BMR Calculator
Find your Basal Metabolic Rate, the calories your body burns at complete rest. Uses Mifflin-St Jeor by default, switches to Katch-McArdle if you enter body fat %. No signup, no email.
What this number means
Want the full picture including activity? Use the TDEE Calculator to add your activity level and get goal-based calorie targets.
How this BMR calculator works
Basal Metabolic Rate is the energy your body uses at complete rest, in a thermally neutral environment, in a fasted state. It is the bare minimum your heart, brain, kidneys, liver, and other organs need to keep functioning. Most people burn somewhere between 1,200 and 2,000 calories per day at rest, depending on size, sex, age, and lean mass.
A common surprise: the brain alone accounts for about 20% of BMR even though it is only 2% of body weight. The liver and kidneys are similarly metabolically expensive. That is why losing weight does not collapse BMR proportionally, you can drop 20 pounds and only lose 100-150 calories of resting burn, because the organs doing the heavy lifting are still the same size.
Knowing your BMR is useful in three ways. First, it sets a hard floor on how aggressively you can cut, eating well below BMR for weeks at a time leads to fatigue, hormonal changes, and metabolic adaptation. Second, it explains why two people with identical scale weight can have very different calorie needs (the one with more muscle has a higher BMR). Third, it lets you reverse-engineer maintenance calories with a single multiplication.
Step 1, Mifflin-St Jeor (the default)
We use the Mifflin-St Jeor equation, published in 1990 and validated in dozens of independent studies. It replaced the older Harris-Benedict equation in most clinical settings because it tends to be 5% closer to measured resting expenditure on average. The formulas:
- Men: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5
- Women: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161
The math handles unit conversion automatically. If you enter weight in pounds and height in feet and inches, we convert to metric internally before plugging into the formula.
A worked example. A 35-year-old man, 180 cm tall, 80 kg: BMR = 10×80 + 6.25×180 − 5×35 + 5 = 800 + 1,125 − 175 + 5 = 1,755 calories. Same person but female: 800 + 1,125 − 175 − 161 = 1,589 calories. The 166-calorie difference between male and female at identical age, height, and weight is the constant baked into the equation, accounting for average differences in lean mass and organ size.
Step 2, Katch-McArdle (if you provide body fat %)
If you enter your body fat percentage, the calculator switches to Katch-McArdle. This formula uses lean body mass instead of total weight, so it is more accurate for very lean people, athletes, and bodybuilders, where Mifflin-St Jeor tends to underestimate.
- Lean mass (kg) = total weight × (1 − body fat % ÷ 100)
- BMR = 370 + 21.6 × lean mass (kg)
For someone with a normal body fat percentage and no athletic background, Mifflin-St Jeor and Katch-McArdle usually agree to within 50-100 calories per day. The gap widens as body fat moves toward either extreme.
Why BMR matters
BMR is the foundation of every calorie target. To find your maintenance calories you multiply BMR by an activity factor. To lose weight you eat below maintenance. To build muscle you eat above. Knowing your BMR also reveals how much room you have to cut, eating well below BMR for long stretches is unsustainable and slows metabolism.
There is also a practical safety floor here. Most registered dietitians draw a line at about 1,200 calories per day for women and 1,500 for men, regardless of what BMR math suggests. Going below those floors for more than a week or two consistently produces nutrient deficiencies, persistent low energy, and rebound bingeing once the diet ends. A larger person with a high BMR can run a deeper dollar deficit; a smaller person with a low BMR has to be patient and accept slower loss.
Accuracy note
Predictive equations are estimates. Mifflin-St Jeor is typically within 10% of measured resting expenditure for most adults, but individual variation is real, two people with identical stats can have BMRs that differ by 200 calories. Treat the number as a starting point, not a hard truth. The proof is in the scale, weigh yourself for two to three weeks while eating consistent calories and adjust if your weight is moving in an unexpected direction.
The gold-standard measurement of BMR is indirect calorimetry, where you breathe into a hood for 15-30 minutes after an overnight fast and the device measures your oxygen consumption. Outside of research labs and a handful of metabolic clinics, almost nobody gets this test. The math you get from a calculator like this one is genuinely close enough for nearly every practical purpose, dieting, training, recomp. The remaining variation is something you account for by tracking and adjusting, not by chasing a more precise formula.
Sources: Mifflin MD, et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990 Feb;51(2):241-7. · Katch FI, McArdle WD. Nutrition, Weight Control, and Exercise. 1977.
Frequently asked questions
What is BMR?
BMR (Basal Metabolic Rate) is the calories your body burns at complete rest, just to keep your organs running, your blood circulating, and your body temperature stable. It does not include any movement, digestion, or exercise. Think of it as the cost of being alive.
What is the difference between BMR and TDEE?
BMR is what you burn at rest. TDEE (Total Daily Energy Expenditure) is BMR plus the calories burned through walking around, exercising, and digesting food. TDEE is what you actually eat to maintain weight; BMR is roughly 60-70% of that. We have a deeper breakdown in TDEE vs BMR.
Which BMR formula is most accurate?
Mifflin-St Jeor is the most accurate equation for the general population. It replaced Harris-Benedict in clinical practice because it tends to be 5% closer to measured resting expenditure. If you know your body fat percentage and you are athletic or lean, Katch-McArdle is more accurate because it uses lean body mass.
Should I eat at my BMR to lose weight?
No. Eating at your BMR means eating as if you stayed in bed all day. For most active people that is a 30-40% deficit, hard to sustain, hunger-inducing, and likely to slow metabolism. A safer cut is 15-25% below your TDEE, not your BMR. Build the deficit on top of TDEE, not below BMR.
Does BMR change with age?
Yes. BMR drops gradually with age, roughly 1-2% per decade after age 20, mostly because lean muscle mass declines. The good news, resistance training and adequate protein intake slow the decline significantly. A 50-year-old who lifts weights regularly often has a BMR similar to a sedentary 30-year-old.
Why is my BMR different on other calculators?
Different sites use different equations. Harris-Benedict (1919, revised 1984) tends to read about 5% higher than Mifflin-St Jeor. Cunningham uses lean mass and tends to read higher than Katch-McArdle on lean people. Differences of 100-200 calories between calculators are normal. Pick one and stick with it for consistency, the trend over weeks matters more than the absolute number.
Can I increase my BMR?
A little, mostly by adding muscle. Each pound of muscle burns about 6-7 calories at rest per day, so adding 10 pounds of muscle adds roughly 60-70 calories of BMR. Stimulants like caffeine bump BMR briefly but the effect is small (1-3%) and tolerance builds. Most "metabolism-boosting" supplements have effects too small to measure on a body weight scale.
Does this calculator store my data?
No. All calculations happen in your browser. Nothing is sent to a server, nothing is stored, no signup is required.
Keep going
BMR is the starting line. Add your activity level for the full daily picture, or learn how the pieces fit together.