Katch-McArdle Calculator

The Katch-McArdle formula estimates basal metabolic rate from lean body mass. Basal metabolic rate, or BMR, is the energy the body is estimated to use each day at complete rest to sustain essential functions such as circulation, breathing, temperature regulation, and cellular activity. Unlike equations based on age, sex, height, and total weight, Katch-McArdle uses body composition directly, making body-fat accuracy especially important. The calculation first converts total body weight into lean body mass. Lean mass equals weight multiplied by one minus body-fat percentage expressed as a decimal. BMR then equals 370 plus 21.6 times lean mass in kilograms. An 80 kg person at 20 percent body fat has 64 kg of lean mass and an estimated BMR of 1,752 kcal per day. Because fat tissue and fat-free tissue have different metabolic demands, two people at the same total weight can receive different estimates. The calculator also multiplies BMR by an activity factor to provide a rough maintenance-calorie estimate. Sedentary uses 1.2, moderately active uses 1.55, and very active uses 1.725. These broad categories combine exercise and everyday movement, so they cannot perfectly represent a specific person. Occupation, step count, training volume, recovery, climate, growth, pregnancy, illness, medication, and adaptive changes during dieting can all affect actual energy expenditure. Body-fat measurements introduce another major source of uncertainty. Consumer scales, skinfold calipers, circumference methods, air displacement, and imaging do not produce identical results. Hydration, recent food, exercise, device settings, and technician skill can alter a reading. For trend tracking, use the same method under similar conditions rather than treating one estimate as exact. A small body-fat error changes lean mass and therefore the calculated BMR. Maintenance calories are best evaluated against observed weight trends over several weeks. Day-to-day scale changes largely reflect water, glycogen, food mass, and digestion rather than immediate fat change. Use the estimate as a starting point, monitor intake and outcomes consistently, and adjust gradually. Athletes, people managing medical conditions or eating disorders, pregnant or breastfeeding people, and anyone pursuing substantial weight change should seek individualized guidance from a qualified health professional. This tool is educational and does not diagnose metabolic health or prescribe nutrition.