How this instrument works
The calculation runs in two steps. First, lean body mass: multiply weight by one minus body fat as a fraction, which strips out the estimated fat and leaves the metabolically active remainder — muscle, organs, bone, water. Second, BMR: multiply that lean mass in kilograms by 21.6 and add 370. The 21.6 figure approximates the energy cost, in kilocalories per day, of maintaining each kilogram of lean tissue at rest; the 370 constant is a baseline capturing the smaller, steadier energy cost of fat mass and other tissue the multiplier doesn't directly count.
Mifflin-St Jeor and Harris-Benedict, this site's other resting-metabolism calculators, infer a rate from height, weight, and age — numbers a scale and a tape measure supply directly, standing in for lean mass through a population-average relationship with total weight. Katch-McArdle instead requires knowing, or reasonably estimating, actual body fat percentage — from a bioimpedance scale, skinfold calipers, or this site's own skinfold and Navy-method body-fat calculators — and rewards low body fat directly. The practical effect: for lean, muscular people, Katch-McArdle tends to run noticeably higher than the height-weight-age formulas, since it credits muscle those formulas can only guess at; for people carrying more body fat, it can run lower, since it isn't leaning on an average that assumes a typical fat-to-muscle ratio for that weight.
Unlike Mifflin-St Jeor or the revised Harris-Benedict, which trace to specific peer-reviewed papers, this exact constant pair — 370 and 21.6 — comes from the exercise-science textbook Exercise Physiology: Nutrition, Energy, and Human Performance by McArdle, Katch, and Katch, rather than one standalone study built solely around it. It's also worth knowing this formula is essentially the same equation John Cunningham proposed independently in a 1991 Am J Clin Nutr review — REE = 370 + 21.6 × fat-free mass — making the two names practically interchangeable, both resting on the same premise that lean mass, not total weight, drives resting energy cost.
- Enter Weight in kilograms.
- Enter Body fat as a percentage — from a bioimpedance scale, skinfold measurement, or this site's Navy or skinfold body-fat calculators.
- Read Lean body mass in kilograms — the working figure the BMR step is built on.
- Read BMR in kcal/day; multiply by an activity factor afterward to estimate total daily calorie needs.
Worked example — three bodies at three body-fat levels
An 80 kg person at 20% body fat: LBM = 80 × (1 − 0.20) = 80 × 0.80 = 64 kg. BMR = 370 + 21.6 × 64 = 370 + 1382.4 = 1752.4 kcal/day.
A 60 kg person at 30% body fat: LBM = 60 × 0.70 = 42 kg. BMR = 370 + 21.6 × 42 = 370 + 907.2 = 1277.2 kcal/day — a smaller frame and a higher fat percentage both pulling the figure down.
A 100 kg person at just 15% body fat: LBM = 100 × 0.85 = 85 kg. BMR = 370 + 21.6 × 85 = 370 + 1836 = 2206.0 kcal/day — the leanest of the three by percentage, and correspondingly the highest resting burn, despite not being much heavier than the first example.
Questions
Why does Katch-McArdle give a different answer from Mifflin-St Jeor for the same person?
Because the two formulas start from different information. Mifflin-St Jeor infers lean mass indirectly, through a population-average relationship between height, weight, age, and typical body composition. Katch-McArdle uses an actual measured or estimated body fat percentage, so it directly credits unusually high or low muscle mass the average-based formulas simply can't see. A lean, muscular person will typically get a higher Katch-McArdle figure than a Mifflin-St Jeor one; someone carrying more body fat than average for their weight will often see a lower one.
Where does the 370 + 21.6 constant pair actually come from?
From the exercise-science textbook Exercise Physiology: Nutrition, Energy, and Human Performance, by McArdle, Katch, and Katch, rather than a single dedicated peer-reviewed paper the way Mifflin-St Jeor or the revised Harris-Benedict trace to one. The nearly identical formula — 370 + 21.6 × fat-free mass — also appears independently in Cunningham's 1991 review in the American Journal of Clinical Nutrition, so the same relationship has been reached from more than one direction.
How accurate does my body fat percentage need to be for this to be useful?
Reasonably accurate, since the entire result is built from it — a body-fat estimate off by five percentage points shifts lean mass, and therefore BMR, by a proportional amount. Bioimpedance scales, skinfold calipers, and the Navy tape method each carry their own margin of error, typically a few percentage points; using the same measurement method consistently over time matters more than chasing perfect precision on any single reading.
Why does this formula reward low body fat so directly?
Because lean tissue — muscle, organs, and the water they hold — is metabolically active in a way fat largely isn't; it costs real energy to maintain at rest, while fat mass sits closer to inert storage. Stripping out the fat percentage and multiplying only the remaining lean mass by a fixed energy cost per kilogram is exactly what makes this formula responsive to muscle in a way height-weight-age formulas structurally cannot be.
Is this the same as the Cunningham equation?
Functionally, yes. John Cunningham published REE = 370 + 21.6 × fat-free mass in a 1991 Am J Clin Nutr review — identical constants, identical logic, arrived at independently of the McArdle-Katch-Katch textbook figure. The two names are commonly used interchangeably in practice, and this calculator's output would match a 'Cunningham equation' tool fed the same lean-mass input.
Should I use this instead of Mifflin-St Jeor?
Only if you have a reasonably trustworthy body fat percentage to feed it — otherwise Mifflin-St Jeor, which needs nothing beyond a scale and a tape measure, is the more practical everyday choice. Katch-McArdle earns its keep specifically for people who already track body composition and suspect a height-weight-age average doesn't reflect their actual muscle mass, in either direction.
Does this number include the calories burned during exercise?
No — like every BMR figure on this site, it's the resting floor, not a daily total. Multiply by an activity factor between roughly 1.2 for sedentary and 1.9 for very active to estimate total daily energy expenditure before using the figure to plan an intake target.
References
- McArdle WD, Katch FI, Katch VL. Exercise Physiology: Nutrition, Energy, and Human Performance, 8th ed. Wolters Kluwer/Lippincott Williams & Wilkins, 2015
- Cunningham JJ. 1991, Am J Clin Nutr — related lean-mass equation (PubMed)
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.