BMR, TDEE and the Mifflin-St Jeor equation
Your basal metabolic rate (BMR) is the energy your body burns at complete rest to keep your heart beating, lungs breathing and organs working. Total daily energy expenditure (TDEE) adds everything else: walking around, digesting food, and exercise. TDEE is the number that matters for weight management, because eating that many calories, on average, keeps your weight stable.
This calculator uses the Mifflin-St Jeor equation, published in 1990 and recommended by the Academy of Nutrition and Dietetics as the most accurate of the common formulas for adults. For men: BMR = 10 x weight (kg) + 6.25 x height (cm) - 5 x age + 5. For women: BMR = 10 x weight (kg) + 6.25 x height (cm) - 5 x age - 161. Pounds and inches are converted behind the scenes (1 lb = 0.4536 kg, 1 in = 2.54 cm). BMR is then multiplied by an activity factor: 1.2 for sedentary, 1.375 for light activity, 1.55 for moderate, 1.725 for very active and 1.9 for extra active people with physical jobs or two-a-day training.
Worked example
Take a 30-year-old man who is 5 ft 9 in and 160 lb, moderately active. His height is 175.3 cm and his weight 72.6 kg. BMR = 10 x 72.6 + 6.25 x 175.3 - 5 x 30 + 5 = 725.7 + 1,095.4 - 150 + 5 = 1,676 calories. Multiply by 1.55 and his TDEE is 2,598 calories per day. To lose about a pound a week he would eat 500 fewer, roughly 2,100 calories; for half a pound a week, 2,350; to gain half a pound a week, about 2,850. A woman of the same age, height and weight would have a BMR of 1,510 and a TDEE of 2,341 at the same activity level.
The 500-calorie rule comes from the estimate that a pound of body fat stores about 3,500 calories, so a daily deficit of 500 is 3,500 a week. Real weight loss is not that tidy: the body adapts, water shifts mask fat loss, and the deficit shrinks as you get lighter. Expect the rule to be roughly right over the first couple of months, then recalculate with your new weight.
Picking an activity level and using the numbers
Most people overestimate their activity. If you sit at a desk and work out three times a week for 45 minutes, you are lightly active, not moderately active; "very active" is for people who train hard almost every day or do physical labor. When in doubt, pick the lower level, eat at that TDEE for two or three weeks, and let the scale tell you which way to adjust. A steady change of 100-200 calories per day is all most people need.
The macro example splits maintenance calories 30% protein, 40% carbohydrate and 30% fat, using 4 calories per gram of protein and carbohydrate and 9 per gram of fat. It is a reasonable general-purpose split; people who lift weights or are dieting often push protein toward 0.7-1 gram per pound of body weight to protect muscle. Very low targets are flagged in the table: going below about 1,200 calories for women or 1,500 for men without medical supervision makes it hard to get enough protein and micronutrients. Pregnancy, breastfeeding, thyroid conditions and medications all change the math, so treat these results as a starting point rather than a prescription.