Basal Metabolic Rate (BMR) is the number of calories your body needs at complete rest. This calculator compares two widely used formulas: the Mifflin-St Jeor equation (considered most accurate) and the original Harris-Benedict equation.
Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest — just keeping the lights on. Heart pumping, brain functioning, kidneys filtering, body maintaining temperature. For a typical adult, BMR represents about 60–75% of total daily calorie burn. The remainder comes from physical activity (the more variable component) and the thermic effect of food (small but real).
Knowing your BMR is the starting point for any calorie planning — weight loss, weight gain, athletic performance, or simply staying at a healthy weight. Total Daily Energy Expenditure (TDEE) is BMR multiplied by an activity factor (typically 1.2 for sedentary, up to 1.9 for very active). Eating at TDEE maintains weight; eating below creates a deficit (loss); eating above creates a surplus (gain). The math is simple in principle, complicated by individual variation and the body's remarkable ability to adjust to chronic over- or under-eating.
This calculator uses two well-known formulas — Mifflin-St Jeor (developed 1990) and the original Harris-Benedict (developed 1919) — to estimate BMR. Modern research has shown Mifflin-St Jeor to be more accurate for the general population, typically within 10% of measured BMR for most healthy adults. Harris-Benedict tends to slightly overestimate. Both are useful as starting estimates; precise individual BMR requires laboratory measurement (indirect calorimetry).
Age 30, female, 140 lb, 5'5" (65 inches). Mifflin-St Jeor BMR: 10(63.5) + 6.25(165.1) − 5(30) − 161 = 635 + 1,032 − 150 − 161 = 1,356 cal/day At moderate activity (1.55 multiplier): TDEE ≈ 2,102 cal/day For 1 lb/week weight loss: eat 1,600 calories/day (500-calorie daily deficit). For maintenance: 2,100 calories/day.
Age 45, male, 200 lb, 6'0" (72 inches). Mifflin-St Jeor BMR: 10(90.7) + 6.25(182.9) − 5(45) + 5 = 907 + 1,143 − 225 + 5 = 1,830 cal/day At sedentary activity (1.2 multiplier): TDEE ≈ 2,196 cal/day A sedentary 45-year-old often needs surprisingly few calories. Aiming for "average" intake recommendations (~2,500–2,800 cal/day) without matching activity levels often leads to slow weight gain over years.
Same person at 25 vs at 65 (170 lb, male, 5'10"): Age 25 BMR: 10(77.1) + 6.25(177.8) − 5(25) + 5 = 771 + 1,111 − 125 + 5 = 1,762 cal/day Age 65 BMR: 10(77.1) + 6.25(177.8) − 5(65) + 5 = 771 + 1,111 − 325 + 5 = 1,562 cal/day Difference: 200 calories/day, or about 73,000 calories/year — roughly 20 lb of theoretical weight gain over a year if eating habits don't adjust. The decline is mostly due to loss of lean muscle mass (sarcopenia). Strength training is the most effective intervention to slow or reverse this decline.
Use this calculator any time you're starting a weight management plan, training for a specific athletic goal, planning recovery from injury or illness, or just trying to understand what your body actually needs to function. The BMR number is the starting point for almost all calorie-related decisions.
It's also useful for revisiting periodically. BMR drops with age (about 1–2% per decade for typical adults), changes with weight (a 200-pound person has higher BMR than a 150-pound person), and decreases with weight loss (one of the reasons sustained weight loss is hard — your body needs fewer calories as you shrink). Re-running every 5–10 lb of weight change keeps your calorie targets calibrated.
Pair this with the TDEE calculator (which applies the activity factor directly), the calorie calculator (which builds in your weight goal), and the macro calculator (which divides total calories into protein, carbs, and fat targets).
A real-world caveat: these formulas estimate BMR for a typical body composition. Athletes with high lean muscle mass typically have higher BMR than the formula suggests; individuals with lower muscle mass (sedentary lifestyle, older age, post-prolonged-illness) typically have lower BMR. For precise measurements, indirect calorimetry (measuring oxygen consumption at rest) is the gold standard but requires specialized equipment — usually only available at fitness research centers or specialty clinics.
Another reality: the body adapts to chronic calorie intake. Sustained large deficits trigger metabolic adaptation — BMR drops, NEAT (non-exercise activity thermogenesis) decreases, and the body becomes more efficient at maintaining weight with less food. This is why aggressive cutting plateaus and why "diet breaks" (eating at maintenance periodically) can paradoxically support continued progress.
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Mifflin-St Jeor
1,706 cal/day
Harris-Benedict
1,780 cal/day
Average BMR
1,743 cal/day
Difference
74 cal