BMR Calculator
Your basal metabolic rate is the energy you burn at complete rest — in a comfortably temperate room, awake but inactive, with an empty digestive system (about 12 hours after your last meal). This calculator estimates it from the formula you choose in Settings, then scales it by six activity levels to show your daily calorie needs.
Results unit:
BMR estimation formula:
BMR =
- Exercise: 15-30 minutes of elevated heart rate activity.
- Intense exercise: 45-120 minutes of elevated heart rate activity.
- Very intense exercise: 2+ hours of elevated heart rate activity.
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(Responsive Fluid)
What BMR measures
Think of an idling car: engine on, going nowhere, still burning fuel. That is BMR for a human — the cost of keeping the heart, brain, kidneys, nervous system, intestines, liver, lungs, sex organs, muscles, and skin running, and nothing else. A laboratory-grade BMR measurement is fussy: you must be awake but completely rested, with the sympathetic nervous system quiet and digestion idle. Because real life rarely cooperates, almost everyone works from equations fitted to statistical data instead — the three under Settings. Harris-Benedict was one of the first (revised in 1984); Mifflin-St Jeor arrived in 1990 and has repeatedly tested more accurate; Katch-McArdle takes lean body mass into account, which the other two ignore, and can be the best of the three for lean people who know their body fat percentage. Multiply the BMR by an activity factor between 1.2 and 1.9 and you get the daily calorie need — the table in your result.
Mifflin-St Jeor
Men: BMR = 10W + 6.25H − 5A + 5
Women: BMR = 10W + 6.25H − 5A − 161
Revised Harris-Benedict
Men: BMR = 13.397W + 4.799H − 5.677A + 88.362
Women: BMR = 9.247W + 3.098H − 4.330A + 447.593
Katch-McArdle
BMR = 370 + 21.6 × (1 − F) × W
W = weight (kg), H = height (cm), A = age, F = body fat fraction.
The variables that move BMR
Muscle mass — Running and cycling don't change BMR directly, but lifting does, indirectly: more muscle means more tissue to maintain at rest, around the clock.
Age — BMR drifts down as the years add up and bodies lose lean tissue; less upkeep, less fuel.
Genetics — Inherited traits account for a real share of the difference between two otherwise similar people.
Weather — Cold forces the body to burn energy holding its temperature; serious heat does too, for cooling. BMR rises roughly 7% for every 1.36 °F increase in internal body temperature.
Diet — Small, regular meals nudge BMR up slightly. Starvation does the opposite, hard: cuts of up to 30% as the body switches to power-saving mode and starts sacrificing energy, mood, and upkeep.
Pregnancy — Growing a second person raises BMR (one reason pregnant women eat more); menopause can push it either way with hormonal change.
Supplements — Some drugs and supplements raise BMR — caffeine is the everyday example — which is why they show up in weight-loss products.
BMR vs RMR, and the limits of any formula
Resting metabolic rate (RMR) is BMR's more relaxed sibling: measured in a rested-but-not-fully-inactive state, under conditions less strict than BMR's demand for total physiological equilibrium, it comes out a little higher. The two terms get used interchangeably, but they are different measurements. Either way, a formula is an estimate. A 2005 meta-analysis (Johnstone et al., American Journal of Clinical Nutrition) found that even after accounting for fat-free mass, fat mass, age, and hormones, about 26% of the variation between people stays unexplained. Serious testing uses calorimetry devices — available at many clinics and gyms — and even those results feed into a TDEE that is still an estimate. The practical answer is the boring one: use BMR to set your starting point, keep a simple journal of food, activity, and weight, and adjust from what actually happens.
FAQ
What is the difference between BMR and RMR?
BMR demands strict conditions — fasted, fully rested, temperature-neutral. RMR relaxes them and reads slightly higher. In practice the terms are often swapped, but the measurements are not identical.
Which formula should I use?
Mifflin-St Jeor for most people. If you know your body fat percentage and are lean, try Katch-McArdle in Settings and compare — our Body Fat Calculator can supply the percentage.
Can I raise my BMR?
Build muscle — it is the one durable lever. And avoid crash diets: severe restriction suppresses BMR by up to 30%, which is part of why aggressive diets rebound.