Katch-McArdle Formula

On this page

1. The formula
2. Step by step
3. Where it comes from
4. Katch-McArdle vs Mifflin-St Jeor
5. Katch-McArdle vs Cunningham
6. Who should use it
7. Which body fat number to enter
8. How much a wrong body fat reading matters
9. Why it ignores age and sex
10. Using Katch-McArdle during a cut
11. From Katch-McArdle to TDEE
12. Common mistakes
13. Limitations
14. Katch-McArdle formula questions

The formula

Lean body mass (kg) = weight (kg) × (1 − body fat % ÷ 100) BMR = 370 + 21.6 × lean body mass (kg) In pounds: BMR = 370 + 9.8 × lean body mass (lb)

Lean body mass is everything that isn’t fat: muscle, organs, bone, water and connective tissue. That is the part of you that does nearly all the work of resting metabolism, which is the idea behind the formula.

Step by step

In kilograms. A woman who weighs 62 kg (137 lb) with 24% body fat:

1. Lean mass = 62 × (1 − 0.24) = 62 × 0.76 = 47.1 kg
2. BMR = 370 + (21.6 × 47.1) = 370 + 1,017.8 = 1,388 kcal
3. If she is moderately active: TDEE = 1,388 × 1.55 = 2,151 kcal

In pounds. A man who weighs 180 lb with 20% body fat:

1. Lean mass = 180 × 0.80 = 144 lb
2. BMR = 370 + (9.8 × 144) = 370 + 1,411 = 1,781 kcal
3. If he is lightly active: TDEE = 1,781 × 1.375 = 2,449 kcal

The body fat TDEE calculator does these steps for you and compares the result with three other equations.

Where it comes from

The formula is named after Frank Katch and William McArdle, whose exercise physiology textbook with Victor Katch made it widely known. It rests on a finding confirmed many times: lean body mass predicts resting metabolism better than total weight. In 1980, Cunningham re-analysed the classic Harris-Benedict data and reached the same conclusion, proposing a similar equation (500 + 22 × lean mass) that usually gives results 100 to 200 kcal higher.

Katch-McArdle vs Mifflin-St Jeor

The two can disagree a lot, and body fat is why. Take a 30-year-old man who is 175 cm (5′9″) and 80 kg (176 lb). Mifflin-St Jeor gives him a BMR of 1,749 kcal whatever his body fat. Katch-McArdle depends on it:

For this man the two agree at about 20% body fat. Leaner than that, Katch-McArdle is higher; fatter, it is lower. For a 35-year-old woman of 165 cm (5′5″) and 65 kg (143 lb), the crossover is around 30% body fat. The exact point depends on height and age, but the pattern holds: lean-mass formulas credit muscle that weight-based formulas can’t see.

For women the pattern is the same. For the 35-year-old woman above (Mifflin-St Jeor BMR 1,345 kcal):

Katch-McArdle vs Cunningham

Cunningham uses the same idea with bigger numbers: 500 + 22 × lean mass. For the 80 kg man above:

Cunningham is consistently about 150 to 160 kcal higher here. It was proposed in 1980 from a reanalysis of the Harris-Benedict data and has held up well in active people; a 2014 study in recreational athletes found it predicted their resting energy well. If you train hard most days, it is worth comparing.

Who should use it

• Lean, muscular people, roughly men under 15% and women under 23% body fat, especially if they lift weights seriously. Weight-based formulas often underestimate them.
• People with a decent body fat measurement: a DEXA scan, skilled skinfold calipers or a careful tape measurement repeated in the same way.
• Anyone curious about how their composition changes the picture, as a second opinion next to Mifflin-St Jeor.

If you don’t know your body fat, or only have a reading from a bathroom scale, Mifflin-St Jeor is usually the safer choice.

Which body fat number to enter

Use the most reliable measurement you have, taken under consistent conditions: in the morning, after the toilet, before eating, drinking or training. Typical errors are about 1 to 2 percentage points for a DEXA scan, 3 to 4 for calipers or a tape method, and 3 to 5 or more for bioimpedance scales, which shift with hydration. If your readings jump around, use an average of several.

Don’t have one? The body fat calculator estimates it from a few tape measurements using the U.S. Navy method.

How much a wrong body fat reading matters

Less than you might fear. Each percentage point of body fat moves lean mass by 1% of your weight. For an 80 kg person, a 3-point error is 2.4 kg of lean mass, which changes BMR by about 52 kcal (2.4 × 21.6) and TDEE by about 80 kcal at a moderate activity level. Picking the wrong activity level usually costs far more.

Why it ignores age and sex

It isn’t an oversight. Men burn more than women of the same weight mainly because they carry more lean mass, and resting metabolism falls with age mainly because people lose muscle. Once lean mass is in the equation, age and sex add little. That makes Katch-McArdle the same formula for everyone, which can be useful for people for whom the sex-specific constants of other equations don’t fit well.

Using Katch-McArdle during a cut

Lean mass changes as you diet, so recalculate as you go. Say the man above starts at 80 kg and 20% body fat (64 kg lean, BMR 1,752) and, after a careful cut with high protein and lifting, reaches 74 kg at 14% body fat. His lean mass is now 74 × 0.86 = 63.6 kg, almost unchanged, and his BMR is about 1,744 kcal: only 8 kcal lower despite losing 6 kg. A weight-based equation would have taken about 60 kcal off.

That is the practical value of a lean-mass formula: it rewards keeping muscle. If his lean mass had dropped to 61 kg instead, his BMR would be 1,688 kcal, a sign that the cut was costing muscle.

From Katch-McArdle to TDEE

Multiply by the same activity factors as any BMR equation: 1.2 sedentary, 1.375 lightly active, 1.55 moderately active, 1.725 very active, 1.9 athlete. Lean, active people often belong in the higher levels, but be honest: three gym sessions a week alongside a desk job is usually lightly or moderately active.

Common mistakes

• Mixing units. Use lean mass in kilograms with 21.6, or in pounds with 9.8. Pounds in the metric version gives a result far too high.
• Switching measurement methods. A DEXA reading one month and a smart scale the next can differ by several points. Stick to one method so changes mean something.
• Entering body fat as a decimal. 18% body fat means multiplying weight by 0.82, not by 0.18.
• Forgetting to update it. After a cut or a bulk, both your weight and body fat have changed. Recalculate with both.

Limitations

• It is only as good as your body fat number.
• It gives no credit for the small amount of energy fat tissue uses, so it can come out low for people with a lot of body fat.
• It isn’t designed for children, pregnancy or medical conditions that affect metabolism.

As with any formula, eat at the result for two to four weeks and let your weight trend confirm or correct it. The formula comparison shows how it stacks up against the others.

Katch-McArdle formula questions

Does Katch-McArdle give TDEE directly?

No, it gives BMR. Multiply it by your activity factor, from 1.2 to 1.9, to get TDEE.

What body fat percentage should I enter?

Your most reliable recent measurement, ideally from DEXA, skilled calipers or a careful tape method. If your readings vary, use an average. If you only have a bathroom-scale reading, consider using Mifflin-St Jeor instead.

Is Katch-McArdle better for muscular people?

Usually, yes. Weight-based equations assume average body composition and tend to underestimate people with more muscle. Katch-McArdle credits that muscle directly.

Is Katch-McArdle the same for men and women?

Yes. It uses only lean body mass, which already reflects most of the difference between men and women.

How often should I measure my body fat?

Every four to eight weeks is plenty. Body fat changes slowly, and frequent measurements mostly show measurement noise.

Do I need to know my body fat exactly?

No. Within about 3 percentage points is good enough: that changes BMR by only about 50 to 65 kcal for most people. Bigger errors are a problem, so avoid single smart-scale readings.

Can I use Katch-McArdle if I have a lot of weight to lose?

You can, but it may come out low, because it gives no credit for the energy used to carry extra weight around. Mifflin-St Jeor is usually the better starting point at higher body fat.

What is the difference between Katch-McArdle and Cunningham?

Both use lean body mass. Cunningham (500 + 22 × lean mass) has a larger constant and usually comes out 100 to 200 kcal higher. It is often suggested for athletes and very active people.

Sources

1. McArdle WD, Katch FI, Katch VL. Exercise Physiology: Nutrition, Energy, and Human Performance. Wolters Kluwer.
2. Cunningham JJ. A reanalysis of the factors influencing basal metabolic rate in normal adults. Am J Clin Nutr. 1980;33(11):2372-2374. doi.org/10.1093/ajcn/33.11.2372
3. ten Haaf T, Weijs PJM. Resting energy expenditure prediction in recreational athletes of 18-35 years: confirmation of Cunningham equation and an improved weight-based alternative. PLoS One. 2014;9(10):e108460. doi.org/10.1371/journal.pone.0108460
4. Mifflin MD, St Jeor ST, Hill LA, et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990;51(2):241-247. doi.org/10.1093/ajcn/51.2.241
5. Elia M. Organ and tissue contribution to metabolic rate. In: Kinney JM, Tucker HN, eds. Energy Metabolism: Tissue Determinants and Cellular Corollaries. Raven Press; 1992:61 to 79.
6. Heyward VH, Wagner DR. Applied Body Composition Assessment. 2nd ed. Human Kinetics; 2004.