BMR Calculator

Basal Metabolic Rate (BMR) is the number of calories your body burns in 24 hours doing absolutely nothing — no walking, no digesting food, no exercise, just keeping your heart beating, your lungs breathing, your brain functioning, your cells repairing themselves, and your body temperature regulated. It is measured, in a laboratory setting, on someone lying completely still in a fasted state after 8 hours of sleep in a thermally neutral room. Almost nobody actually experiences a day at true BMR, but it serves as the essential baseline for every other calorie estimate: your Total Daily Energy Expenditure (TDEE) is your BMR plus the calories burned through digestion, movement and exercise on top of it, which is why BMR is the first number any calorie or macro calculation needs to start from. Because a true lab measurement (indirect calorimetry, which analyses the oxygen and carbon dioxide in your breath) is impractical for everyday use, researchers have developed predictive equations that estimate BMR from a handful of easily measured variables: weight, height, age and sex. The oldest widely used version, the Harris-Benedict equation, was originally published in 1919 and revised in 1984 to correct a systematic overestimate found in the original formula. It remained the default clinical and fitness-industry standard for decades. In 1990, Mifflin and St Jeor published a new equation, developed from a larger and more contemporary sample of the population, which numerous validation studies since have found to be more accurate than Harris-Benedict for most people today — likely because average body composition and lifestyle have shifted since 1919. Mifflin-St Jeor is now the equation most commonly recommended by dietitians and used as the default in clinical nutrition software, which is why this tool leads with it and shows the revised Harris-Benedict figure alongside for comparison rather than as the primary result. Both equations take the same four inputs — weight, height, age and sex — and combine them with different coefficients derived from their respective research populations. Neither equation directly measures body composition (the ratio of muscle to fat), which is the single biggest limiter of accuracy for both formulas: muscle tissue burns meaningfully more calories at rest than fat tissue, so someone who is unusually muscular for their weight will have a real BMR somewhat higher than either formula predicts, and someone with unusually low muscle mass for their weight will run somewhat lower. A separate formula, Katch-McArdle, addresses this by using lean body mass directly instead of total weight — but it requires a body fat percentage measurement as an input (from calipers, a bioelectrical impedance scale, or a DEXA scan), which most people do not have on hand, so it is not included here. BMR naturally declines with age (roughly 1–2% per decade after your twenties, largely due to gradual muscle loss unless resistance-trained against), and varies with genetics, hormone levels (thyroid function in particular has a large effect), and body size — a larger body, even at the same body-fat percentage, has more tissue to maintain and therefore a higher BMR. Once you have a BMR figure from this tool, the natural next step is to multiply it by an activity multiplier to estimate your full daily calorie needs (TDEE) — that calculation, along with calorie targets for weight loss, maintenance and gain, is covered by the separate Calorie Calculator, and turning a calorie target into grams of protein, carbohydrate and fat is covered by the Macro Calculator.