What your BMR tells you
Your Basal Metabolic Rate (BMR) is the energy your body uses just to stay alive — breathing, circulating blood, regulating temperature, repairing cells — if you did absolutely nothing all day. For most people it's the biggest single chunk of daily calorie burn, around 60–70% of the total. It's the foundation every other number is built on: multiply BMR by an activity factor and you get your TDEE; adjust that for a goal and you get your daily calorie target.
The three formulas, compared
This calculator shows all three side by side so you can see how they differ:
- Mifflin-St Jeor (1990): the modern standard and the most accurate for the general population. This is the one to use unless you have a reason not to.
- Harris-Benedict (revised 1984): the classic equation. It tends to read slightly higher than Mifflin, especially for people carrying more body fat.
- Katch-McArdle: uses your lean body mass instead of total weight, so it needs your body-fat percentage. If you know your body fat and you're lean or muscular, it's often the most accurate of the three — enter your body fat % to unlock it.
Differences of 50–150 calories between formulas are normal and nothing to worry about. Pick one, stay consistent, and let real-world results fine-tune it.
BMR, RMR and what to do next
You'll sometimes see RMR (Resting Metabolic Rate) used almost interchangeably with BMR. RMR is measured under slightly less strict conditions and usually comes out marginally higher, but for planning purposes they're close enough to treat the same. The important thing is what you do with the number: BMR alone isn't your calorie target — you move every day, so you need more than your BMR. Take it to the TDEE calculator to fold in activity, then the macro calculator to turn it into a daily plan.