BMR calculator — your basal metabolic rate.

Your BMR is the number of calories your body burns at complete rest over 24 hours just to keep you alive — breathing, circulating blood, repairing cells, and regulating temperature. It is the single biggest part of your daily calorie burn, typically 60 to 70 percent of the total, before any movement or food digestion is added.

It uses the Mifflin-St Jeor equation, established in 1990 and the most accurate validated formula for modern adults. Studies find it lands within 10 percent of measured BMR more often than the older Harris-Benedict equation, which dates to 1919 and tends to overestimate by 5 to 15 percent in sedentary people.

BMR is your burn at rest. TDEE — total daily energy expenditure — is BMR multiplied by an activity factor between 1.2 and 1.9 to account for movement, exercise, and digesting food. You eat to your TDEE, not your BMR. Use the TDEE calculator to get that number.

As a rule, no. Eating below your BMR for long stretches is hard to sustain, can cost you muscle, and tends to drive fatigue and rebound eating. For fat loss, set a deficit against your TDEE — not your BMR — so you still cover the calories your body needs at rest.

For most people the Mifflin-St Jeor estimate is within about 10 percent of a lab-measured value. It cannot see your exact muscle mass, genetics, or hormones, so treat it as a strong starting point. Track your weight for two to three weeks and adjust if reality drifts from the estimate.