What this BMR calculator does
The BMR calculator estimates how many calories your body burns at complete rest, using two of the best-known equations in nutrition science. Mifflin-St Jeor (developed in 1990) is the modern default — it was designed to be more accurate for contemporary populations than its predecessor. Harris-Benedict (1919, revised in 1984) is the historical standard still appearing in many textbooks and older tools. Showing both side by side lets you see the small difference for your stats and understand which figure other calculators or labels are based on.
The two formulas, written out
Mifflin-St Jeor:
- Men: BMR = 10 × kg + 6.25 × cm − 5 × age + 5
- Women: BMR = 10 × kg + 6.25 × cm − 5 × age − 161
Harris-Benedict (revised 1984):
- Men: BMR = 88.362 + 13.397 × kg + 4.799 × cm − 5.677 × age
- Women: BMR = 447.593 + 9.247 × kg + 3.098 × cm − 4.330 × age
The two formulas usually agree within 3-5%. Harris-Benedict tends to give slightly higher numbers for typical inputs, which is one reason research in the 1990s pushed the field toward Mifflin-St Jeor.
What BMR is — and isn't
BMR is the calorie cost of being alive — keeping your heart beating, your lungs breathing, your kidneys filtering, your body temperature steady. It accounts for 60-75% of total daily calorie burn for most people. The remainder comes from the thermic effect of food (the calories you spend digesting), non-exercise activity thermogenesis (fidgeting, standing, walking around), and structured exercise. To go from BMR to total daily energy expenditure, multiply by an activity factor (1.2 sedentary to 1.9 athlete) — that's what the TDEE calculator does.
Why BMR matters for weight goals
BMR is the foundation of every calorie target. Eating consistently below maintenance is what creates fat loss; eating above is what supports muscle gain. Without a reasonable BMR estimate you're guessing. Pair this calculator with the calorie calculator for goal-adjusted targets, or the macro calculator for protein, carb and fat splits.
Accuracy and caveats
Mifflin-St Jeor is accurate within about 10% for most adults. The biggest sources of error are extremes of muscle mass (very lean athletes have a higher true BMR than the formula predicts; very high body fat can mean a lower one), pregnancy, breastfeeding, and metabolic disorders like hypo- or hyperthyroidism. If accuracy really matters for medical or athletic reasons, indirect calorimetry — measured at a clinic or sports science lab — is the gold standard.