What this calculator does
Most resting metabolic rate formulas work from height, weight, age and sex, because those are what people know. Katch-McArdle works from lean body mass instead, on the reasoning that fat tissue burns very little energy at rest.
That makes it more accurate when body fat is actually measured, and it responds to composition rather than to weight. A 75 kg person at 10 per cent body fat is estimated at 1,828 kcal a day, and the same weight at 30 per cent fat at 1,504.
The formula
Lean body mass is body weight less the fat portion. The estimate is 370 kcal plus 21.6 kcal per kilogram of that lean mass.
| Term | Meaning |
|---|---|
| RMR | Resting metabolic rate: energy burned at complete rest. |
| Lean body mass | Body weight excluding fat, which is what drives the estimate. |
| Katch-McArdle | The formula used here, which requires a measured body fat percentage. |
The inputs explained
| Field | What to enter |
|---|---|
| Weight (kg) | Body weight in kilograms. |
| Body fat (%) | Body fat as a percentage. This needs to be measured rather than guessed for the formula to be worth using. |
When to use it
After a DEXA or body composition scan
Once body fat is measured, this formula uses it rather than discarding it.
For a muscular or very lean person
Weight-based formulas underestimate people carrying unusual amounts of muscle.
Setting a calorie baseline
RMR is the starting point that activity multipliers are applied to.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much does body composition change the estimate?
The same weight at three body fat percentages.
| Body fat | RMR | Lean body mass used |
|---|---|---|
| 10% | 1,828 kcal/day | 67.5 kg |
| 20% | 1,666 kcal/day | 60.0 kg |
| 30% | 1,504 kcal/day | 52.5 kg |
Questions
Is this better than Mifflin-St Jeor?
Where body fat is genuinely measured, usually yes, particularly at the extremes of body composition. Where body fat is guessed, the guess introduces more error than the formula removes, and a weight-based equation is the safer choice.
Why does fat tissue count for so little?
Because it is metabolically far less active than muscle, organ and brain tissue. It is not literally zero, which is why the formula still has a 370 kcal constant, but per kilogram it burns a small fraction of what lean tissue does.
Is RMR the same as BMR?
Close but not identical. Basal metabolic rate is measured under stricter conditions, typically after an overnight fast and full rest. Resting metabolic rate is slightly higher and easier to measure, and the two are often used interchangeably.
How do I get to a daily calorie figure?
Multiply by an activity factor, which is what a TDEE calculation does. RMR alone describes energy burned at rest and will be well below what an active person actually needs.
For the weight-based equivalent, see the BMR calculator. For total daily needs, see the TDEE calculator.