Your basal metabolic rate (BMR) is the energy your body uses just to stay alive — breathing, circulating blood, maintaining body temperature and running your organs — while you lie still and digest nothing. For most people who are not very active it accounts for roughly 60–70% of the calories burned in a day, so it is the foundation of every calorie estimate.
How to use the BMR calculator
- Choose US or Metric units and select your sex.
- Enter your age, height and weight.
- If you know your body fat percentage from a scan or a reliable estimate, add it to get a third, lean-mass-based result.
- Read the Mifflin–St Jeor BMR on the tape, compare it with the other equations, and use the table to see maintenance calories for each activity level.
BMR formulas
All three equations use kilograms, centimeters and years; the calculator converts US entries first.
Mifflin–St Jeor (1990) — the primary result:
Harris–Benedict, revised by Roza and Shizgal (1984):
Women: 447.593 + 9.247W + 3.098H − 4.330A
Katch–McArdle, which ignores sex and age and uses lean body mass (LBM = weight × (1 − body fat %)):
Worked example
A 40-year-old man who is 180 cm tall and weighs 80 kg:
Mifflin–St Jeor: 10 × 80 + 6.25 × 180 − 5 × 40 + 5 = 800 + 1,125 − 200 + 5 = 1,730 kcal/day
Harris–Benedict: 88.362 + 13.397 × 80 + 4.799 × 180 − 5.677 × 40 ≈ 1,797 kcal/day
With 18% body fat, lean mass is 65.6 kg, so Katch–McArdle gives 370 + 21.6 × 65.6 ≈ 1,787 kcal/day.
The three answers sit within about 70 kcal of each other, which is typical for someone of average build. For a woman of 35 who is 5 ft 5 in and 140 lb, Mifflin–St Jeor gives 1,331 kcal/day and Harris–Benedict about 1,395.
Which equation should you trust?
Each equation was fitted to a particular group of volunteers, so each carries some bias. The original Harris–Benedict study dates to 1919, when average body sizes and lifestyles were different; Mifflin and St Jeor built their equation from about 500 adults in the late 1980s, including people with obesity. Evidence reviews have generally found Mifflin–St Jeor the most reliable for the general adult population, but even it misses the measured value by more than 10% for a meaningful minority of people. Treat any single number as the middle of a range, not a precise reading.
Katch–McArdle is attractive for athletes because muscle, not body weight, drives most resting energy use. Its weakness is the input: home body fat estimates can easily be off by several percentage points, and each 5-point error shifts the result by roughly 60–80 kcal for an average adult.
BMR, RMR and TDEE
| Term | What it measures |
|---|---|
| BMR | Energy at complete rest after fasting, under lab conditions |
| RMR | Energy at rest under everyday conditions; usually slightly higher |
| TEF | Energy used to digest food, about 10% of intake |
| TDEE | Total daily energy expenditure: BMR plus activity plus TEF |
Activity multipliers fold TEF and movement into one factor, which is how the table under the result turns BMR into maintenance calories. For goals such as gradual weight loss, use the calorie calculator.
What raises or lowers BMR
Lean mass is the biggest lever: muscle and organs are metabolically active, while fat tissue burns comparatively little. Larger bodies burn more in absolute terms, men usually burn more than women of the same size because they carry more lean mass, and BMR drifts down with age. Thyroid hormones, certain medications, fever, pregnancy and breastfeeding can all move it noticeably, which no general equation can capture.
This calculator is informational only and is not medical advice. If you are planning a diet, managing a medical condition or are pregnant, consult a healthcare professional or registered dietitian.
Frequently asked questions
What is the difference between BMR and RMR?
Basal metabolic rate is measured under strict lab conditions: after an overnight fast, a full night's sleep and complete rest in a warm room. Resting metabolic rate is measured under looser conditions and usually comes out a little higher. The equations on this page are used for both, and the difference is small compared with person-to-person variation.
Which BMR formula is the most accurate?
In a 2005 review of predictive equations by the American Dietetic Association, Mifflin–St Jeor estimated resting energy within 10% of measured values for more adults than any other equation tested, which is why it is the primary result here. Katch–McArdle can do better for lean, muscular people if you know your body fat percentage accurately.
Why does BMR go down with age?
Most of the decline comes from losing lean tissue, especially muscle, which burns more energy than fat. The equations build this in with an age term — Mifflin–St Jeor subtracts 5 kcal per year — but people who keep their muscle mass tend to lose less.
How do I turn BMR into daily calorie needs?
Multiply BMR by an activity factor between 1.2 (sedentary) and 1.9 (very hard daily exercise or a physical job). The table under the result does this for every level; the calorie calculator adds weight-change targets on top.