What this calculates
Enter your sex, age, height and weight, in metric or imperial units, and this gives your basal metabolic rate — the calories your body burns in a day at complete rest — by the Mifflin-St Jeor equation, with the revised Harris-Benedict equation beside it, the same figure in kilojoules, and what that works out to per hour.
The formula
Mifflin-St Jeor, men
BMR = 10 × weight in kg + 6.25 × height in cm − 5 × age + 5Mifflin-St Jeor, women
BMR = 10 × weight in kg + 6.25 × height in cm − 5 × age − 161Harris-Benedict, men
BMR = 88.362 + 13.397 × kg + 4.799 × cm − 5.677 × ageHarris-Benedict, women
BMR = 447.593 + 9.247 × kg + 3.098 × cm − 4.330 × ageBoth give kilocalories a day — the "calories" on food labels. One kilocalorie is 4.184 kilojoules.
Worked example
The revised Harris-Benedict equation gives the same man 1,696 kcal/day. A woman of 25, 160 cm and 55 kg has a Mifflin-St Jeor BMR of 550 + 1,000 − 125 − 161 = 1,264 kcal/day.
When to use it, and the mistakes to avoid
Use it as the starting point for a diet or fitness plan: multiply by your activity factor to estimate your daily calorie needs, then eat a little below that to lose weight or above it to gain.
The mistakes that cost the most:
- Eating only your BMR. BMR is what you burn lying still all day. Your real needs are higher — about 1,979 kcal rather than 1,649 even for someone who sits most of the day.
- Overestimating activity. Most office workers are nearer 1.2 than 1.55, even with a few gym sessions a week.
- Treating it as exact. Any equation can be off by 10% or more for one person, especially very muscular people or those with a lot of body fat.
- Mixing units. Use kilograms and centimetres together, or pounds with feet and inches — not pounds with centimetres.
- Forgetting to update it. Your BMR drops as you lose weight and as you age; recalculate when either changes.
FAQ
What is BMR?
Basal metabolic rate is the energy your body uses in a day at complete rest - to breathe, keep your heart beating, hold your temperature and run your organs. It is usually most of what you burn in a day, but not all of it: moving, exercising and digesting food come on top.
Which BMR formula is the most accurate?
Studies comparing the equations with measured metabolic rates have found Mifflin-St Jeor (1990) the most reliable for most adults, so it gives the headline figure here. The revised Harris-Benedict equation (1984) is shown alongside; it usually comes out a little higher.
How many calories should I eat a day?
Multiply your BMR by an activity factor: about 1.2 if you sit most of the day, 1.375 for light exercise, 1.55 for moderate exercise three to five days a week, 1.725 for hard exercise most days, and 1.9 for heavy physical work. A BMR of 1,649 kcal at 1.2 is about 1,979 kcal a day; at 1.55, about 2,556.
Why does BMR go down with age?
Mostly because people tend to lose muscle as they age, and muscle burns more energy than fat. The Mifflin-St Jeor equation takes 5 kcal off for every year: the same 175 cm, 70 kg man burns 1,649 kcal at rest at 30 and about 1,549 at 50.
Why is BMR different for men and women?
At the same height and weight, men on average carry more muscle and less fat. In the Mifflin-St Jeor equation that shows up as a constant: +5 for men and −161 for women, a gap of 166 kcal a day.
Should I eat less than my BMR to lose weight?
Generally not without medical advice. Your BMR is what your body needs just to keep running; a sensible deficit comes off your total daily needs - BMR times your activity factor - rather than going below BMR itself.