BMR Calculator: Mifflin-St Jeor Resting Calories

The Mifflin-St Jeor equation

This calculator uses the equation published by Mifflin and colleagues in the American Journal of Clinical Nutrition in 1990. In metric units: men, BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5. Women use the same formula but subtract 161 instead of adding 5. The tool takes inches and pounds, converts them (1 lb = 0.4536 kg, 1 in = 2.54 cm), and runs the numbers.

The 166-calorie gap between the male and female versions reflects average differences in lean mass at the same height and weight. The equation was developed on healthy adults aged roughly 19 to 78, so results for teenagers, pregnant women, and people with medical conditions affecting metabolism are rougher estimates.

Worked examples

Man, 35 years old, 5 feet 9 inches (69 in), 180 lb. Weight: 81.6 kg. Height: 175.3 cm. 10 × 81.6 = 816.5, 6.25 × 175.3 = 1,095.4, minus 5 × 35 = 175, plus 5. BMR is about 1,742 kcal a day, or about 73 calories an hour lying still.

Woman, 30 years old, 5 feet 4 inches (64 in), 140 lb. Weight: 63.5 kg. Height: 162.6 cm. 635 + 1,016 − 150 − 161 = 1,340 kcal a day. Same formula shape, smaller body, lower number.

From BMR to the calories you actually burn

Nobody lives at BMR. Multiply it by an activity factor to estimate total daily energy expenditure (TDEE): 1.2 if you sit most of the day, 1.375 for light exercise one to three days a week, 1.55 for moderate exercise three to five days, 1.725 for hard exercise six or seven days, and 1.9 for a physical job plus training.

For the man above, that is about 2,090 kcal sedentary, 2,700 kcal moderately active, and 3,005 kcal very active. The table under the result shows all five levels. Most people overestimate their activity, so if you are unsure between two levels, pick the lower one and adjust after a few weeks of real data.

How accurate is it?

In validation studies Mifflin-St Jeor predicted resting metabolic rate within 10 percent of measured values for most healthy adults, which is why it replaced the older Harris-Benedict equation (1919) as the default in most nutrition software. Within 10 percent still means a 1,742 estimate could be 1,570 or 1,915 for a given person.

Body composition is the main source of error. Muscle burns more energy at rest than fat, so a lean, muscular person often has a higher BMR than predicted, and someone with a high body fat percentage a lower one. If you know your body fat percentage, the Katch-McArdle equation (370 + 21.6 × lean mass in kg) can be more accurate for athletes.

Using BMR for weight goals

Your BMR is a floor, not a target. Eating at BMR means running a deficit equal to all your daily activity, often 500 to 1,000 kcal or more, which is aggressive and hard to sustain. A better approach: estimate TDEE, subtract about 500 kcal for roughly a pound a week of loss, and keep intake above common minimums of about 1,200 kcal for women and 1,500 for men without medical supervision.

BMR drops as you lose weight, since a smaller body costs less to maintain, and the formula subtracts 5 kcal per year of age. Recalculate every 10 to 15 pounds. Strength training two or three times a week helps preserve muscle, which keeps your real BMR closer to or above the prediction.

Get the most accurate number

Weigh yourself in the morning after using the bathroom and measure height without shoes. Then treat the result as a starting estimate. Track your intake and weekly average weight for two to three weeks: if weight is stable, your intake is your true maintenance, whatever the formula said. This calculator is educational, not medical advice, and runs entirely in your browser.