Katch-McArdle Calculator

What is the Katch-McArdle formula?

The Katch-McArdle equation estimates basal metabolic rate from lean body mass rather than total body weight. Formula: BMR = 370 + 21.6 × lean body mass (kg), where lean body mass equals total weight minus fat mass.

McArdle, Katch, and Katch presented the formula in exercise physiology textbooks rather than a primary equation paper indexed in PubMed. It resembles Cunningham's fat-free-mass approach but uses a different intercept and coefficient. The names and equations should not be treated as interchangeable.

Fat-free mass is a strong determinant of resting expenditure, but adding body composition does not guarantee a better individual estimate. Accuracy depends on the equation's source population and the quality of the body-fat measurement.

The formula assumes that organs, bone, and muscle collectively determine resting needs while adipose tissue contributes minimally. Lean mass-based prediction works only when body fat measurement is trustworthy.

Resting expenditure also varies within fat-free mass because organs consume far more energy per kilogram than skeletal muscle. Two people with equal lean mass can have different organ proportions, age, health, and measured expenditure.

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Calculating lean body mass

Lean body mass (LBM) = body weight × (1 − body fat percentage / 100). Example: 80 kg at 20% body fat → fat mass = 16 kg → LBM = 64 kg → BMR = 370 + 21.6 × 64 = 1,752 kcal.

Body fat percentage accuracy determines result quality. DXA scans, hydrostatic weighing, and multi-site skinfold measurements provide reasonable estimates. Visual guessing or generic online charts introduce error that propagates directly into BMR.

This calculator requires body fat percentage as an input. If you do not know your body fat, use a BMR equation based on weight, height, age, and sex instead, such as Mifflin-St Jeor.

A 5% error in body fat percentage on an 80 kg person shifts lean mass by 4 kg, changing Katch-McArdle BMR by roughly 86 kcal. Small measurement errors compound into meaningful calorie differences over weeks of dieting.

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When to use Katch-McArdle

Use this formula when you have a reliable body fat estimate and your physique differs from population averages. For example, if you carry substantially more muscle or fat than typical for your weight and height.

Athlete studies often favor fat-free-mass equations, particularly Cunningham, but results depend on sport, sex, and measurement method. Katch-McArdle should remain one estimate rather than an assumed best formula for every muscular person.

Obese individuals may also benefit because weight-based equations overestimate resting needs by counting fat mass as equally metabolically active as lean tissue.

NIH nutrition summaries emphasize that fat-free mass is the primary driver of resting expenditure. Katch-McArdle applies that principle directly when you supply a trustworthy body fat percentage.

[2][4][8]

Katch-McArdle vs other BMR equations

Mifflin-St Jeor uses weight, height, age, and sex without body composition data. It works well for average adults and is less precise at extremes of leanness or adiposity.

Harris-Benedict (revised) uses anthropometrics only and tends to overestimate modern resting expenditure.

Comparing all three equations in the RMR calculator shows the spread of estimates. When Katch-McArdle diverges significantly from Mifflin-St Jeor, body composition is likely driving the difference. Katch-McArdle adds lean mass, the composition variable the weight-based formulas omit.

If you are average weight with unknown body fat, Mifflin-St Jeor remains the evidence-based default from the Academy of Nutrition and Dietetics. Reserve Katch-McArdle for measured or professionally assessed body fat.

[4][5][9]

How to use the Katch-McArdle result

The 370 kcal constant and 21.6 multiplier were derived from specific study populations. Individual variation in organ size, thyroid function, and genetics still produces error of 10% or more even with accurate lean mass.

Lean body mass includes bone, water, and organ weight, not just skeletal muscle.

Changes in hydration or glycogen stores alter estimated LBM without changing true metabolic rate. Validate results through 2 to 3 weeks of food and weight tracking. Measured energy balance overrides the formula.

For recomposition goals (losing fat while building muscle), Katch-McArdle paired with regular body fat remeasurement keeps calorie targets aligned as lean mass changes. Static weight-based equations handle that poorly over months of training.

Recalculate Katch-McArdle BMR whenever body fat percentage changes by 3 or more points. During a 12-week cut or bulk, lean mass shifts enough to warrant updated calorie targets at least once midway through the phase.

Use body-fat data collected under repeatable conditions. Enter weight and body-fat percentage from the same time point, compute resting expenditure, then apply an activity estimate to obtain maintenance. Do not compare resting output directly with food intake.

Compare Katch-McArdle with Mifflin-St Jeor and investigate large gaps. A 200 kcal difference may reflect body-fat error or a physique outside one equation's typical data. Indirect calorimetry is the better test when precision affects care.

[1][6]

Body-fat measurement assumptions

DXA, bioimpedance, skinfolds, and circumference methods estimate different body compartments with different assumptions. Hydration, glycogen, food intake, device algorithms, and technician skill can shift results. A decimal displayed by a device is not proof of decimal-level accuracy.

Repeat the same method under similar conditions and focus on changes larger than normal measurement noise. Switching from a home bioimpedance scale to DXA can change the reported percentage without a biological change, which then moves the calculated RMR.

Lean body mass includes water, bone, organs, and muscle. The formula does not identify which compartment changed. A rapid lean-mass drop during dieting may be glycogen and water rather than lost muscle.

[2][3][7]

Validate the estimate with energy balance

Multiply resting expenditure by an activity factor, log food intake, and compare seven-day average weights for 14 to 28 days. Stable weight at a repeatable intake supplies a better personal maintenance estimate than agreement between two equations.

If calculated TDEE is 2,500 kcal but weight falls 0.25 kg per week at 2,200 kcal intake, static arithmetic suggests observed expenditure near 2,475 kcal/day. Water changes and the composition of loss make this approximate, so confirm across several weeks.

Adjust intake by 100 to 200 kcal and hold the change. Recalculate when body weight, body composition, or activity changes materially. Avoid using short-term body-fat readings to make weekly calorie changes.

[2][4][6]

Population and clinical limitations

The Katch-McArdle formula lacks the kind of original, clearly described primary derivation paper available for Mifflin-St Jeor. Validation results differ across populations. Cunningham has stronger primary documentation and should be distinguished when research cites an FFM equation.

General resting equations do not cover children, pregnancy, lactation, critical illness, severe malnutrition, edema, or major changes in hydration. Body-composition methods also perform poorly with fluid imbalance, making this formula especially unsuitable in those settings.

Seek clinical guidance for unexplained weight change, eating-disorder history, kidney disease, diabetes medication, pregnancy, or symptoms of under-fueling. A lean-mass protein or calorie target should not override prescribed nutrition.

Sex does not appear in the formula because fat-free mass is the only input after body composition is calculated. That does not make the result universally sex-neutral: organ mass, fat distribution, age, hormones, and the method used to estimate body fat can still affect agreement with measured expenditure.

The output is resting expenditure, not the energy cost of a full day. A person with an estimate of 1,750 kcal may maintain at a much higher intake once work, walking, digestion, and training are included. Applying a generic activity factor adds another source of error.

Do not use repeated bioimpedance readings to chase daily calorie changes. Hydration can move both the reported body-fat percentage and scale weight in the same direction, creating a convincing but false signal. Hold the plan and compare multiweek trends.

[4][7][10][11]

How it works

BMR = 370 + 21.6 × lean body mass (kg). Requires accurate body fat measurement.

Frequently asked questions

• When should I use Katch-McArdle?Use it when you know your body fat percentage, especially if you are muscular or lean.

Related calculators

References

1. McArdle, Katch & Katch. Human energy expenditure during rest and physical activity
2. Frankenfield et al.. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults
3. McArdle, Katch & Katch. Essentials of Exercise Physiology
4. Mifflin et al.. A new predictive equation for resting energy expenditure in healthy individuals
5. Roza & Shizgal. The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass
6. Frankenfield et al.. Validation of several established equations for resting metabolic rate in obese and nonobese people
7. Frankenfield et al.. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults
8. NIH/National Academies. Energy - Recommended Dietary Allowances
9. Academy of Nutrition and Dietetics. Academy of Nutrition and Dietetics: Assess Energy Needs
10. Cunningham. A reanalysis of the factors influencing basal metabolic rate in normal adults
11. Cunningham. Body composition as a determinant of energy expenditure: a synthetic review and a proposed general prediction equation