BMR Calculator
Plug in age, sex, weight, and height. See your basal metabolic rate via two well-known formulas — three if you add an optional body fat % to unlock Katch-McArdle. The number is the calories your body would burn at complete rest — the floor of your daily energy needs. The table underneath translates it into daily calories at five activity levels.
What this means for daily calories
Mifflin-St Jeor BMR × activity multiplier ≈ total daily burn (TDEE).
| Activity level | Multiplier | kcal/day |
|---|---|---|
| Sedentary — little or no exercise | ×1.2 | — |
| Light (1–3×/wk) | ×1.375 | — |
| Moderate (3–5×/wk) | ×1.55 | — |
| Heavy (6–7×/wk) | ×1.725 | — |
| Athlete — daily training + physical job | ×1.9 | — |
Want the activity step with goals and sensitivity ranges? → TDEE calculator.
What BMR is and isn't
BMR is the floor. It's the calories your body burns to keep your heart beating, brain firing, kidneys filtering, and muscles maintained — at total rest, fasted, in a thermoneutral environment. Nobody actually consumes only their BMR worth of calories on a normal day; total daily expenditure (TDEE) is BMR plus everything you do.
To get from BMR → TDEE, multiply by an activity factor (1.2 for sedentary, 1.55 for moderate, 1.725 for active). Use the TDEE calculator for that step.
The Mifflin-St Jeor formula
Published in 1990 by M. D. Mifflin and S. T. St Jeor in the American Journal of Clinical Nutrition. They derived the formula from indirect-calorimetry measurements on 498 healthy adults. It's been validated multiple times since against metabolic-chamber data and consistently outperforms the older Harris-Benedict equation for accuracy.
The formula:
- Men:
BMR = 10 × kg + 6.25 × cm − 5 × age + 5 - Women:
BMR = 10 × kg + 6.25 × cm − 5 × age − 161
The +5 / −161 difference reflects average body composition differences — men typically carry more lean mass, which burns more calories per kg than fat does.
The 10% rule
Both formulas predict BMR within ±10% for any given individual — that's just the inherent variance from body composition, genetics, thyroid function, and other factors no formula can capture. Treat the number as a starting estimate, not a target.
Related
- TDEE calculator — multiply BMR by activity
- Macro calculator — calorie + protein/carb/fat split
- BMI calculator
- Health hub
FAQ
Is anything I enter sent to a server?
No. The math is two formulas in 30 lines of TypeScript — runs in your browser. No weight, age, or sex data is uploaded.
Which formula should I trust?
Mifflin-St Jeor (1990). It's what dietitians use today. It tracks better against modern indirect-calorimetry measurements than the older Harris-Benedict (1919, revised 1984). They typically agree within 5%; Harris-Benedict tends to overestimate slightly. Use Mifflin as the default, treat Harris as a reasonability check.
What's BMR vs RMR?
Resting Metabolic Rate (RMR) is measured under less-strict conditions than BMR (you can be sitting up, recently woken). RMR is typically 5–10% higher than BMR. The Mifflin-St Jeor formula was actually derived from RMR data, so what we call 'BMR' here is technically closer to RMR. The names are used interchangeably in most fitness contexts.
Why does my BMR feel high or low compared to a Fitbit / Apple Watch estimate?
Wearables typically use heart-rate-based estimates, not the Mifflin formula. Their numbers vary by person and by device — they're often 5–15% off from a real metabolic chamber measurement. The Mifflin formula is also an estimate (±10% for any individual) but has the advantage of being deterministic and reproducible.
I'm very muscular / lean. Is the formula still accurate?
Mifflin-St Jeor doesn't directly account for body composition. For very lean / muscular people (under 12% body fat for men, under 18% for women), the Katch-McArdle formula (which uses lean body mass) is more representative. Enter your body fat % in the optional field and it appears as a third result row.
How does the Katch-McArdle formula work?
BMR = 370 + 21.6 × lean body mass (kg), where lean mass = weight × (1 − body fat %). Because it keys off lean mass instead of total weight, it doesn't need a sex term — and it's only as good as your body-fat estimate. A ±5-point error in body fat % shifts the result by roughly 80–110 kcal/day for most adults, so treat gym-scale bioimpedance readings as rough.
How do I turn BMR into the calories I actually burn per day?
Multiply by an activity factor — that's the table under the results: sedentary ×1.2, light exercise 1–3×/week ×1.375, moderate 3–5×/week ×1.55, heavy 6–7×/week ×1.725, athlete ×1.9. The product is your TDEE (total daily energy expenditure). When you've entered a body fat %, the table uses Katch-McArdle as its base; otherwise Mifflin-St Jeor. For goal targets and sensitivity ranges, use the TDEE calculator.
What's the kJ toggle for?
Most of the world outside the US labels food energy in kilojoules (Australia and NZ exclusively). The toggle converts every number on the page at the standard factor 1 kcal = 4.184 kJ. A 1,700 kcal BMR is ≈ 7,113 kJ.