BMR Katch-McArdle Calculator

What the Katch-McArdle equation does differently

Most BMR formulas ask for your weight, height, age and sex. Katch-McArdle asks for one thing: how much of you is not fat. The equation multiplies lean body mass in kilograms by 21.6 and adds 370, and that is the whole calculation.

The logic behind it is that fat tissue burns little. Muscle, organs, bone and the water around them do most of the resting work, and fat is close to inert. Two people who weigh the same can have lean masses 40 pounds apart, and their resting burn follows the lean mass rather than the scale. A formula based on total weight has to guess at body composition. This one is told.

The equation comes from the exercise physiology literature associated with Katch and McArdle, and it has been a standard in fitness testing for decades. It is not a medical device and it is not a measurement. It is a prediction, and like every prediction it is only as good as the body fat number you feed it.

The formula, step by step

BMR = 370 + (21.6 x lean body mass in kg). That is it. The only work is turning your weight and body fat percentage into lean mass in kilograms.

Lean mass is your total weight minus the fat. At 180 pounds and 20% body fat, fat mass is 180 x 0.20 = 36 lb, so lean mass is 180 - 36 = 144 lb. The shortcut is 180 x 0.80 = 144 lb, which gives the same answer because the two percentages add to 100.

Convert pounds to kilograms by multiplying by 0.4536. That gives 144 x 0.4536 = 65.32 kg. Then multiply by 21.6 and add 370: 21.6 x 65.32 = 1,410.9, and 370 + 1,410.9 = 1,781 kcal a day. That is the energy your body uses at complete rest, before you walk to the kitchen.

One detail worth keeping straight: the 21.6 is per kilogram, not per pound. If you skip the conversion and multiply pounds by 21.6, you get 3,110 kcal, which is roughly the burn of a 300 pound athlete. The unit error is the most common mistake with this formula.

Worked example: 180 pounds at 20% body fat

Here is the full run for a 40 year old man, 5 feet 10 inches, 180 pounds, 20% body fat, who trains three to five times a week. The calculator on this page returns these numbers with the default inputs.

• Fat mass: 180 x 0.20 = 36 lb
• Lean mass: 180 - 36 = 144 lb
• Lean mass in kg: 144 x 0.4536 = 65.32 kg
• BMR: 370 + 21.6 x 65.32 = 1,781 kcal
• TDEE at 1.55: 1,781 x 1.55 = 2,760 kcal
• Mifflin-St Jeor: 10 x 81.65 + 6.25 x 177.8 - 5 x 40 + 5 = 1,733 kcal
• Difference: 1,781 - 1,733 = +48 kcal

The two formulas land 48 calories apart, which is 2.8% and well inside the error of any prediction equation. That is a good sign: when Katch-McArdle and Mifflin-St Jeor agree, the body fat estimate and the height and weight are telling a consistent story.

Now change one input. Set body fat to 25% while keeping the same 180 pounds, and lean mass drops to 135 lb, or 61.23 kg. The BMR becomes 370 + 1,322.7 = 1,693 kcal, which is 88 calories lower than the 20% version. Same person, same scale, different answer, all because of a body fat estimate.

Why body fat error moves the answer

Every kilogram of lean mass is worth 21.6 kcal in this equation. Every kilogram of fat is worth zero. So an error in body fat percentage moves the answer by 21.6 kcal for every kilogram of lean mass you misplace, which works out to about 9.8 kcal per pound.

Five percentage points of body fat is worth about 88 kcal a day in this example, which is 2,640 kcal a month, or roughly three quarters of a pound of body fat. Over a year that is 9 pounds of drift if you eat to the wrong number.

How accurate are body fat estimates? A DEXA scan is the clinical reference and still carries a few points of error depending on the machine and the technician. Calipers are only as good as the person using them, and the same person can read 3 to 5 points differently on different days. Consumer scales that send a current through your feet can be off by 5 to 8 points, and hydration moves them more than fat does. If your only body fat number comes from a smart scale, treat the BMR as a rough starting point and adjust from real weight change.

Katch-McArdle against Mifflin-St Jeor

Mifflin-St Jeor is the equation most dietitians reach for, and a systematic review of resting metabolic rate equations found it the most accurate for both nonobese and obese adults. The Frankenfield review in the Journal of the American Dietetic Association is the usual citation, and it is the reason Mifflin shows up in so many clinical tools.

The two equations agree closely when body composition is average, and they separate at the edges. Katch-McArdle runs higher for lean, muscular people, because lean mass is high relative to total weight. It runs lower for people with high body fat, because the fat contributes nothing in this model while Mifflin still credits total weight.

The lean 200 pound man gets a 189 kcal boost from Katch-McArdle, and the 130 pound woman at 38% body fat gets a 129 kcal cut. That pattern is the equation working as designed. It also means the choice between the two formulas matters most for people at the extremes of body composition, and hardly at all for everyone in the middle.

From BMR to daily calories

BMR is the floor, not the total. Multiply it by an activity factor to get total daily energy expenditure, which is the number you eat against. The multipliers below are the standard ones used in fitness testing, and they are rough buckets rather than measurements.

The multiplier is the weakest link in the chain for most people. Two people with the same BMR can differ by 600 kcal a day in expenditure, and the bucket you pick is a guess about a week you have already lived. A step counter and a food log for two weeks will beat any multiplier, because they measure what happened instead of estimating it.

Who should use it, and who should not

Katch-McArdle is the better choice when you have a body fat number you trust. That usually means a DEXA scan, a hydrostatic test, or a caliper reading from someone who does it often. It is also the better choice for athletes and lifters, because their lean mass is high and a total weight formula tends to underfeed them.

It is a poor choice when the body fat number is a guess from a bathroom scale. The equation amplifies that guess, and a five point error moves the answer by 88 kcal in the example above. In that case Mifflin-St Jeor is more forgiving, because it spreads its estimate across height, weight, age and sex instead of betting everything on one measurement.

Neither equation is a medical tool. Resting metabolic rate can be measured directly with indirect calorimetry, which is accurate to about 5% when the test is run properly, and that is the only way to know your own number rather than an estimate of it. If you have a thyroid condition, are pregnant, or are recovering from surgery, talk to a clinician before you build a calorie target around any formula.

What to do with the number

Use the BMR as a starting point and let the scale correct it. Eat at your estimated TDEE for two weeks, weigh yourself at the same time each morning, and average the readings. If your weight holds steady, the estimate was close. If you gain, the real number is lower than the estimate, and if you lose, it is higher.

For fat loss, a deficit of 500 kcal a day is the usual starting point, which is about a pound a week. From a TDEE of 2,760 that is 2,260 kcal, still well above the BMR of 1,781, which matters: eating below your BMR for long stretches is hard to sustain and tends to cost lean mass along with fat. The NHLBI guidance on aiming for a healthy weight is a reasonable plain language reference for the pace of change.

Recheck the body fat number every three months or so, since lean mass changes with training. A 10 pound gain that is mostly muscle raises the BMR by roughly 98 kcal, and a formula stuck on an old body fat percentage will miss it.

This is an estimate for planning, not medical advice. Body fat measurement and calorie needs vary by person, so confirm anything important with a clinician or a registered dietitian.