Most BMR formulas only know your total weight. The Katch-McArdle formula knows something better: how much of that weight is lean tissue. By building its estimate around lean body mass instead of the number on the scale, it can outperform the standard equations for lean and muscular people, the exact group where total-weight formulas tend to slip. The catch is that it needs an honest body-fat number to work.
The Katch-McArdle formula estimates your Basal Metabolic Rate (BMR) from your lean body mass (LBM), everything in your body that isn't fat: muscle, bone, organs, and water. The logic is clean. Fat tissue is metabolically quiet, burning only a couple of calories per pound per day. Lean tissue is where almost all of your resting energy expenditure happens. So if you base the estimate on lean mass alone, you sidestep the biggest source of error in total-weight formulas.
That's the trade-off. Katch-McArdle ignores age and sex as separate inputs, because their effects are already baked into your body composition. What it can't do without is a body-fat percentage, and that number is exactly what most people don't have lying around.
Katch-McArdle is a two-step calculation. First find lean body mass, then plug it into the BMR equation. Weight is in kilograms and body fat is expressed as a decimal:
The two constants do all the work. The 370 is a baseline floor for essential organ function, and the 21.6 is how many calories each kilogram of lean mass burns per day at rest. Notice what's missing: no age term, no separate male/female version. One equation covers everyone, because your lean mass already encodes those differences.
| Step | What you calculate | You need |
|---|---|---|
| 1. Lean body mass | weight × (1 − body fat %) | Weight (kg), body fat % |
| 2. BMR | 370 + (21.6 × LBM) | LBM from step 1 |
Take a 28-year-old man who weighs 175 pounds at 14% body fat. Convert weight to kilograms first: 175 ÷ 2.205 = 79.4 kg.
Step one, lean body mass:
Step two, BMR:
To turn that into a daily target, multiply by an activity factor. Training four or five days a week puts him around 1.55: 1,845 × 1.55 = 2,860 calories to maintain. Subtract 250–500 to cut, add 250–500 to build.
| Value | Result |
|---|---|
| Weight | 79.4 kg (175 lb) |
| Body fat | 14% |
| Lean body mass | 68.3 kg |
| BMR | 1,845 cal/day |
| TDEE (× 1.55) | 2,860 cal/day |
The calculator supports Katch-McArdle when you enter a body-fat percentage, no manual math needed.
Calculate My BMR →Standard formulas like Mifflin-St Jeor see only total weight, so they can't tell muscle from fat. Two people at the same height, weight, age, and sex get the same estimate even if one is a shredded athlete and the other isn't, despite the athlete carrying far more metabolically active tissue. For the lean and muscular, that means the standard formula tends to underestimate true BMR.
Katch-McArdle solves this directly. By calculating from lean mass, it gives the athlete full credit for their muscle and produces a higher, more realistic number. The same advantage applies in reverse: a person carrying a lot of fat has less lean mass than their scale weight suggests, and Katch-McArdle won't over-credit that fat the way a total-weight formula can.
Because the formula leans entirely on body fat, the quality of your estimate is everything. Here's how the common methods stack up:
Whatever method you use, measure under consistent conditions, ideally first thing in the morning, fasted, and well hydrated, so changes you track later reflect your body and not the measurement noise.
Neither formula is universally "better." The right one depends on whether you have a trustworthy body-fat number and how your body is composed.
| Your situation | Better formula | Why |
|---|---|---|
| Lean or muscular, know your body fat | Katch-McArdle | Credits your lean mass; more accurate at the extremes |
| Average build, no body-fat data | Mifflin-St Jeor | Reliable from age, sex, height, weight alone |
| Only guessing at body fat | Mifflin-St Jeor | Avoids garbage-in error from a bad body-fat input |
| Tracking a recomposition with regular DEXA | Katch-McArdle | Updates cleanly as lean mass changes |
For a deeper look at the everyday default, see our guide to the Mifflin-St Jeor equation. And if you're still untangling the resting versus total burn distinction, TDEE vs BMR covers it cleanly.
Use it when you're lean or muscular and have a reliable body-fat measurement. Those are the conditions where it outperforms total-weight formulas, because it can account for the extra muscle they'd otherwise miss. For an average build with no body-fat data, Mifflin-St Jeor is the safer pick.
A DEXA scan is the most accurate practical option and a great one-time anchor. Skinfold calipers are reliable when used consistently. Smart scales and handheld impedance devices are convenient but noisy, so trust the trend over weeks rather than any single reading.
It can be, but only with a good body-fat number. For lean and athletic people, basing BMR on lean mass is genuinely more accurate. With a guessed or sloppy body-fat figure, that advantage disappears and Mifflin-St Jeor usually wins.
Then don't use Katch-McArdle. It depends entirely on that input, and a bad estimate produces a bad BMR. Default to Mifflin-St Jeor, which needs only age, sex, height, and weight, and revisit Katch-McArdle once you've measured your body fat properly.
Enter a body-fat percentage to run Katch-McArdle, or skip it for Mifflin-St Jeor. Either way, get your targets in seconds.
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