Why does Katch-McArdle ignore age and sex?

The formula assumes that lean body mass alone determines resting metabolic rate. Age and sex correlate with BMR largely because they track changes in muscle mass and organ size; by measuring body composition directly, Katch-McArdle bypasses demographic proxies. Research shows lean mass explains most of the variance in BMR across individuals.

How accurate does my body-fat measurement need to be?

Each 1% error in body-fat percentage shifts lean mass by roughly 0.75 kg for a 75 kg person, altering BMR by about 16 kcal/day. A 5% measurement error moves the estimate by 80 kcal. DEXA, hydrostatic weighing, and multi-site skinfold protocols offer the tightest precision; single-frequency BIA scales can drift by 3–5% session to session.

Can I use this formula if I am obese or very lean?

The Katch-McArdle equation was derived from adults in moderate body-fat ranges. At very high body-fat percentages (above 40%), adipose tissue contributes more to resting metabolism than the constant 21.6 factor assumes, potentially underestimating BMR. Elite athletes below 8% body fat may have organ and metabolic adaptations not captured by the equation.

What is the difference between BMR and TDEE?

BMR is the energy required for basic physiological functions—heartbeat, respiration, cellular repair—measured at complete rest. Total daily energy expenditure (TDEE) adds physical activity, non-exercise movement, and the thermic effect of digesting food. TDEE is typically 1.2–2.5 times BMR, depending on lifestyle and training volume.

How often does BMR change?

BMR shifts whenever lean body mass changes. Gaining muscle through resistance training raises BMR proportionally; losing muscle during a calorie deficit or aging lowers it. Short-term factors like sleep deprivation, illness, or extreme dieting can also alter metabolic rate by 5–15%, though these effects are not captured by static body-composition equations.

Body Composition

BMR Calculator (Katch-McArdle)

Estimate basal metabolic rate using lean body mass instead of age and sex.

Last updated: 22 April 2026

What this tool does

This calculator estimates basal metabolic rate (BMR) using the Katch-McArdle equation, which relies on lean body mass rather than age or biological sex. It requires total body weight in kilograms and body fat percentage as inputs, then applies the formula BMR = 370 + (21.6 × lean body mass) to produce a daily resting energy expenditure in kilocalories. The method is most accurate when body fat percentage is measured reliably, as the equation derives lean mass by subtracting fat mass from total weight.

Inputs

Result

—

Formula Used

BMR = 370 + 21.6 \times LBM

w

Weight in kg

b

Body-fat percentage

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This calculator implements the Katch-McArdle formula to estimate basal metabolic rate (BMR) based on lean body mass. Unlike age- and sex-based equations, Katch-McArdle relies exclusively on body composition: total weight and body-fat percentage. The tool subtracts fat mass from total weight to determine lean body mass, then applies a fixed constant (21.6) to reflect the higher metabolic cost of maintaining muscle, organs, bone, and other fat-free tissue.

How BMR Calculator (Katch-McArdle) works

The calculator takes two inputs: body weight in kilograms and body-fat percentage. It multiplies weight by (1 − body-fat percentage / 100) to isolate lean body mass. That lean mass figure is then inserted into the Katch-McArdle equation, which adds a base constant of 370 kcal to 21.6 times lean body mass in kilograms. The result is an estimated daily calorie expenditure at complete rest—no physical activity, digestion, or thermic effect of food included.

The formula

BMR = 370 + (21.6 × lean body mass in kg)

Lean body mass is calculated as: LBM = weight (kg) × (1 − body-fat % / 100). For example, a 75 kg individual at 20% body fat has 60 kg of lean mass, yielding a BMR of 370 + (21.6 × 60) = 1,666 kcal/day.

Where this method is most accurate

Katch-McArdle tends to perform best when body-fat percentage is measured with precision—via DEXA, hydrostatic weighing, or multi-site skinfold calipers administered by trained technicians. Bioelectrical impedance scales and single-site calipers introduce enough measurement error that small changes in estimated body fat can shift BMR predictions by 100+ kcal. The formula was developed using metabolic-chamber data from adults with moderate body-fat levels; it may overestimate BMR in individuals carrying very high body fat or underestimate in elite athletes with single-digit body-fat percentages.

What this tool does not do

This calculator returns BMR only—the energy cost of essential physiological processes. It does not account for activity, non-exercise movement (NEAT), the thermic effect of food, or adaptive thermogenesis. It does not provide meal plans, macronutrient splits, training advice, or health diagnoses. Every output is an estimate derived from a single equation; individual metabolic rate can vary by 10–15% even among people with identical lean mass. The tool does not assess metabolic health, thyroid function, or hormonal status.

Disclaimer

This tool is for educational and informational purposes only. It is not medical, nutritional, or training advice. Results are estimates from a publicly available equation and may not reflect individual metabolic reality. Consult a physician, registered dietitian, or certified exercise professional before making decisions about diet, supplementation, or training based on any calculator output.

Questions

Why does Katch-McArdle ignore age and sex?: The formula assumes that lean body mass alone determines resting metabolic rate. Age and sex correlate with BMR largely because they track changes in muscle mass and organ size; by measuring body composition directly, Katch-McArdle bypasses demographic proxies. Research shows lean mass explains most of the variance in BMR across individuals.
How accurate does my body-fat measurement need to be?: Each 1% error in body-fat percentage shifts lean mass by roughly 0.75 kg for a 75 kg person, altering BMR by about 16 kcal/day. A 5% measurement error moves the estimate by 80 kcal. DEXA, hydrostatic weighing, and multi-site skinfold protocols offer the tightest precision; single-frequency BIA scales can drift by 3–5% session to session.
Can I use this formula if I am obese or very lean?: The Katch-McArdle equation was derived from adults in moderate body-fat ranges. At very high body-fat percentages (above 40%), adipose tissue contributes more to resting metabolism than the constant 21.6 factor assumes, potentially underestimating BMR. Elite athletes below 8% body fat may have organ and metabolic adaptations not captured by the equation.
What is the difference between BMR and TDEE?: BMR is the energy required for basic physiological functions—heartbeat, respiration, cellular repair—measured at complete rest. Total daily energy expenditure (TDEE) adds physical activity, non-exercise movement, and the thermic effect of digesting food. TDEE is typically 1.2–2.5 times BMR, depending on lifestyle and training volume.
How often does BMR change?: BMR shifts whenever lean body mass changes. Gaining muscle through resistance training raises BMR proportionally; losing muscle during a calorie deficit or aging lowers it. Short-term factors like sleep deprivation, illness, or extreme dieting can also alter metabolic rate by 5–15%, though these effects are not captured by static body-composition equations.

Sources & Methodology

Applies the Katch-McArdle equation: BMR = 370 + (21.6 × lean body mass in kg). Lean body mass is derived by multiplying total weight by (1 − body-fat % / 100). Originally published by Katch and McArdle in exercise-physiology texts using metabolic-chamber measurements.

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