Formula, in Fat Loss

Katch-McArdle Formula

Katch-McArdle estimates BMR from lean body mass alone, skipping the height/age/sex inputs that Mifflin-St Jeor uses as proxies. For people with accurate body-fat measurements (DXA, BIA, or hydrostatic weighing), it's more accurate than mass-based equations — and especially better for muscular athletes whose Mifflin-St Jeor estimate underrates them.

4 variables. Published May 14, 2026. By AI Fit Hub.

Education, not medical advice. The result comes only from the published formula and your inputs. How we check our formulas.

Formula

BMR = 370 + 21.6 · LBM_kg
   LBM_kg = mass_kg × (1 − body_fat_pct / 100)

Variables

BMR

Basal Metabolic Rate

Calories burned per 24 h at complete rest. Drives 60–75% of total daily expenditure for sedentary adults; lower fraction for highly active people.

LBM_kg

Lean Body Mass

Kilograms of fat-free mass (muscle, organs, bone, water). Requires a body-composition measurement — caliper skinfolds run ±3-5% absolute error, BIA scales ±5-8%, DXA ±1-2%.

mass_kg

Body mass

Total body weight in kg. Used with body-fat % to derive lean mass.

body_fat_pct

Body fat percentage

Total mass percentage that is adipose tissue. Healthy ranges: 8–24% men, 21–35% women (ACSM 2018). The accuracy of this number is the accuracy of the whole equation.

Step by step

1

Measure or estimate body fat percentage. DXA scan > hydrostatic weighing > skinfolds > BIA > visual estimate.

DXA result: 18.5% body fat.
2

Compute lean body mass: mass × (1 − body_fat/100).

82 kg × (1 − 0.185) = 82 × 0.815 = 66.83 kg LBM.
3

Multiply LBM by 21.6 and add 370.

370 + 21.6 × 66.83 = 370 + 1,443.5 = 1,813.5 BMR.
4

Apply activity factor (same as Mifflin-St Jeor): 1.2 sedentary to 1.9 athlete.

Athlete training twice daily: 1,813.5 × 1.9 = 3,445.7 TDEE.

Worked example

30-year-old male athlete, 82 kg, 18.5% body fat (DXA), training twice daily

Body mass

82 kg

Body fat %

18.5%

Lean body mass

66.83 kg

BMR = 370 + 21.6 × 66.83 = 1,813.5 cal/day TDEE = 1,813.5 × 1.9 = 3,445.7 cal/day

Katch-McArdle estimates ~1,813 BMR vs Mifflin-St Jeor at the same weight/age/height (~1,775). The 38-cal/day gap is the muscular-athlete correction — Mifflin underrates lean people.

Common variations

Cunningham (1980): BMR = 500 + 22·LBM_kg. Derived from elite-athlete data — runs ~10% higher than Katch-McArdle at the same LBM. Use only for elite endurance athletes.

Mifflin-St Jeor: uses total mass + height + age + sex. More practical when body composition unknown but underrates lean populations and overrates obese ones.

Owen equation (1986): BMR = 879 + 10.2·mass_kg for women, 879 + 9.99·mass_kg for men. Older, less accurate than Mifflin-St Jeor — included only for historical reference.

Run the numbers next

Nutrition TDEE Calculator
Estimate your daily energy expenditure with Mifflin-St Jeor + activity factors.
Body Composition Body Fat Percentage Calculator
Estimate body fat percentage using the U.S. Navy circumference method.
Body Composition Lean Body Mass Calculator
Estimate lean body mass using Boer, James, and Hume formulas from height and weight.

Sources

Nutrition, Weight Control, and Exercise (Katch & McArdle, 1977) , Lea & Febiger — original publication of the equation
Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults , Journal of the American Dietetic Association (2005) — covers Katch-McArdle vs Mifflin-St Jeor accuracy
ACSM's Guidelines for Exercise Testing and Prescription (10th ed.) , American College of Sports Medicine — body-fat percentage reference ranges