BMR Calculator
Your basal metabolic rate is the energy you burn at complete rest. Estimate it with the Mifflin-St Jeor equation, plus daily maintenance calories for each activity level.
Your BMR is the energy you burn at complete rest, best estimated with the Mifflin-St Jeor equation; multiplying it by an activity factor of 1.2–1.9 estimates your daily maintenance calories (TDEE).
What your BMR is
Basal metabolic rate is the energy your body uses at complete rest just to stay alive — running your heart, brain, lungs and other organs. For most people it is the single biggest chunk of daily calorie burn. This calculator estimates it with the Mifflin-St Jeor equation, the most validated formula for healthy adults, using your age, sex, height and weight.
BMR is not the same as the calories you actually need in a day. Add everything else you do — digesting food, walking around, training — and you get your TDEE (total daily energy expenditure), or maintenance calories. That is why the result shows your BMR alongside maintenance figures for each activity level: your TDEE is simply BMR multiplied by an activity factor from 1.2 to 1.9.
Using your BMR
Your BMR is the floor of your energy needs, and eating below it for long stretches is generally discouraged. To lose weight, take a moderate deficit from your maintenance figure rather than from BMR — commonly 300–500 kcal/day for a steady, sustainable loss. If you know your body-fat percentage, the tool also shows a Katch-McArdle estimate, which is driven by lean mass and often fits leaner, more muscular people better.
These equations are estimates, typically within about 10% of a lab measurement, and BMR drifts down with age and changes with body composition. Recalculate whenever your weight shifts, then split your calories with the macro calculator or map goal targets with the TDEE calculator.
Activity multipliers (BMR → maintenance calories)
| Activity level | Description | × BMR |
|---|---|---|
| Sedentary | Little or no exercise; desk job | 1.2 |
| Lightly active | Light exercise 1–3 days/week | 1.375 |
| Moderately active | Moderate exercise 3–5 days/week | 1.55 |
| Very active | Hard exercise 6–7 days/week | 1.725 |
| Extra active | Very hard exercise or a physical job | 1.9 |
Maintenance calories (TDEE) = BMR × multiplier. Most people overestimate activity — when unsure, pick the lower band.
The multiplier does not just scale your BMR — it scales the error too
Mifflin-St Jeor is the best of the common equations, and the systematic review cited below is what establishes that: across the studies it examined, Mifflin-St Jeor predicted resting metabolic rate within 10% of the measured value in more people than any other equation tested, with the narrowest error range. The same review is blunt about what that leaves: “noteworthy errors and limitations exist when it is applied to individuals.”
Ten per cent is worth converting into calories, because it is easy to read a four-digit number as if it were measured. Take a 30-year-old man at 80 kg and 180 cm. This calculator returns a BMR of 1,780 kcal. Within that ±10% band, his real resting rate is somewhere between 1,602 and 1,958 — a spread of 356 kcal before he has stood up.
Then the activity multiplier arrives, and because it multiplies, it stretches the band along with the number. It cannot narrow the uncertainty; it can only magnify it.
| Activity level | Maintenance shown | Range implied by ±10% | Width |
|---|---|---|---|
| Sedentary (×1.2) | 2,136 kcal | 1,922–2,350 | 428 kcal |
| Moderate (×1.55) | 2,759 kcal | 2,483–3,035 | 552 kcal |
| Very active (×1.9) | 3,382 kcal | 3,044–3,720 | 676 kcal |
Computed for the same 30-year-old man, 80 kg and 180 cm, using this page's own Mifflin-St Jeor arithmetic and the ±10% accuracy band from the systematic review cited below. The multiplier is applied to both ends of the band.
At a moderate activity level the honest reading of “2,759” is closer to somewhere between 2,483 and 3,035 — a 552 kcal spread, which is wider than the 500 kcal deficit most people would set out to run. That does not make the number useless. It makes it a starting point rather than a measurement, and it explains something people find baffling: two people can follow the same calculated target with identical discipline and get different results, without either of them having done anything wrong.
The way out is not a better equation, it is your own data. Hold intake near the figure for two to three weeks and watch the weight trend rather than the daily readings. If the trend is flat when you expected loss, your real maintenance sits at the lower end of the band, and you now know that about yourself with more confidence than any formula can offer. The weight-loss timeline calculator turns a rate into a date once you have that figure.
BMR uses Mifflin-St Jeor (1990); the optional Katch-McArdle estimate uses lean body mass. Activity multipliers are the standard Harris-Benedict factors. Estimates only.
Frequently asked questions
What is the difference between BMR, RMR, and TDEE?
BMR (basal metabolic rate) is the energy your body uses at complete rest just to keep you alive — measured after sleep, fasted, and fully relaxed. RMR (resting metabolic rate) is measured under slightly looser conditions and runs about 10% higher. TDEE (total daily energy expenditure) is your BMR plus everything else you do — digestion, movement, and exercise — which is why we multiply BMR by an activity factor of 1.2 to 1.9 to estimate it.
Which BMR formula should I use?
For most people Mifflin-St Jeor is the recommended default; it is the most validated equation for healthy adults and needs only weight, height, age, and sex. If you know your body-fat percentage and are leaner or more muscular than average, Katch-McArdle (based on lean body mass) may fit you better. This calculator shows the Katch-McArdle figure automatically when you enter a body-fat percentage.
How accurate is a calculated BMR?
These are population-based estimates, not measurements. Even the best equation is typically within roughly 10% of a lab measurement for most people, but individuals with unusual body composition, older adults, or those on certain medications can fall further from the estimate. Treat the result as a well-informed starting point and adjust based on how your weight actually responds over a few weeks.
Should I ever eat below my BMR to lose weight?
Eating consistently below your BMR is generally discouraged. A safer, more sustainable approach is a moderate deficit from your TDEE — commonly around 300–500 kcal/day, targeting roughly 0.25–0.5 kg (about 0.5–1 lb) of loss per week. Very-low-calorie plans should only be done under professional supervision. This tool is for general information, not medical or dietary advice.
Why did my BMR change over time?
BMR falls gradually with age (the equations subtract calories per year) and changes with weight and body composition — more lean muscle raises it, losing weight lowers it. Aggressive dieting can also reduce it somewhat. Recalculate whenever your weight changes meaningfully to keep your calorie targets realistic.
Sources
- Mifflin MD, St Jeor ST, et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990;51(2):241-247. — Default Mifflin-St Jeor BMR equation
- Roza AM, Shizgal HM. The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass. Am J Clin Nutr. 1984;40(1):168-182. — Revised Harris-Benedict BMR equation
- Cunningham JJ. A reanalysis of the factors influencing basal metabolic rate in normal adults. Am J Clin Nutr. 1980;33(11):2372-2374. — Lean-body-mass BMR basis (Katch-McArdle)
- Frankenfield D, Roth-Yousey L, Compher C. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. J Am Diet Assoc. 2005;105(5):775-789. — Validates Mifflin-St Jeor as most accurate
The thresholds and formulas used by this calculator follow the primary sources above. This tool provides general information, not personalised medical advice.