What is BMR and which formula is most accurate?
Your basal metabolic rate (BMR) is the calories you burn at complete rest, just to stay alive. It powers your heart, brain, kidneys and cell repair while you do nothing at all, and it makes up the largest share of daily energy use, typically 60 to 70 percent. For estimating it from simple measurements, the Mifflin-St Jeor equation is the most reliable choice for most people. If you know your body-fat percentage, the Katch-McArdle formula can do slightly better because it works from lean mass.
Calories burned at complete rest.
Mifflin-St Jeor equation.
Use the calculator.
BMR is the floor, not the whole picture
Think of BMR as the idle cost of running your body. Even asleep, you burn a steady stream of calories, and that stream is set mostly by your size and how much lean tissue you carry. It is not the same as your full daily burn: multiply BMR by an activity factor and you get Total Daily Energy Expenditure, which adds digestion, daily movement and exercise. If you want that fuller breakdown, see what TDEE is and the four things that make it up.
The three formulas, and when each wins
| Formula | Uses | Best for |
|---|---|---|
| Mifflin-St Jeor | Weight, height, age, sex | The reliable default for most people |
| Harris-Benedict | Weight, height, age, sex | Older formula, tends to overestimate slightly |
| Katch-McArdle | Lean body mass | People who know their body-fat percentage |
Mifflin-St Jeor replaced Harris-Benedict as the everyday standard because studies found it predicted resting energy more accurately across modern populations. Katch-McArdle sidesteps the sex and age adjustments by going straight to lean mass, which is why it shines for lean, muscular people whom the general formulas can under-serve. For everyone else, the extra measurement rarely changes the answer enough to matter.
Why muscle and size raise BMR
A larger body has more tissue to maintain, so it burns more at rest. Muscle is metabolically active, meaning it costs energy to keep, which is one reason strength training slowly nudges BMR upward and why crash dieting that sheds muscle can drag it down. This is also why two people of the same weight can have different BMRs: the one carrying more muscle and less fat usually burns more at rest. Supporting that muscle with enough protein from foods like Greek yogurt, salmon and a balanced plate with a starchy side helps protect it during weight loss.
How to use your BMR number
Do not eat at your BMR to lose weight. BMR is resting burn only, so eating at that level while also moving and exercising creates an unnecessarily steep deficit. Instead, use BMR as the input to your TDEE, set a moderate deficit or surplus from there, and adjust with real data. The calorie calculator handles the arithmetic, but treat any formula output as a starting estimate rather than a precise measurement, because individual metabolism varies by roughly 5 to 10 percent around the prediction.
Bottom line
BMR is the calories your body spends doing nothing, the biggest and most stable part of your daily burn. Mifflin-St Jeor is the sensible default, Katch-McArdle wins if you know your lean mass, and either way the number is a launchpad for your calorie target, not a food limit in itself.
Sources
Common questions
What is BMR?
Basal metabolic rate is the number of calories your body uses at complete rest to keep organs running. It is the largest part of daily energy expenditure, usually 60 to 70 percent.
Which BMR formula is most accurate?
For most people the Mifflin-St Jeor equation is the most reliable using weight, height, age and sex. If you know your body-fat percentage, Katch-McArdle can be more accurate because it uses lean mass.
Is BMR the same as TDEE?
No. BMR is resting burn only. TDEE is your full daily burn, which multiplies BMR by an activity factor to add digestion, daily movement and exercise.