Katch-McArdle BMR Calculator

The Katch-McArdle equation: BMR from lean body mass

Developed by exercise physiologists Frank Katch and Victor McArdle and published in their 1977 textbook Nutrition, Weight Control, and Exercise, the Katch-McArdle equation takes a fundamentally different approach from the Harris-Benedict and Mifflin-St Jeor formulas. Instead of using total body weight, age, and height as predictors, it uses lean body mass (LBM) alone. The formula is simply: BMR = 370 + 21.6 × LBM(kg) for both men and women. The single-sex form reflects the underlying physiology, lean tissue, not sex itself, drives metabolic rate.

Why lean body mass is the right predictor

Almost all of the variation in resting energy expenditure between individuals of the same weight is explained by differences in body composition. Skeletal muscle burns roughly 13 kcal/kg/day at rest, organs (heart, liver, kidneys, brain) burn 100-440 kcal/kg/day, while adipose tissue burns only about 4.5 kcal/kg/day. A 90 kg bodybuilder at 8% body fat has dramatically more metabolically active tissue than a 90 kg sedentary individual at 30% body fat, even though both weigh the same. The Katch-McArdle equation captures this distinction by working from LBM rather than total weight.

How to compute lean body mass

Lean body mass = total body weight × (1 − body fat fraction). For example, an 80 kg person at 15% body fat has LBM = 80 × (1 − 0.15) = 68 kg. Their Katch-McArdle BMR is 370 + 21.6 × 68 = 370 + 1468.8 = 1838.8 ≈ 1839 kcal/day. Body fat percentage can be estimated from skinfold calipers (Jackson-Pollock 3-site or 7-site), bioelectrical impedance analysis (BIA) scales, DEXA scans, hydrostatic weighing, or our Body Fat Calculator (which uses the U.S. Navy circumference method). The accuracy of your BMR estimate is only as good as your body fat measurement.

How the 370 kcal constant was derived

The intercept term of 370 kcal/day represents the baseline energy cost of organs and essential metabolic processes that are not proportional to total lean mass, brain metabolism, liver function, kidney filtration, and the energy required simply to keep a cell alive. The 21.6 coefficient represents the average resting metabolic rate of lean tissue, which is a weighted average of muscle (low metabolic rate per kg) and organs (high metabolic rate per kg). This simplified two-parameter model captures about 70-80% of the variance in measured BMR between individuals of similar body composition.

When Katch-McArdle is the best choice

Katch-McArdle excels in three populations. First, athletes and bodybuilders, whose high lean mass is poorly captured by weight-based equations. Second, very lean individuals (under 12% body fat for men, under 20% for women), where the weight-based equations systematically under-predict BMR. Third, individuals undergoing significant body recomposition, losing fat while gaining muscle, whose weight may stay constant but whose true energy expenditure changes as their lean mass shifts. In all three cases, the Katch-McArdle equation tracks the underlying physiological change more faithfully than weight-based alternatives.

When Katch-McArdle is the wrong choice

If you don't have a reliable body fat measurement, the Katch-McArdle equation is no better than guessing. BIA scales can be off by 4-8 percentage points depending on hydration status, and skinfold measurements require trained technicians. For most general-population adults without body composition data, the Mifflin-St Jeor equation (which uses weight, height, age, and sex) is more practical and only slightly less accurate on average. Katch-McArdle is also less validated in obese populations, where the assumption that "all lean mass is metabolically active at 21.6 kcal/kg" may not hold, obese individuals often have metabolically "inert" lean mass supporting the extra weight.

Comparison with the Cunningham equation

The Cunningham equation, published in 1991 specifically for athletes, uses a similar structure: BMR = 500 + 22 × LBM(kg). Cunningham's coefficient is slightly higher (22 vs 21.6) and the intercept is higher (500 vs 370), reflecting the higher metabolic rate observed in trained athletes whose muscle tissue has adapted to chronic training with more mitochondria and greater oxidative capacity. For trained athletes, Cunningham typically predicts 3-8% higher than Katch-McArdle. For non-athletes, Katch-McArdle is preferred. You can compare both side-by-side using our BMR Formula Comparison calculator.

From BMR to TDEE for athletes

Athletes should use higher activity factors than the general population. The standard 1.2 (sedentary) through 1.9 (extra active) scale was developed for non-athletes. An endurance athlete training 10-15 hours per week may have a TDEE of 2.0-2.4 × BMR, and a strength athlete in heavy training may reach 1.9-2.2 × BMR. For precise tracking, use a heart rate monitor or wearable to measure exercise energy expenditure and add it to your BMR-based estimate of resting expenditure.

Practical accuracy and limitations

Even with accurate body composition data, the Katch-McArdle equation has a standard error of estimate of roughly 150-200 kcal/day compared to measured indirect calorimetry. That means your true BMR could be 10-15% above or below the predicted value. Individual variation in thyroid function, brown fat activity, mitochondrial efficiency, and genetics explains the residual variance. Use the result as a starting point, then track your weight and adjust intake by 100-200 kcal every 2-3 weeks based on real-world outcomes.

How to measure body fat accurately for Katch-McArdle

Because Katch-McArdle depends entirely on lean body mass, the accuracy of your body fat measurement determines the accuracy of your BMR estimate. Five methods are commonly available, ranked from most to least accurate. DEXA (dual-energy X-ray absorptiometry) scans cost $50-$150 and provide 1-2% accuracy, making them the gold standard for body composition. Hydrostatic weighing (underwater weighing) is similarly accurate (1-2%) but requires specialized facilities. Air displacement plethysmography (BodPod) is comparable to hydrostatic weighing in accuracy and more convenient. Skinfold calipers, when used by a trained technician with the Jackson-Pollock 7-site protocol, achieve 3-4% accuracy, but accuracy drops to 4-6% with the simpler 3-site protocol or inexperienced technicians. Bioelectrical impedance analysis (BIA) scales are the most accessible but least accurate, with errors of 4-8% depending on hydration status, recent meals, and time of day.

For practical purposes, the U.S. Navy circumference method (used in our Body Fat Calculator) offers a reasonable compromise, it requires only a tape measure and gives results within 3-5% of DEXA for most adults. Whatever method you choose, measure under consistent conditions (same time of day, same hydration status, same meal timing) and track the trend over time rather than relying on a single measurement. For Katch-McArdle, an error of 3 percentage points in body fat translates to roughly 100-150 kcal/day error in BMR, significant for precise nutrition planning but acceptable for general tracking.

Tracking BMR during body recomposition

Body recomposition, losing fat while gaining muscle simultaneously, is the goal of many intermediate lifters. During recomposition, body weight may stay constant or change only slightly, but body composition shifts substantially. Katch-McArdle is uniquely suited to tracking BMR during recomposition because it accounts for the change in lean mass directly. As you gain muscle, your LBM rises and your Katch-McArdle BMR rises with it, even if your weight stays the same. This is invisible to weight-based equations like Mifflin-St Jeor, which would report the same BMR before and after a successful recomposition.

To track BMR during recomposition, re-measure your body composition every 4-8 weeks using the same method and the same conditions. Recompute your Katch-McArdle BMR each time and adjust your calorie intake accordingly. A typical recomposition adds 0.25-0.5 kg of muscle per month while losing 0.25-0.5 kg of fat, small changes individually, but they compound to significant shifts in BMR over 6-12 months. A successful 6-month recomposition might add 1.5-3 kg of LBM, raising Katch-McArdle BMR by 30-65 kcal/day. That may sound small, but over a year it represents 11,000-24,000 additional kcal burned at rest, equivalent to 1.5-3 kg of fat loss with no other changes.

Common errors in Katch-McArdle calculations

Three errors commonly undermine Katch-McArdle calculations. First, using body fat percentage as a decimal instead of a percentage, entering 0.15 instead of 15 will compute LBM as 99.85% of body weight instead of 85%, producing wildly inaccurate BMR values. Second, mixing units, entering weight in pounds but treating it as kilograms (or vice versa) produces errors of about 2.2x. Third, using outdated body fat measurements, if you've gained or lost significant weight since your last measurement, your Katch-McArdle BMR is no longer accurate. Re-measure every 4-8 weeks during periods of weight change, and always use consistent measurement conditions.

When Katch-McArdle and weight-based equations give the same answer

For an individual at the population-average body fat percentage (about 15% for young men, 25% for young women), Katch-McArdle and Mifflin-St Jeor give nearly identical BMR estimates, typically within 50-100 kcal of each other. The two equations diverge only when body composition deviates from the average. For leaner-than-average individuals (under 10% body fat for men, under 18% for women), Katch-McArdle predicts higher BMR than Mifflin because it captures the elevated metabolic cost of the extra lean mass. For fatter-than-average individuals (over 25% body fat for men, over 35% for women), Katch-McArdle predicts lower BMR than Mifflin because it correctly identifies that much of the excess weight is metabolically inactive fat. This is why Katch-McArdle is preferred for athletes, bodybuilders, and anyone whose body composition deviates significantly from average, it gives a more accurate estimate of true resting energy expenditure.

Using Katch-McArdle alongside indirect calorimetry

If you have access to indirect calorimetry (a BreathGem, FitMate, or metabolic cart), you can validate your Katch-McArdle prediction against direct measurement. For most healthy adults, the measured BMR will be within 5-10% of the Katch-McArdle prediction. If the discrepancy is larger, three explanations are likely. First, your body fat measurement was inaccurate (most common. BIA scales can be off by 4-8 percentage points). Second, you have atypical muscle mitochondrial density, endurance athletes often have measured BMR 5-10% higher than Katch-McArdle predicts, while sedentary individuals often have measured BMR 5-10% lower. Third, you may be in a state of metabolic adaptation (recent calorie restriction, sleep deprivation, or high stress), which suppresses measured BMR by 5-15% relative to prediction. Use the discrepancy between prediction and measurement as a diagnostic signal, not a flaw in either method.