What your BMR pays for
Lie perfectly still for 24 hours and your body still spends a remarkable amount of energy. The liver and brain each claim roughly a fifth of it. The heart beats around 100,000 times. Kidneys filter about 180 litres of blood, lungs cycle 11,000 litres of air, and every cell runs the ion pumps that keep it alive. That baseline spend is your basal metabolic rate, and for most people it accounts for 60–70% of everything they burn in a day.
The practical point: the majority of your calorie burn is rent, paid whether or not you exercise. A 1,600 kcal BMR means your workouts sit on top of a large fixed budget, not the other way round. This is why two people who train identically can have very different calorie needs.
The three formulas, side by side
Mifflin-St Jeor (the default)
Published in 1990 and recommended by the Academy of Nutrition and Dietetics for the general population:
- Men: 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5
- Women: 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161
Harris-Benedict (the veteran)
The 1918 original, revised by Roza and Shizgal in 1984. It reads about 5% higher than Mifflin-St Jeor for modern, less active populations, which is exactly why it was replaced as the clinical default. We show it because plenty of apps and older references still use it, and the comparison explains many why-is-my-number-different moments.
Katch-McArdle (for known body fat)
BMR = 370 + 21.6 × lean mass (kg). By working from lean mass instead of total weight, it ignores fat mass almost entirely, a better model for very muscular or very lean people. Enter a body fat percentage above and the calculator adds this estimate; without one it cannot run.
BMR or RMR: does the difference matter?
A true BMR measurement is strict: overnight fast, eight hours of sleep, measured on waking in a temperature-controlled lab. Resting metabolic rate (RMR) loosens those rules and lands about 10% higher. Every formula on this page predicts something between the two, and the terms get swapped freely across apps and articles. Since formula error runs to ±10% anyway, the distinction is academic for everyday use. Pick one number, use it consistently, and let real-world results correct it.
From BMR to the calories you actually burn
Nobody spends the day at complete rest, so BMR is never your calorie target. Multiply it by an activity factor and you get total daily energy expenditure, the number food targets are built from. The table under your result previews this; the TDEE calculator does the full job with cut and bulk targets, and the maintenance calorie calculator focuses on the hold-steady number.
Why long stretches below BMR backfire
BMR is not a tripwire, and an occasional day under it is harmless. The problem is what a below-BMR target implies: a deficit large enough that your body starts economising. In prolonged aggressive diets, resting metabolism falls 10–15% beyond what the lost weight alone predicts, muscle goes down the drain alongside fat, and intakes under about 1,200 kcal make protein and micronutrient targets nearly impossible to hit. The result is a diet that gets harder every week while burning less.
The sustainable route runs the other way: take a 250–500 kcal deficit from your TDEE, keep protein high, and let the calorie deficit calculator map the pace. Slower on paper, faster in practice, because you actually finish.
