example 1: office worker, 35, female
Take a woman of 35 who is 165 cm tall, weighs 68 kg, works in an office, drives everywhere, doesn't exercise and would like to lose 5 kg. The calculation for her goes like this:
| Step | Calculation | Result |
|---|---|---|
| BMR | 10 × 68 + 6.25 × 165 - 5 × 35 - 161 | 1,375 kcal |
| Maintenance, sedentary | 1,375 × 1.2 | 1,650 kcal |
| Weight loss, 15% deficit | 1,650 × 0.85 | 1,400 kcal |
That leaves a deficit of only 250 kcal a day, or 0.2-0.25 kg of fat a week, which is slow. Cutting to 1,320 kcal with a 20% deficit is one option, but walking more is a far better one, since at 8-10 thousand steps a day the factor rises to 1.375, maintenance climbs to 1,890 kcal and the same 1,400 kcal gives almost twice the deficit.
Example 2: active man, 28
The second example is a 28-year-old programmer, 182 cm and 80 kg, who lifts four times a week, plays football at weekends and wants to lean out a little:
| Step | Calculation | Result |
|---|---|---|
| BMR | 10 × 80 + 6.25 × 182 - 5 × 28 + 5 | 1,803 kcal |
| Maintenance, 4-5 workouts a week | 1,803 × 1.55 | 2,795 kcal |
| Weight loss, 15% deficit | 2,795 × 0.85 | 2,375 kcal |
Here the deficit is about 420 kcal, or 0.35-0.4 kg of fat a week, and it's better not to go deeper, since with that much training a big deficit soon eats into strength and recovery. Protein should be 1.6-2 g per kilogram, so 130-160 g a day, and the macros for this target can be split in the macro calculator.
Example 3: a woman over 50
The third case is a familiar one: a woman of 55, 162 cm and 74 kg, with a desk job, an hour's dog walk every day and 8 kg gained over five years, even though she "eats the same as before":
| Step | Calculation | Result |
|---|---|---|
| BMR | 10 × 74 + 6.25 × 162 - 5 × 55 - 161 | 1,317 kcal |
| Maintenance, desk job and daily walks | 1,317 × 1.375 | 1,810 kcal |
| Weight loss, 15% deficit | 1,810 × 0.85 | 1,540 kcal |
The formula answers the "why" directly. Age alone has taken 100 kcal off her BMR in twenty years, 5 kcal for every year, and muscle and everyday movement usually shrink along with it, so "eating the same as before" means eating 100-200 kcal a day more than needed, which is exactly 1-2 kg a year. At that age protein and strength work at least twice a week matter most, because they slow the loss of muscle and with it the drop in metabolism.
Comparison with the Harris-Benedict equation
Until the 1990s targets were calculated with the equation James Harris and Francis Benedict derived back in 1918 at the Carnegie Institution's laboratory in Boston, and you can still find it in older calculators. In its 1984 revision it reads 447.6 + 9.25 × weight + 3.10 × height - 4.33 × age for women and 88.4 + 13.40 × weight + 4.80 × height - 5.68 × age for men. Here's what it gives for the three examples:
| Person | Mifflin-St Jeor BMR | Harris-Benedict BMR | Difference |
|---|---|---|---|
| Woman, 35 | 1,375 | 1,436 | +61 (4%) |
| Man, 28 | 1,803 | 1,875 | +72 (4%) |
| Woman, 55 | 1,317 | 1,396 | +79 (6%) |
Harris-Benedict comes out 4-6% higher, and that's no accident, since it was derived from people in the early 20th century who lived and ate differently. In 2005 David Frankenfield compared the various equations with real measurements of metabolism and concluded that Mifflin-St Jeor is the most accurate and lands within ±10% of the true value more often than any other, which is why dietitians and most apps, MyFoodMate AI included, use it today.
Where the formula goes wrong
Mifflin and St Jeor measured people in late-1980s Nevada, and their equation describes the average of those people rather than you in particular. Athletes with a lot of muscle have a higher metabolism than it predicts, because muscle burns more energy than fat. At very high body weights the error grows in both directions, accuracy drops after seventy, and the equation knows nothing at all about thyroid conditions or medicines that change metabolism. The biggest weak point, though, is the activity factor, as even a perfectly calculated BMR multiplied by the wrong factor ends up 200-400 kcal out.
For most people the real margin of error is about ±10%, so the first woman's true target could be anywhere between 1,485 and 1,815 kcal, and the formula's number is best treated as a starting point. MyFoodMate AI uses the same equation but with more modest activity factors, because people nearly always rate themselves as more active than they are, and if you keep a diary and weigh yourself, after a few weeks the app compares what you ate with how your weight changed and suggests adjusting your target, as explained in the guides How your daily target is calculated and Recalculating your target.
The simplest way to test the formula is on yourself: start with its number, log your food honestly and step on the scale regularly, and after three or four weeks you'll see whether it was right. If your weight drops at the expected pace, it was, if it stalls, your energy use is lower than calculated, and if it drops too fast, it's higher, and what to do when the scale stops moving is covered in Weight not moving: why it happens and what to do.
Sources
- Mifflin MD, St Jeor ST et al. A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition, 1990.
- Frankenfield D, Roth-Yousey L, Compher C. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. Journal of the American Dietetic Association, 2005.
- Roza AM, Shizgal HM. The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass. American Journal of Clinical Nutrition, 1984.
We check blog materials against current WHO recommendations.
