Your Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest — the energy required just to keep you alive, breathing, circulating blood, and maintaining body temperature. It is the single largest component of your total daily calorie expenditure, typically accounting for 60-75% of everything you burn in a day.
Knowing your BMR is the foundation of scientifically sound nutrition. Weight loss requires eating below your total expenditure; muscle gain requires eating above; and both start with understanding the baseline your body demands. This free BMR calculator applies the Mifflin-St Jeor equation — the most extensively validated formula for most adults — to your height, weight, age, and sex in seconds.
The Mifflin-St Jeor Equation
For men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5. For women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161. The equation was published in 1990 and has been validated in multiple independent studies as more accurate than the older Harris-Benedict formula, which tends to overestimate.
To apply it with imperial units, weight in pounds is converted to kilograms (÷ 2.205) and height in inches to centimeters (× 2.54). The calculator handles both unit systems automatically.
A worked example: a 35-year-old man weighing 180 lb (81.6 kg) at 5'10" (178 cm) computes BMR = (10 × 81.6) + (6.25 × 178) − (5 × 35) + 5 = 816 + 1112.5 − 175 + 5 = 1,758 calories per day. A 30-year-old woman at 140 lb (63.5 kg), 5'5" (165 cm): (10 × 63.5) + (6.25 × 165) − (5 × 30) − 161 = 635 + 1031 − 150 − 161 = 1,355 calories.
From BMR to Total Daily Energy Expenditure
BMR is only part of the picture. Your total daily energy expenditure (TDEE) multiplies BMR by an activity factor: sedentary (× 1.2), lightly active (× 1.375), moderately active (× 1.55), very active (× 1.725), and extra active (× 1.9). The man above with a BMR of 1,758 who is moderately active needs about 2,725 calories daily to maintain weight.
TDEE shapes every goal. Losing about 1 lb per week requires a daily deficit near 500 calories — eating roughly 500 below TDEE. Gaining lean mass typically uses a 200-500 calorie surplus with adequate protein. Maintenance is simply eating at TDEE.
Activity level is the most error-prone input: desk workers who exercise 1-2 days weekly should use "lightly active," not "moderately." Most people overstate activity, which inflates TDEE and undermines weight-loss plans. When in doubt, use the lower factor and adjust based on observed weight trends over 2-3 weeks.
What Affects Your BMR
Muscle mass is the most controllable driver of BMR. Each pound of muscle burns more calories at rest than a pound of fat, and lean mass is metabolically active tissue. This is why resistance training raises BMR over time — it is the only activity that structurally increases your resting energy burn.
Age typically reduces BMR because lean mass tends to decline with years; the −5 × age term reflects this. Sex differences are built into the equations (men average higher lean mass). Height increases BMR by increasing body volume, and weight raises it directly through the 10 × weight term.
Hormones and temperature also matter: thyroid function, illness, extreme environments, and adaptations from prolonged calorie restriction all modulate BMR. A sustained large deficit causes metabolic adaptation — BMR drops as the body conserves energy — which is why extreme diets plateau and why periodic diet breaks support long-term success.
BMR is the keystone of personalized nutrition — the metabolic baseline every diet and training goal must be built around. Calculate yours, multiply by an honest activity factor to find TDEE, and set calorie targets from that foundation. Pair it with real-world feedback from the scale over several weeks, and you convert generic dietary advice into a plan calibrated to your own body.