What it means
The formula is 370 plus 21.6 times lean body mass in kilograms. Because it skips age and sex, it relies entirely on how much lean mass you carry.
It can be more accurate than weight-based formulas for very lean or very muscular people, but only if your body fat estimate is reasonably good. A body fat reading that is 5 points off will throw the result off too.
Sunny Weigh switches to Katch-McArdle automatically when you enter a body fat percentage, and shows both results so you can compare.
If you do not know your body fat, Sunny Weigh's body fat calculator can give a tape-measure estimate you can plug in. Both formulas are estimates, so let your weight trend over a few weeks guide final adjustments.
Why it matters
It gives a more personal BMR for people who know their body fat percentage.