BMR Calculator
Estimate your basal metabolic rate (BMR) — the calories your body burns at complete rest — using the revised Harris-Benedict equation. The starting point for calculating how many calories you need to eat to lose, maintain, or gain weight.
Last updated: May 2026
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About this calculator
BMR is the energy your body burns over 24 hours doing nothing but staying alive — keeping your heart beating, lungs breathing, brain firing, organs running, and core temperature stable. For most people it represents 60–75% of total daily energy expenditure. This calculator uses the revised Harris-Benedict equation (Roza & Shizgal, 1984), which gives BMR in kcal/day: Male = 88.362 + (13.397 × weight kg) + (4.799 × height cm) − (5.677 × age years); Female = 447.593 + (9.247 × weight kg) + (3.098 × height cm) − (4.330 × age years). Variables: weight in kilograms, height in centimetres, age in whole years, biological sex (male or female). Why these equations and not Mifflin-St Jeor? Mifflin (1990) is slightly more accurate for the modern population (narrower 95% confidence interval) and many nutritionists prefer it; revised Harris-Benedict remains widely used in textbooks and clinical contexts and produces results within ~5% of Mifflin for typical body sizes. Edge cases: the formulas were derived from average healthy adults aged 18–65 and lose accuracy at the extremes — very lean athletes underestimate (high muscle mass burns more), very obese individuals overestimate (excess adipose is less metabolically active per kg), and elderly adults often have lower BMR than predicted due to muscle loss. Real measured BMR via indirect calorimetry can differ from any prediction equation by 10–15% even in healthy adults, so treat the output as a starting estimate to refine by tracking weight against actual intake over 2–3 weeks. Pregnancy, lactation, hyperthyroidism, hypothyroidism, fever, and recent surgery all materially shift BMR and warrant clinical assessment rather than a calculator.
How to use
Example 1 — Adult male. A 30-year-old man weighing 80 kg and standing 180 cm tall. Enter Weight = 80, Height = 180, Age = 30, Gender = Male. BMR = 88.362 + (13.397 × 80) + (4.799 × 180) − (5.677 × 30) = 88.362 + 1071.76 + 863.82 − 170.31 ≈ 1853.6 kcal/day. ✓ That is his resting energy cost; multiply by an activity factor (typically 1.4–1.7) to get total daily energy expenditure. Example 2 — Adult female. A 45-year-old woman weighing 65 kg and standing 165 cm tall. Enter Weight = 65, Height = 165, Age = 45, Gender = Female. BMR = 447.593 + (9.247 × 65) + (3.098 × 165) − (4.330 × 45) = 447.593 + 601.06 + 511.17 − 194.85 ≈ 1365 kcal/day. ✓ Her resting needs are lower than the male example reflecting both body size and the formula coefficients; eating at this level would cause weight loss because it doesn't include energy for daily activity.
Frequently asked questions
What's the difference between BMR and resting metabolic rate (RMR)?
They're very similar but measured under different conditions. BMR is the strict scientific term — measured first thing in the morning after a 12-hour fast, in a thermally neutral environment, with the person fully rested and lying down. RMR is a less strict measurement taken under more normal conditions (no fast required, person reasonably rested but not necessarily after waking up). RMR tends to be about 10% higher than true BMR because of residual digestion (thermic effect of food) and slightly elevated metabolic activity from being upright or moving. In practice, prediction equations like Harris-Benedict and Mifflin-St Jeor produce estimates that fall between true BMR and RMR, and the terms are often used interchangeably in popular fitness literature. For weight-management purposes, the small difference rarely matters — what matters is consistency and calibration against actual weight change over weeks.
Which BMR equation is most accurate?
For non-obese healthy adults, the Mifflin-St Jeor equation (1990) is considered the gold standard prediction equation — multiple validation studies show it has the narrowest error band of the major formulas. Revised Harris-Benedict (used here) is close second and typically estimates BMR a few percent higher than Mifflin for the same person. The Katch-McArdle equation is more accurate than either for very lean or very muscular people because it incorporates lean body mass directly: BMR = 370 + (21.6 × lean mass kg). Schofield equations are widely used in clinical paediatric and geriatric settings. For most adults any of these gets within 5–10% of true BMR; the bigger source of error in real-life calorie tracking is the activity multiplier (a guess) and food intake (almost always under-reported). Don't obsess over which equation — pick one, track for 2–3 weeks, and adjust empirically.
How does BMR change with age, weight loss, and exercise?
BMR declines roughly 1–2% per decade after age 20, mostly due to gradual loss of lean muscle mass (sarcopenia). It also drops with weight loss — you literally have less body tissue to maintain — at roughly 15–25 kcal per kg lost. This "metabolic adaptation" is the main reason dieters often stall: the BMR you calculated at 90 kg is no longer valid at 80 kg, so the deficit you computed becomes smaller than planned. Strength training partially offsets both effects by preserving or building muscle, which is metabolically active tissue (each kg of skeletal muscle burns ~13 kcal/day at rest, vs ~4 kcal/day for fat). Cardiovascular exercise burns calories during the workout but has little lasting effect on BMR. The practical upshot: recompute BMR every 5–10 kg of weight change, and prioritise strength work alongside any calorie deficit to preserve metabolic rate.
What are the most common mistakes people make using BMR calculators?
The first is treating the output as exact when individual variation is 10–15% — calibrate against actual weight change over 2–3 weeks before trusting the number. The second is forgetting BMR is not total calorie need — multiply by an activity factor to get TDEE before designing any meal plan. The third is using the male formula by accident (or vice versa) because the gender field defaults wrong; the equations are different enough that this distorts the answer by 100–300 kcal/day. The fourth is mixing units — entering weight in pounds while the calculator expects kilograms (or height in inches when it wants centimetres) produces wildly wrong values. The fifth is not recomputing as you lose weight; the BMR you started with no longer applies once you've dropped 10 kg, and the gap explains many "I plateaued" complaints. Finally, people often expect BMR to predict weight loss perfectly; it gives a starting hypothesis to test, not a guarantee.
When should I not use this calculator?
Skip it for children and adolescents (under 18) — pediatric BMR uses different equations (Schofield is standard) and accounts for growth energy costs. Don't use it during pregnancy or lactation, which add roughly 300 kcal/day in pregnancy and 500 kcal/day during breastfeeding to baseline needs; dedicated calculators exist for those. It's unreliable for people with significant medical conditions affecting metabolism — untreated thyroid disorders (hyperthyroidism elevates BMR 10–60%, hypothyroidism depresses it 10–30%), recent major surgery, severe burns, fever, cancer cachexia, or eating-disorder recovery — those situations need clinical indirect calorimetry. It's also a poor fit at the extremes of body composition: elite bodybuilders with very high lean mass underestimate (use Katch-McArdle), and severely obese individuals (BMI > 40) tend to overestimate because excess adipose is less metabolically active than the linear formula assumes. For these groups, a registered dietitian or sports medicine physician should interpret needs in context.