Katch–McArdleBMR calculator.

§ 01 · The formula, named

BMR = 370 + (21.6 × LBM kg), where LBM (lean body mass) = weight × (1 − bodyFat/100).

The 370 is a baseline metabolic floor. The 21.6 multiplier is the energy cost per kg of lean tissue — primarily muscle and organ mass. Fat tissue is much less metabolically active, so by zeroing it out the formula gets closer to your real resting energy expenditure than weight-only formulas.

§ 02 · When Katch–McArdle beats Mifflin–St Jeor

Mifflin–St Jeoruses total body weight. It works fine for most people, but it can't distinguish between two people of the same weight where one is lean and the other isn't.

Katch–McArdle works in lean mass directly. That matters most at the tails of body composition — for very lean athletes (Mifflin under-estimates) and for higher body fat (Mifflin over-estimates). If you train regularly, Katch–McArdle is usually closer to the truth.

§ 03 · Don't have a body-fat percentage?

Use the US Navy body fat calculator — three or four circumference measurements get you a usable estimate in two minutes. Plug the result back into this calculator.

Or, if you have a recent BMI, the Deurenberg formula (built into BiteDeck's app, alongside US Navy) gives a rough estimate without measurements.

§ 04 · How BiteDeck uses this

The forecast inside BiteDeck runs Katch–McArdle daily — using your saved body-fat % and the latest weight reading from HealthKit. The 5-state model (locked / unlocked / goal-reached / maintaining / reversed) sits on top of that BMR. See On-device weight forecasting for the rest of the wiring.

For comparison with the body-mass-aware approach: Mifflin–St Jeor TDEE calculator.

§ 05 · FAQ

• Is Katch–McArdle more accurate than Mifflin–St Jeor?

  When you have a body-fat number you trust, usually yes. Katch–McArdle works in lean body mass directly, so it tells two people of the same weight apart when one is lean and the other is not. Without a reliable body-fat figure, Mifflin–St Jeor is the safer default because it only needs weight, height, age, and sex.

• What body fat percentage should I use?

  A measured one — DEXA, calipers, or a smart scale. If you do not have a measurement, the US Navy body fat calculator gets you a usable estimate from a tape measure in two minutes. A guessed body-fat number makes the BMR no better than a weight-only formula.

• Does Katch–McArdle need my height?

  No. Unlike Mifflin–St Jeor, the Katch–McArdle formula uses only weight and body fat % — together they give lean body mass, which is all the equation needs. Height is already baked into your body composition.

• How do I turn BMR into TDEE?

  Multiply BMR by an activity factor: 1.2 (sedentary), 1.375 (lightly active), 1.55 (moderately active), 1.725 (very active), or 1.9 (extra active). The result is your total daily energy expenditure — the calories that hold your weight steady.

• What do the 370 and 21.6 in the formula mean?

  The 370 is a baseline metabolic floor in kcal. The 21.6 is the estimated energy cost, in kcal per day, of each kilogram of lean tissue. Because fat is far less metabolically active, the formula effectively ignores it and works from lean mass alone.