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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​

  1. Enter biometrics — Sex, age, height, weight in metric or imperial.
  2. Optional body-fat % — Required for Katch-McArdle: BMR = 370 + 21.6 × lean body mass (kg).
  3. Pick equation — Mifflin-St Jeor (default), Harris-Benedict (legacy), Katch-McArdle (with body-fat).
  4. Compute — Returns BMR in kcal/day plus the intermediate numbers (so the math is auditable).
  5. 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.

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.

Try it now​

The full BMR Calculator runs in your browser at https://ztools.zaions.com/bmr-calculator — no signup, no upload, no data leaves your device.

Open the tool ↗

Last updated: 2026-05-05 · Author: Ahsan Mahmood · Edit this page on GitHub