Mifflin-St Jeor Calculator (BMR)

BMR, or basal metabolic rate, is the number of calories your body burns at complete rest. It's the cost of your heartbeat, breathing, internal organ function and temperature regulation. Even with a full day of bed rest, your body would burn roughly the amount of energy this calculator shows.

The Mifflin-St Jeor equation dates back to 1990 and replaced the older Harris-Benedict equation from 1919. For men it's calculated as: 10 × weight in kg + 6.25 × height in cm − 5 × age + 5. For women the last term is −161 instead of +5. A man weighing 80 kg, 180 cm tall, aged 35 gets 800 + 1125 − 175 + 5, which equals 1755 kcal per day. The deviation from calorimetric measurement is usually below 10 percent for people without extreme obesity.

BMR alone isn't your full energy requirement. Full daily requirement, or TDEE, is calculated like this, multiply your BMR by an activity factor: 1.2 for a sedentary job with no exercise, 1.375 for light activity twice a week, 1.55 for training three to five times a week, 1.725 for daily exertion, and 1.9 for physical work combined with training. Going back to our example: 1755 × 1.55 gives roughly 2720 kcal per day.

It's worth knowing the limitations of this calculation. It doesn't distinguish muscle from fat, and muscle tissue burns noticeably more energy at rest than fat tissue. A bodybuilder will see a value below their actual burn, while for people with severe obesity it will be overestimated. Different equations apply to children and teenagers, for pregnant or breastfeeding women, or for conditions like thyroid disease that alter metabolic rate. The number is an estimate, not a measurement — actual BMR is measured through indirect calorimetry under laboratory conditions.