The formula
Lean body mass = weight × (1 − body fat % ÷ 100)
BMR = 370 + 21.6 × lean body mass (kg)
Then multiply BMR by your activity factor to get TDEE, exactly as with the other equations.
How body fat changes the result
Same person, 180 lb, at different body-fat levels. For comparison, Mifflin-St Jeor gives a 32-year-old man who is 5′10″ and 180 lb about 1,773 calories whatever his body fat.
| Body fat | Lean mass | Katch-McArdle BMR |
|---|---|---|
| 12% | 158 lb | 1,922 |
| 18% | 148 lb | 1,816 |
| 25% | 135 lb | 1,693 |
| 32% | 122 lb | 1,569 |
Leaner people get a higher estimate at the same weight, since lean tissue uses more energy. A 5-point error in body fat moves this example by about 88 calories, so treat the result as a range.
Where the formula comes from
The equation is associated with exercise physiologists Frank Katch and William McArdle and appears in their textbook Exercise Physiology: Nutrition, Energy, and Human Performance. It belongs to a family of lean-mass equations; a closely related one by Cunningham (1980) uses 500 + 22 × lean body mass. Both are popular in sports nutrition, where body-fat measurements are more common.
Frequently asked questions
What is the Katch-McArdle formula?
BMR = 370 + 21.6 × lean body mass in kilograms. Lean body mass is your weight minus the weight of your body fat. Because it works from lean mass, the formula uses the same equation for men and women and doesn’t need age.
Is Katch-McArdle more accurate?
It can be for people whose body composition is far from average, such as very muscular athletes, because weight-based equations can’t tell muscle from fat. But it is only as good as your body-fat number, and home scales and visual estimates can be several percentage points off.
How do I find my body-fat percentage?
Options range from DEXA scans and hydrostatic weighing (more precise, usually paid) to skinfold calipers measured by a trained person, and bioelectrical impedance scales (convenient but variable). Use the same method each time if you want to track change.
Do I need to choose Katch-McArdle in the formula menu?
No. With the formula set to Auto, entering a body-fat % switches the calculator to Katch-McArdle automatically. Clear the field to go back to Mifflin-St Jeor.