What BMR actually measures
Basal metabolic rate is the energy your body uses at complete rest to run its essential functions — heartbeat, breathing, brain activity, temperature control and cell renewal. It is measured in calories per day and typically accounts for 60–70% of everything you burn, which is why it is the foundation of any calorie plan. BMR is not the same as the calories you can eat to maintain weight; that is your total daily energy expenditure (TDEE), which multiplies BMR by an activity factor. Knowing your BMR tells you the floor — the calories your body needs before you move at all.
The Mifflin-St Jeor equation
This calculator uses the Mifflin-St Jeor equation, published in 1990 and shown in later reviews to predict resting energy expenditure more accurately than the older Harris-Benedict formula for most people. For men it is 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5; for women the final constant is −161 instead of +5. It needs only four inputs you already know, which is why clinicians and dietitians use it as the default. It assumes an average body composition, so very muscular or very lean people may burn slightly more or less than it predicts.
Turning BMR into a calorie target
To find the calories that maintain your weight, multiply BMR by an activity factor: 1.2 if you are sedentary, up to 1.9 for a physically demanding job plus daily training. That gives your TDEE. To lose weight, eat below TDEE — a deficit of about 500 calories a day targets roughly half a kilogram (one pound) of fat loss per week. To gain, eat above it. Use the TDEE calculator for the activity-adjusted number, and revisit the figure as your weight changes, because BMR falls as you get lighter.
Why the estimate is only a starting point
BMR formulas predict an average; your real number depends on muscle mass, genetics, hormones and past dieting. Treat the result as a well-founded estimate, track your weight over two to three weeks, and adjust calories based on what actually happens on the scale rather than the prediction alone. These tools use standard, published formulas and the figures you enter. They are estimates for general fitness and planning, not a medical assessment — check with a doctor or dietitian before making health decisions.
Frequently asked questions
What is a normal BMR?
It varies with size, age and sex, but many adults fall between roughly 1,400 and 1,900 calories a day. Larger, younger and more muscular bodies have a higher BMR; it declines with age and weight loss.
Is BMR the same as calories I should eat?
No. BMR is what you burn at rest. To maintain weight you eat your TDEE, which is BMR multiplied by an activity factor (1.2–1.9). Eating only your BMR would usually create a deficit.
Which BMR formula is most accurate?
The Mifflin-St Jeor equation, used here, is the most accurate for the general population in published comparisons, edging out the older Harris-Benedict equation.
Does BMR change over time?
Yes. BMR falls as you age and as you lose weight, and rises if you add muscle. Recalculate every few kilograms of change to keep your calorie target realistic.