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The Mifflin-St Jeor equation (1990) estimates your resting calorie burn from weight, height, age and sex, repeatedly validated as the most accurate standard prediction for healthy adults. Multiplying by an activity factor (1.2–1.9) gives TDEE, your total daily energy expenditure. Results are planning estimates, not medical advice.
Published in 1990 and repeatedly validated since, Mifflin-St Jeor is the equation doctors and dietitians use most: Men: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5. Women: same, but − 161 instead of + 5.
Worked examples: a 30-year-old man, 180 cm, 80 kg: 800 + 1,125 − 150 + 5 = 1,780 kcal/day. A 30-year-old woman, 165 cm, 60 kg: 600 + 1,031.25 − 150 − 161 = 1,320 kcal/day. The only difference between the two is the final constant, a 166 kcal gap reflecting average body-composition differences.
Enter your weight, height, age and sex, then pick the activity level that matches your usual week, be honest; most people overestimate. Press Calculate for your BMR, your TDEE (daily maintenance calories), and calorie targets for gradual loss or gain.
This calculator is designed for adults aged 18 and over. Teenagers need extra energy for growth, which adult equations do not account for.
BMR is what you burn at complete rest. To get TDEE, what you actually need per day, multiply by an activity factor: sedentary (desk job, little exercise) × 1.2; lightly active × 1.375; moderately active (exercise 3–5 days/week) × 1.55; very active × 1.725; extra active (physical job or two-a-day training) × 1.9.
For the worked example above (BMR 1,780): sedentary = 2,136; moderate = 2,759; extra active = 3,382 kcal/day. People overestimate activity far more often than they underestimate, a desk job with three workouts a week is usually 'lightly active', and choosing too high a factor is the most common reason calorie targets fail.
BMR (basal metabolic rate): energy at complete rest, measured under strict lab conditions after fasting, the floor. RMR (resting metabolic rate): similar, measured less strictly, usually slightly higher; in practice used interchangeably with BMR. TDEE (total daily energy expenditure): BMR plus activity and digestion, the number that matters for weight goals.
The one rule that matters: BMR is not how much you should eat. Your daily needs are your TDEE, which is always higher.
Muscle: lean tissue burns more energy at rest than fat, the biggest reason two people of the same weight can have different BMRs. Body size: more tissue to maintain means a higher BMR. Age: the equation subtracts 5 kcal a year, so 30 extra years costs about 150 kcal, mostly from muscle loss. Sex: the separate constants (+5 / −161) reflect average composition differences. Hormones and health: thyroid conditions, illness, fever, pregnancy and some medications all shift metabolic rate.
The one reliable long-run lever is strength training: building or keeping muscle raises the energy your body uses at rest, and it matters most as a way of limiting the decline that comes with age and dieting.
Start from TDEE and adjust by 10–20%: that is the moderate-deficit band most plans use. The familiar rule that a 500 kcal daily deficit loses about 0.45 kg (1 lb) a week is a rough starting point, not a promise, as you lose weight your BMR falls too, and the body adapts, so progress usually slows after the first few weeks.
Eating well below your BMR for long periods is not recommended: very low-calorie diets can cause muscle loss, nutritional deficiencies and gallstones, and should only be followed under medical supervision. Track your weight over three or four weeks and adjust the target, real-world response beats any formula.
For most healthy adults, Mifflin-St Jeor lands within about 10% of a measured BMR, roughly ±180 kcal a day at a BMR of 1,780. That is good enough for a starting estimate but meaningful over weeks, which is why you adjust from real results.
Accuracy is lower at the extremes: very muscular athletes, people with high body fat, older adults and anyone with a metabolic condition. The older Harris-Benedict equation (1919, revised 1984) tends to read slightly higher for many adults; the Academy of Nutrition and Dietetics recommends Mifflin-St Jeor for healthy adults when body fat percentage is unknown. The only exact measurement is indirect calorimetry at a clinic.
Eating to your BMR: BMR is rest-only energy; your needs are your TDEE. Choosing too high an activity level: inflates TDEE and every target built on it, be conservative. Treating the result as exact: expect around 10% margin. Counting exercise twice: picking an active multiplier and then eating back fitness-tracker calories double-counts. Mixing units: make sure the weight unit matches what you entered, pounds versus kilograms is the classic slip. Not adjusting: if your weight does not move as expected after three or four weeks, the estimate needs updating, not your willpower.
Basal metabolic rate is the calories your body burns at complete rest just to stay alive, breathing, circulating blood, keeping organs running. It is usually 60–70% of everything you burn in a day, the largest single piece of your energy budget.
With the Mifflin-St Jeor equation: 10 × weight(kg) + 6.25 × height(cm) − 5 × age, then +5 for men or −161 for women. A 30-year-old man at 180 cm and 80 kg: 800 + 1,125 − 150 + 5 = 1,780 kcal/day.
BMR is energy at complete rest, the floor. TDEE (total daily energy expenditure) is BMR multiplied by an activity factor (1.2 sedentary to 1.9 very active) to include movement and digestion. TDEE, not BMR, is the number to base calorie targets on.
Almost. BMR is measured under strict lab conditions after fasting and a night of rest; resting metabolic rate (RMR) is measured less strictly and comes out slightly higher. The gap is smaller than the error in any estimate, so they are used interchangeably.
Mainly body composition: at the same height and weight, women carry more fat and less muscle on average, and muscle uses more energy at rest. The equations reflect it with +5 for men versus −161 for women, a 166 kcal difference.
Yes, the equation subtracts 5 kcal per year, so the same man at 60 instead of 30 has a BMR about 150 kcal lower, mostly from losing muscle. Strength training is the main way to slow the decline.
Modestly, by building or keeping muscle through strength training, since lean tissue burns more energy at rest than fat. The effect is real but small; its main value is limiting the decline that comes with age and dieting. Crash diets do the opposite, severe restriction can lower BMR.
Mifflin-St Jeor lands within about 10% of a measured BMR for most healthy adults, roughly ±180 kcal a day at a BMR of 1,780. Accuracy is lower for very muscular people, people with high body fat, older adults and metabolic conditions. Indirect calorimetry at a clinic is the only exact measurement.