BMR Calculator

The basal metabolic rate (BMR) is the energy your body burns purely to stay alive at rest — breathing, pumping blood, maintaining organ function — before any activity or digestion. This tool estimates it with three widely used equations and then shows how that number translates into rough daily calorie targets across six activity levels. Everything runs in your browser; no measurement data is sent to a server.

How to use

1. Choose the unit system — Metric (centimeters and kilograms) or US (feet, inches, and pounds). Switching tabs converts your current numbers, so you can jump between systems without retyping.
2. Pick a sex — the equations have different constants for men and women. The Katch-McArdle formula ignores sex and uses body-fat percentage instead, which is why it is offered for both.
3. Enter age, height, and weight — the result updates live as you type.
4. Choose a formula under Settings. The default is Mifflin-St Jeor. Katch-McArdle reveals an extra Body fat (%) input because it requires a fat percentage.
5. Pick the results unit — Calories (kcal) or Kilojoules (kJ); the big BMR figure and the activity table both switch.
6. Read the results: the large BMR value and the daily calorie needs by activity level table underneath.

The component warns when the age falls outside 15–80 or when a body-fat value looks implausible, because the formulas were derived from adult populations and lose accuracy outside their range.

The three formulas

Mifflin-St Jeor (the default, considered the most accurate of the three for most people):

• Men: BMR = 10W + 6.25H − 5A + 5
• Women: BMR = 10W + 6.25H − 5A − 161

Revised Harris-Benedict (an older 1984 revision, kept for comparison):

• Men: BMR = 13.397W + 4.799H − 5.677A + 88.362
• Women: BMR = 9.247W + 3.098H − 4.330A + 447.593

Katch-McArdle (lean-body-mass formula):

• BMR = 370 + 21.6 × (1 − F) × W, where F is body fat as a decimal

where W is weight in kg, H is height in cm, and A is age in years. Because Katch-McArdle feeds on lean mass, it can be more accurate for athletic or very lean people who know their body-fat percentage; for everyone else Mifflin-St Jeor is usually the safest choice. The three often land within a few percent of each other — for a 30-year-old woman of 165 cm and 60 kg, Mifflin-St Jeor gives about 1320 Calories/day and the Revised Harris-Benedict about 1384.

Activity levels and TDEE

Your total daily energy expenditure (TDEE) is the BMR multiplied by an activity factor. The table uses the six standard factors:

The table multiplies your computed BMR by each factor and rounds to a whole number, so you can compare what different activity levels would mean for your intake without switching settings back and forth.

What to use this for

BMR estimates feed into nutrition and weight planning: a maintenance target is roughly your TDEE at your real activity level, a deficit of about 500 Calories/day tends to produce around half a kilogram of weekly loss, and a surplus supports muscle growth. The activity table lets you see all of those numbers at once. To put your body weight itself in context, pair this with the BMI calculator.

Limitations — read this first

BMR equations are statistical averages, not measurements. A 2005 analysis (Johnstone et al., Am J Clin Nutr) found roughly 26% of the variance in true BMR remains unexplained even after controlling for known factors. Genetics, muscle mass, recent diet, stress, thyroid function, and even ambient temperature all shift the real number. For a precise value you would need indirect calorimetry at a clinic or gym, not an equation. Use the results as starting points and calibrate against what your scale, tape measure, and training journal actually show over a few weeks.