BYTETOOLS

BMR Calculator

Calculate your basal metabolic rate with Mifflin-St Jeor, revised Harris-Benedict or Katch-McArdle, in metric or imperial, and compare all three side by side.

1,780 kcal
Your BMR
74 kcal
Per hour at rest
22.3 kcal
Per kg of body weight

How this number was calculated

BMR = 10 × 80 + 6.25 × 180 − 5 × 30 + 5 = 1780 kcal/day

All three formulas compared

EquationBMRNotes
Mifflin-St Jeor (1990)1,780 kcalBest general-purpose accuracy
Revised Harris-Benedict (1984)1,854 kcalTends to read a little high
Katch-McArdle1,752 kcalBest if your body fat % is reliable

The spread between these three is a fair picture of how uncertain any predicted BMR is. Mifflin-St Jeor lands within about 10% of measured resting metabolic rate for most healthy adults — but not all. Estimates only, not medical advice.

What is the BMR Calculator?

Basal metabolic rate (BMR) is the number of calories your body burns at complete rest to keep basic functions running. The Mifflin-St Jeor equation estimates it as 10 × weight(kg) + 6.25 × height(cm) − 5 × age, plus 5 for men or minus 161 for women.

  • Mifflin-St Jeor, revised Harris-Benedict and Katch-McArdle
  • Side-by-side comparison of all three estimates
  • Shows the substituted formula so the number is auditable
  • Metric and imperial input
  • Also reports calories per hour and BMR per kilogram
  • Entirely offline — no uploads, no accounts, no tracking

How to use the BMR Calculator

  1. 1

    Pick metric or imperial units.

  2. 2

    Enter your sex, age, height and weight.

  3. 3

    Choose an equation, or leave Mifflin-St Jeor selected.

  4. 4

    For Katch-McArdle, enter your body-fat percentage.

  5. 5

    Read your BMR in calories per day and compare all three formulas.

About the BMR Calculator

The ByteTools BMR Calculator estimates your basal metabolic rate — the calories your body burns at complete rest to keep your heart, brain, lungs and cells running. It defaults to the Mifflin-St Jeor equation, which is the one most clinicians and dietitians reach for, and shows the exact arithmetic so you can check it.

You can also switch to the revised Harris-Benedict equation or to Katch-McArdle, which uses lean body mass and needs a body-fat percentage. All three are shown together in a comparison panel, because seeing the spread between them is the honest way to understand how much uncertainty is baked into any single number.

Height and weight can be entered in centimetres and kilograms or feet, inches and pounds. Every calculation happens locally in your browser and nothing is uploaded or saved. The result is an estimate only and is not medical advice — a metabolic cart test at a clinic is the only way to actually measure your resting metabolism.

Frequently asked questions

What is BMR?

BMR is your basal metabolic rate: the calories your body uses over 24 hours doing nothing but staying alive — breathing, circulating blood, maintaining temperature and repairing cells. For most adults it accounts for 60 to 70 percent of total daily energy use.

How accurate is the Mifflin-St Jeor equation?

It predicts measured resting metabolic rate within 10% for roughly 70 to 80 percent of healthy adults, which makes it the most reliable of the common prediction equations. That still leaves a meaningful minority whose real BMR is 15 to 20 percent away from the prediction, and accuracy degrades for people who are very lean, very obese, elderly or ill.

What is the Mifflin-St Jeor formula?

For men, BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5. For women it is the same expression ending in − 161. A 30-year-old man at 80 kg and 180 cm gets 800 + 1125 − 150 + 5 = 1,780 calories per day.

Should I eat my BMR in calories?

No. BMR is what you would burn lying in bed all day, so eating exactly your BMR usually means a substantial deficit once ordinary movement is counted. Multiply BMR by an activity factor to get maintenance calories, then apply a deficit from there.

Why do the three formulas disagree?

Each was fitted to a different population using different measurement methods, decades apart. Harris-Benedict was derived from early 20th-century subjects and tends to run high; Katch-McArdle only works from lean mass and so ignores sex and age entirely. The spread between them is a fair picture of the real uncertainty.

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