BMR Calculator Guide

A BMR calculator estimates your Basal Metabolic Rate — the calories your body burns at complete rest just to stay alive. It is the foundation of every calorie plan, but the raw number is easy to misread: three common formulas give three slightly different answers, and none of them is a calorie target by itself. This guide explains what our calculator computes, which formula to trust, and what to actually do with the result.

Open BMR Calculator

What the Calculator Computes

Our BMR calculator runs three validated equations side by side from the same inputs (sex, age, height, weight, and optional body fat percent):

Mifflin-St Jeor is shown as the primary result. The Harris-Benedict value comes from the revised Roza & Shizgal coefficients, not the 1919 original. Katch-McArdle only appears when you enter a body fat percent, because it works from lean body mass instead of total weight — if you don't know yours, the Navy-method body fat calculator gives a reasonable tape-measure estimate.

Which Formula Should You Trust?

For most people, Mifflin-St Jeor. In validation studies it predicts measured resting metabolic rate within 10% for more adults than the alternatives. Katch-McArdle becomes the better choice when your body composition is far from average — very lean or very muscular — and your body fat number is trustworthy. A guessed body fat percent makes Katch-McArdle less accurate than Mifflin, not more. Differences of 50–150 kcal between the three results are completely normal; if they roughly agree, you can be confident in the estimate.

A Worked Example

A 30-year-old man, 80 kg, 180 cm: Mifflin-St Jeor gives 10×80 + 6.25×180 − 5×30 + 5 = 1,780 kcal/day. Revised Harris-Benedict gives about 1,854 kcal. At 20% body fat his lean mass is 64 kg, so Katch-McArdle gives 370 + 21.6×64 ≈ 1,752 kcal. All three cluster within about 100 kcal — that cluster, not any single decimal, is his real BMR estimate.

What to Do With Your BMR

Nothing directly — and that is the most important point. BMR is not a calorie target; it is the input for one. Multiply it by an activity factor to get your TDEE, and set calorie goals from there. The full comparison lives in the BMR vs TDEE guide. BMR's one direct practical use is as a floor: if a diet plan has you eating well below your BMR for weeks, the plan is more aggressive than it needs to be.

Frequently Asked Questions

Which BMR formula should I use?
Mifflin-St Jeor is the best default for most people. If you have a reliable body fat measurement, Katch-McArdle is often more accurate for lean or athletic builds.

Why do the three formulas give different numbers?
Each was fitted to a different study population and uses different inputs. Differences of 50 to 150 kcal between formulas are normal and rarely change a practical decision.

Is my calculated BMR exact?
No formula is exact — individual metabolisms vary roughly 10% around any estimate. Treat the number as a starting point and calibrate with real-world weight data, as described in the maintenance calories guide.