How to calculate your BMR by hand
- Convert to metric. Use kilograms and centimeters: pounds × 0.4536 gives kilograms, inches × 2.54 gives centimeters.
- Multiply. Work out 10 × your weight in kg, 6.25 × your height in cm and 5 × your age in years.
- Combine. Add the first two numbers and subtract the third.
- Add the sex constant. Add 5 for men or subtract 161 for women. The result is your BMR in kcal per day. For example, a 30-year-old man who is 180 cm and 80 kg: 800 + 1,125 − 150 + 5 = 1,780 kcal.
How the BMR calculator works
The calculator runs three published equations on the same details and puts them side by side. Mifflin-St Jeor is the headline number because it’s the most reliable for most adults, and because it’s exactly what the Calorix app uses, so your BMR here matches your BMR in the app.
Mifflin-St Jeor (1990): the default
Built from resting energy measurements of 498 healthy adults aged 19 to 78, including people of normal weight and people with obesity (Mifflin et al., 1990):
- Men: 10 × weight in kg + 6.25 × height in cm − 5 × age + 5
- Women: 10 × weight in kg + 6.25 × height in cm − 5 × age − 161
- Other: the same formula with −78, the midpoint of the two
Harris-Benedict, revised (1984)
The original Harris-Benedict equations date from 1919. In 1984, Roza and Shizgal refitted them to a larger data set (Roza and Shizgal, 1984), and that revised version is the one used here:
- Men: 88.362 + 13.397 × weight in kg + 4.799 × height in cm − 5.677 × age
- Women: 447.593 + 9.247 × weight in kg + 3.098 × height in cm − 4.330 × age
- Other: the average of the two
Katch-McArdle: from lean mass
Katch-McArdle, from McArdle, Katch and Katch’s Exercise Physiology, ignores sex, height and age and works from lean body mass instead. It only appears when you enter a body fat percentage:
- Lean mass = weight in kg × (1 − body fat % ÷ 100)
- BMR = 370 + 21.6 × lean mass in kg
Because it depends on lean mass, it’s only as good as your body fat estimate, and home scales’ readings shift with hydration and can differ noticeably from a lab measurement.
Worked examples
| Person | Mifflin-St Jeor | Harris-Benedict (revised) | Katch-McArdle |
|---|---|---|---|
| Man, 30, 180 cm, 80 kg, 15% body fat | 1,780 kcal | 1,854 kcal | 1,839 kcal |
| Woman, 25, 165 cm, 60 kg, 25% body fat | 1,345 kcal | 1,405 kcal | 1,342 kcal |
For the man, Mifflin-St Jeor gives 10 × 80 + 6.25 × 180 − 5 × 30 + 5 = 1,780 kcal. His lean mass is 80 × 0.85 = 68 kg, so Katch-McArdle gives 370 + 21.6 × 68 = 1,839 kcal. The three results sit within about 75 kcal of each other, a typical spread.
Which equation should you trust?
In a systematic review of predictive equations for healthy adults, Mifflin-St Jeor was the most likely to land within 10% of measured resting metabolism, in people with and without obesity, and it had the narrowest range of errors. The same review warns that individual errors can still be large, and that older adults and several ethnic groups were underrepresented in the research.
Harris-Benedict tends to run a little higher than Mifflin-St Jeor for the same person, as the examples show. Katch-McArdle performed comparatively well in some groups: in a 2021 study of healthy women measured by indirect calorimetry, it was within 10% of the measured value more often than the other equations tested, though that was still only for 39.5% of the women. No formula replaces a measurement, which is why the calculator shows all three and why a BMR is best read as a range.
BMR, RMR and TDEE
- BMR (basal metabolic rate): energy at complete rest, measured under strict conditions.
- RMR (resting metabolic rate): the same idea measured with fewer restrictions. In practice the two are close.
- TDEE (total daily energy expenditure): everything you burn in a day. It’s your BMR multiplied by an activity factor:
| Activity level | Factor | Example: BMR 1,500 kcal |
|---|---|---|
| Sedentary | 1.2 | 1,800 kcal |
| Lightly active | 1.375 | 2,063 kcal |
| Moderately active | 1.55 | 2,325 kcal |
| Very active | 1.725 | 2,588 kcal |
| Extremely active | 1.9 | 2,850 kcal |
Your TDEE is your maintenance calories. To see yours with your own activity level, and targets for losing or gaining weight, use the TDEE calculator.
Why your scale or watch shows a different number
Smart scales usually estimate BMR from a bioimpedance reading of your lean mass, and watches from your profile and heart rate, each with their own formula. Most don’t publish it, and bioimpedance readings shift with hydration, the time of day and a recent meal. A different number isn’t wrong so much as a different estimate. What matters is to pick one source and judge it against your real weight trend over a few weeks.
BMR for women and men
The equations treat sex as a stand-in for body composition: at the same height, weight and age, men carry more lean mass on average, and lean tissue uses more energy at rest than fat. In Mifflin-St Jeor that’s a fixed difference of 166 kcal a day. If you know your body fat, Katch-McArdle skips sex altogether and works from your lean mass, which can suit people whose body composition is far from average, such as strength athletes.
What changes your BMR
- Body size. More weight and more height mean a higher BMR.
- Lean mass. Muscle is more metabolically active than fat, which is why strength training helps protect your BMR while you lose weight.
- Age. Mifflin-St Jeor subtracts 5 kcal a day for every year of age, which mostly reflects gradual changes in body composition.
- Weight loss. As you get lighter, your BMR falls with your body size, so the same intake produces a smaller deficit. Modelling research in The Lancet describes how slowly the body settles after a change in intake.
Teens
This calculator is for adults. For teenagers, the Calorix app uses the National Academies’ estimated energy requirement, which includes the energy needed for growth, and never sets a calorie deficit for anyone under 18.
How Calorix uses your BMR
When you set up Calorix, it calculates your BMR with this exact Mifflin-St Jeor equation, multiplies it by your activity factor and turns the result into a daily calorie budget with protein, carbs, fat, fiber and water targets. Targets update as your weight changes. To plan a goal, try the calorie deficit calculator; to split your calories, the macro calculator.
