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RMR (Katch-McArdle) Calculator

Estimate resting metabolic rate from your lean body mass.

How it is calculated

First the lean body mass is found: LBM = weight × (1 − bodyfat / 100), with weight in kilograms and body fat as a percentage. Then RMR = 370 + 21.6 × LBM, in kcal per day.

The tool also multiplies the RMR by 1.2 for a sedentary day and by 1.55 for an active one, and shows both. RMR and LBM are rounded to whole kcal and two decimals of a kilogram. Weight must be above zero and body fat from 0 up to (but not including) 100, otherwise no result is shown. Age, height and sex play no part, because the formula assumes body composition already captures them.

Worked example

A person weighs 70 kg with 20% body fat. Lean mass = 70 × (1 − 0.20) = 56.00 kg. RMR = 370 + 21.6 × 56 = 370 + 1209.6 = 1,580 kcal/day. Multiplying the unrounded RMR by 1.2 gives 1,896 kcal/day for a sedentary level, and by 1.55 gives 2,448 kcal/day for an active level.

Try this example →

Common mistakes

  • Entering the body weight in pounds: the formula's constants are for kilograms, so a value in lb overstates the lean mass by about 2.2 times.
  • Typing body fat as a fraction (0.2 instead of 20): the tool reads a percentage, so 0.2 means 0.2% and the RMR comes out almost as if you had no fat at all.
  • Using an unreliable body-fat figure, for example from a bathroom scale: the result depends directly on it, since a 5-point error shifts LBM by 3.5 kg at 70 kg and RMR by roughly 75 kcal.
  • Treating the sedentary or active line as a precise target: those are fixed multipliers (1.2 and 1.55) applied to an estimate.

About this calculator

The Katch-McArdle formula, RMR = 370 + 21.6 × LBM (kg), estimates resting metabolic rate from lean body mass (total weight minus fat mass) rather than total body weight — unlike the Harris-Benedict or Mifflin-St Jeor equations, which use weight, height, age, and sex without accounting for body composition directly.

Because it's built on lean mass, it tends to be more accurate for people who are noticeably leaner or more muscular than average — athletes and bodybuilders in particular — and it's widely used by trainers and nutrition coaches as the base for daily calorie targets, then multiplied by an activity factor to estimate sedentary through very-active maintenance calories.

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