What is BMR (Basal Metabolic Rate)?
Basal metabolic rate (BMR) estimates how many calories your body burns at complete rest to keep vital functions like breathing, circulation, and cell production running. A BMR calculator is the starting point for estimating your daily calorie needs because BMR feeds into Total Daily Energy Expenditure (Total Daily Energy Expenditure (TDEE)) along with activity and the Thermic Effect of Food (Thermic Effect of Food (TEF)).
BMR is not a calorie intake target — it’s the baseline number used to estimate TDEE and maintenance calories.
What affects BMR?
- Body size and lean mass are the biggest drivers of basal metabolic rate.
- Lean mass (muscle and organs) drives most of BMR differences; fat mass contributes less per kg.
- Age, sex, and hormones shift energy needs over time.
- Sleep, illness, and recent dieting can temporarily lower basal metabolic rate.
BMR vs RMR: what’s the difference?
Resting metabolic rate (RMR) is measured under less strict conditions than BMR and typically lands slightly higher. In practice, the terms are used interchangeably in nutrition apps, but this page focuses on basal metabolic rate because it is the core input for most BMR calculator formulas and daily calorie needs estimates.
BMR formulas
The calculator automatically chooses the best-fit BMR formula: it defaults to Mifflin–St Jeor and upgrades to Katch–McArdle whenever you provide a valid body-fat percentage (3–60%).
Mifflin–St Jeor
Default — balances height, weight, age, and sex for most people.
BMR = 10w + 6.25h − 5a + s (s = +5 male, −161 female)
- Modern clinical default
- Sensitive to weight changes, no body-fat requirement
Katch–McArdle
Best when you know body-fat % and want lean-mass driven predictions.
BMR = 370 + 21.6 × LeanMass(kg)
- Lean mass (LBM) = weight × (1 − body fat %)
- Requires body-fat between 3–60%
- Responsive to recomposition plans
How the calculator chooses a formula
- Mifflin–St Jeor — runs automatically when the Body fat % field is blank. Common default for general use, macro coaching, and comparisons with modern nutrition apps.
- Katch–McArdle — activates when you supply a 3–60% body-fat value. Often preferred for recomposition, athletic cuts, or any plan that leans on lean-mass calculations.
- The calculator only uses the stats you provide, so accuracy depends on consistent measurements for weight, height, and body fat.
- No setting is required - the calculator makes the decision based on your profile inputs.
- Clearing the Body fat % input reverts to Mifflin–St Jeor without affecting other data.
Interpreting your BMR results
- Your basal metabolic rate is one piece of the daily calorie needs puzzle, not a target intake by itself.
- Use the full TDEE estimate to set maintenance calories, then adjust for weight loss or weight gain goals.
- Recheck your BMR calculator results after big changes in weight, body fat, or training volume.
These are estimates. The most accurate signal is how your real-world scale trends respond over 2–4 weeks.
Need a refresher? Return to the calculator, enter or clear Body fat %, and check which formula was used in your BMR results summary.