Mifflin-St Jeor Calculator (BMR)

BMR — Basal Metabolic Rate, is the energy needed just to keep you alive. It covers your heartbeat, breathing, keeping your body temperature and brain function. Even if you lay in bed motionless for a full day, your body would burn roughly the amount of energy this calculator shows.

The Mifflin-St Jeor formula was published in 1990 and superseded the old Harris-Benedict formula from over a hundred years ago. 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. In validation studies the formula matches actual BMR with an error below 10 percent for people without extreme obesity.

BMR is not the same as the daily caloric intake you actually eat. 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. The man from our example, training three times a week, needs about 2720 kcal.

It's worth knowing the limitations of this calculation. It treats a kilogram of muscle the same as a kilogram of 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 result is an estimate — actual BMR is measured through indirect calorimetry under laboratory conditions.