BMR Calculator — Basal Metabolic Rate (Mifflin-St Jeor Equation)
Estimate your basal metabolic rate (BMR) — calories burned at complete rest. Mifflin-St Jeor + Katch-McArdle. Free.
About BMR Calculator
A BMR (basal metabolic rate) calculator estimates the calories your body burns at complete rest in 24 hours just to keep vital organs running — heart, lungs, brain, kidneys, basic cell maintenance — and is the foundation of every calorie-target calculation. The ZTools BMR Calculator offers the Mifflin-St Jeor equation (the modern default), Harris-Benedict (legacy), and Katch-McArdle (for users who know their body-fat percentage), shows the formula and intermediate numbers so the result is transparent, and runs entirely in your browser without storing personal data.
Use cases
- Foundation for daily calorie planning. BMR is the floor your TDEE builds on. Knowing it helps choose realistic deficits (never below BMR for sustained dieting).
- Comparing equations. Curious how Mifflin-St Jeor vs Harris-Benedict vs Katch-McArdle differ for the same person. See all three side-by-side.
- Tracking BMR change over time. Significant weight changes shift BMR. Recalculate quarterly to keep targets honest.
- Educational reference. Nutrition students and personal trainers reference the underlying equations and can teach the math from this tool.
How it works
- Enter biometrics. Sex, age, height, weight in metric or imperial.
- Optional body-fat %. Required for Katch-McArdle: BMR = 370 + 21.6 × lean body mass (kg).
- Pick equation. Mifflin-St Jeor (default), Harris-Benedict (legacy), Katch-McArdle (with body-fat).
- Compute. Returns BMR in kcal/day plus the intermediate numbers (so the math is auditable).
- Compare equations. Side-by-side results for the same input — typical spread is 50–150 kcal/day.
Examples
Input: Female, 30 yr, 165 cm, 65 kg
Output: Mifflin-St Jeor ≈ 1,381 · Harris-Benedict ≈ 1,455 · ~5% spread
Input: Male, 25 yr, 180 cm, 80 kg, 18% body fat
Output: Katch-McArdle ≈ 1,803 · Mifflin-St Jeor ≈ 1,830 · ~30 kcal difference
Input: Female, 50 yr, 160 cm, 70 kg
Output: Mifflin-St Jeor ≈ 1,289 kcal/day
Frequently asked questions
Is BMR the same as RMR?
Almost. BMR is measured under strict conditions (overnight fast, total rest, room temperature, just-awake). RMR (resting metabolic rate) is the same idea under less strict conditions and runs ~10% higher. The equations here estimate BMR.
Why do men have higher BMR than women on average?
Greater lean body mass — muscle and organs are metabolically more active than fat. Adjusting for body composition narrows the gap considerably.
What lowers BMR?
Aging (~2% per decade after 30), severe caloric restriction (adaptive thermogenesis), loss of lean mass, certain medications, hypothyroidism.
Can I increase my BMR?
Building lean mass (~10–15 kcal per kg of muscle per day). High-intensity training has a small post-exercise effect. Most "boost your metabolism" claims overstate the effect.
Should I eat below BMR?
Generally no for sustained periods — long-term sub-BMR intake risks lean mass loss and metabolic adaptation. Brief medically supervised very-low-calorie diets are exceptions.
Why does Katch-McArdle give different results?
It accounts for lean body mass directly rather than using sex/height/age proxies. More accurate for very lean or very heavy individuals.
Pro tips
- Treat BMR as a baseline, not a goal — daily targets should be TDEE-based.
- Recalculate after significant weight change (5+ kg) — old BMR estimates drift.
- For lean / muscular individuals, prefer Katch-McArdle if body-fat % is known.
- BMR-based maintenance underestimates intake for active people — always add an activity multiplier.
- BMR is an estimate; metabolic ward studies put equation accuracy at ~10%. Calibrate against real-world adherence.
Reviewed by Ahsan Mahmood · Last updated 2026-05-05 · Part of ZTools.
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