BMR Calculator

What BMR actually measures

Basal Metabolic Rate (BMR) is the number of calories your body needs to perform its most basic, involuntary functions — keeping your heart beating, your lungs breathing, your cells repairing themselves and your brain running — if you did absolutely nothing else all day. It typically accounts for 60–75% of the total calories most people burn, which makes it the foundation of any calorie or nutrition plan.

The Mifflin-St Jeor formula

This calculator uses the Mifflin-St Jeor equation, which research has repeatedly shown to be more accurate than the older Harris-Benedict formula for most adults. It uses your weight, height, age and sex, since these four factors are the strongest predictors of resting metabolism available without specialised lab equipment such as indirect calorimetry. Muscle tissue burns more calories at rest than fat tissue, which is one reason two people of the same height and weight can have noticeably different BMRs.

What to do with your BMR number

BMR alone isn't a calorie target — it's a baseline. To get a realistic daily calorie need, that number is multiplied by an activity factor reflecting how much you move throughout the day, which is exactly what our Calorie Calculator does automatically. Knowing your BMR on its own is still useful, though: it tells you the absolute minimum a very low-calorie diet should rarely fall below, since eating far under your BMR for extended periods can affect energy, hormones and muscle mass.

Factors that change your BMR over time

Metabolism naturally slows with age, largely due to gradual muscle loss, and can also shift with major weight changes, thyroid conditions, pregnancy and certain medications. Strength training is one of the few interventions shown to meaningfully preserve or raise BMR over time, since it helps maintain metabolically active muscle mass. If you haven't recalculated your BMR in over a year, or after a significant weight change, it's worth running the numbers again.