Katch-McArdle BMR: Estimating Calories From Lean Body Mass

Step by Step

First estimate body fat percentage using a method you trust: skinfold calipers, DEXA, a tape-measure formula, or a bioimpedance scale. Then calculate fat mass (weight × body fat fraction) and lean body mass (weight − fat mass). Finally apply the formula. For a 55 kg woman with 28% body fat, lean mass is 39.6 kg and BMR = 370 + 21.6 × 39.6 = 1,225 calories. For a 100 kg man with 25% body fat, lean mass is 75 kg and BMR = 1,990.

As with all BMR equations, multiply the result by an activity factor to estimate total daily energy expenditure.

When It Beats Other Equations

The advantage appears in people whose body composition differs from the average. Two people with the same weight, height, and age can have very different lean mass, and weight-and-height formulas cannot tell them apart. A muscular person with low body fat will burn more at rest than Mifflin-St Jeor predicts, and a person with high body fat and little muscle will burn less. For an athlete who is 95 kg at 10% body fat, Katch-McArdle predicts 2,217 calories, while Mifflin-St Jeor (for a 30-year-old man of 185 cm) gives 1,961, a difference of about 250 calories.

For people with obesity, Katch-McArdle can overestimate if the body fat measurement is too low, and Mifflin-St Jeor often performs better in population studies for the general public.

The Weak Link: Body Fat Accuracy

The formula is only as good as the body fat estimate. An error of 5 percentage points in body fat shifts lean mass by 4 kg for an 80 kg person, which changes the BMR estimate by 86 calories. Consumer devices can be off by more than that. Use the same method consistently, and treat the BMR as an estimate. Direct measurement by indirect calorimetry is the gold standard, though it is not widely available.

Whatever the equation, check your estimate against real-world results: eat at the estimated maintenance level for two to three weeks and see whether weight stays stable.

Related Equations

The Cunningham equation, BMR = 500 + 22 × lean mass in kg, is a variant developed for athletes and gives slightly higher values at higher lean mass (for 68 kg: 1,996). The Harris-Benedict, Mifflin-St Jeor, and Schofield equations use body weight. In practice, the different equations vary by 5% to 10% for most people, which is within the margin of error. Pick one, track results, and adjust.

Using the Result

Multiply BMR by an activity factor (1.2 to 1.9) to get TDEE. For the 80 kg man with a BMR of 1,839 and a lightly active factor of 1.375, TDEE is about 2,529 calories. A 20% deficit would be around 2,023 calories. Remember that muscle retention in a deficit depends on protein intake and resistance training as much as on calorie targets.