BMR Calculator

BMR (basal metabolic rate) is the energy your body uses while lying still in a thermoneutral environment, fasted. It's roughly 60–70% of your total daily energy use for sedentary people. RMR (resting metabolic rate) is a slightly looser, easier-to-measure version and runs about 10% higher than true BMR. Most calorie calculators (including this one) actually estimate RMR-equivalent values and call it BMR.

TDEE = BMR × activity multiplier. The multipliers approximate everything you do beyond resting: standing, walking to your car, fidgeting, exercising, even digestion (which alone consumes about 10% of intake). Sedentary = 1.2 (desk job, no formal exercise). Lightly active = 1.375 (1–3 sessions/week). Moderately active = 1.55. Very active = 1.725. Extra active = 1.9 (manual labor + daily training).

This calculator uses the Mifflin-St Jeor equation, published in 1990 and validated as the most accurate prediction equation for non-obese and obese adults in a 2005 American Dietetic Association systematic review. It's accurate within ~10% for ~80% of people.

Older formulas like Harris-Benedict (1919, revised 1984) tend to overestimate BMR by 5–10% in modern populations because they were derived from a smaller, more active sample. Katch-McArdle uses lean body mass instead of total weight, which is theoretically better for very lean or very muscular people — but only if you have an accurate body fat measurement to feed it.

For weight maintenance, eat at TDEE. For fat loss, subtract 15–25% (a 500 kcal/day deficit produces about 1 lb/week of fat loss). For muscle gain, add 10–15% (a 250 kcal/day surplus is enough for most natural lifters; bigger surpluses just add fat).

Reality check: BMR equations are estimates, and individual metabolism varies by ±200 kcal/day even between people of the same age, sex, and body composition. Use the calculator's number as a starting point, then track weight and waist measurements over 2–3 weeks. If the trend isn't moving the way the math predicted, adjust calories by 100–200/day and recheck. Your real-world data beats the formula every time.

BMR declines roughly 2–3% per decade after age 30, mostly due to sarcopenia — the gradual loss of muscle mass that accompanies aging. Resistance training can offset or even reverse this decline by preserving lean mass. A 70-year-old who strength-trains regularly may have a BMR comparable to a sedentary 50-year-old.

Chronic dieting and significant weight loss can lower BMR beyond what the formula predicts — a phenomenon often called adaptive thermogenesis or metabolic adaptation. Studies of extreme weight-loss contestants showed BMRs hundreds of calories below predicted years after the diet ended. This doesn't mean 'calories don't matter,' but it does mean the calculator's starting point may need a larger adjustment after prolonged restriction.

Hormonal factors also matter. Thyroid disorders, PCOS, and certain medications affect BMR independently of body composition. If your real-world weight trend diverges from the calculator's prediction by more than 200 kcal/day consistently, consider medical screening rather than simply cutting harder.