What BMR measures
Basal metabolic rate (BMR) is the energy your body uses to stay alive when you are doing nothing at all: lying still, awake, after a night’s sleep and without food for about 12 hours. It powers your heart, lungs, brain, liver and kidneys, keeps your temperature steady and repairs cells. For most adults it is the largest part of daily energy use, often around 60% to 70% of the total.
Measuring BMR properly needs a lab and a breathing hood. Most people use a prediction equation instead. This calculator runs the three equations you are most likely to see and puts them side by side, so you can see how much they agree for you.
The three equations
Mifflin–St Jeor (1990) was developed from resting measurements of 498 healthy adults. A 2005 systematic review for the American Dietetic Association found it predicted resting energy within 10% of the measured value for more people than the other common equations, which is why it is the default here.
Women: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161
Revised Harris–Benedict (Roza and Shizgal, 1984) is an update of the original 1919 equation. It usually gives a figure a little higher than Mifflin–St Jeor.
Women: 447.593 + 9.247 × kg + 3.098 × cm − 4.330 × age
Katch–McArdle works from lean body mass (everything that is not fat), so it ignores sex and age. It can suit very lean or muscular people, but it is only as good as your body fat number.
Worked examples
A 30-year-old man, 180 cm, 80 kg.
- Mifflin–St Jeor: 10 × 80 + 6.25 × 180 − 5 × 30 + 5 = 800 + 1,125 − 150 + 5 = 1,780 kcal a day.
- Harris–Benedict: 88.362 + 1,071.76 + 863.82 − 170.31 = 1,854 kcal a day.
- With 20% body fat, lean mass is 80 × 0.8 = 64 kg, so Katch–McArdle gives 370 + 21.6 × 64 = 1,752 kcal a day.
The three answers sit within about 100 kcal of each other, which is typical.
A 40-year-old woman, 165 cm, 60 kg.
- Mifflin–St Jeor: 600 + 1,031.25 − 200 − 161 = 1,270 kcal a day (about 5,315 kJ).
- Harris–Benedict: 447.593 + 554.82 + 511.17 − 173.2 = 1,340 kcal a day.
From BMR to daily calories
You burn more than your BMR every day, because you digest food, move around and exercise. A common shortcut multiplies BMR by an activity factor to estimate total daily energy expenditure (TDEE).
| Activity level | Multiplier | 1,780 kcal BMR becomes |
|---|---|---|
| Sedentary | 1.2 | 2,136 kcal |
| Lightly active | 1.375 | 2,448 kcal |
| Moderately active | 1.55 | 2,759 kcal |
| Very active | 1.725 | 3,071 kcal |
| Extra active | 1.9 | 3,382 kcal |
These multipliers are conventions, not measurements. For a fuller estimate with goals and macros, use the TDEE calculator.
Tips and limits
- Do not eat at your BMR to lose weight. BMR is the floor at rest, not a diet target. Plan any deficit from your TDEE with the calorie deficit calculator.
- Expect an error of around 10%. All three equations predict the average for people with your measurements. Two people with identical numbers can differ by a few hundred calories.
- Muscle matters. Muscle uses more energy than fat, so two people of the same weight can have different BMRs. If you know your body fat, compare Katch–McArdle with the others. You can estimate it with the body fat calculator.
- Age changes the number slowly. Mifflin–St Jeor subtracts 5 kcal a day per year of age, so ten years lowers BMR by 50 kcal if nothing else changes.
- Some situations are not covered. These equations were built from healthy adults. They are not designed for children, pregnancy, breastfeeding, serious illness or recovery from surgery, where a clinician should assess energy needs.
kcal or kJ?
Calories on food labels are kilocalories (kcal). Many countries also show kilojoules (kJ). One kilocalorie equals 4.184 kilojoules, so a BMR of 1,780 kcal is about 7,448 kJ. The calculator shows both.