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.
Formula
Copy the exact expression or work through it step by step below.
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
Try These Tools
Run the numbers next
TDEE Calculator
Estimate your daily energy expenditure with Mifflin-St Jeor + activity factors.
Body Fat Percentage Calculator
Estimate body fat percentage using the U.S. Navy circumference method.
Lean Body Mass Calculator
Estimate lean body mass using Boer, James, Hume, and Peters formulas from height and weight.
Sources & References
- 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