TDEE Calculator with Body Fat

TDEE calculator, body fat and lean mass: how the math works

The calculation has three steps. First, fat mass is your weight times your body fat percentage, and lean body mass is what is left: bone, muscle, organs and water. Second, BMR = 370 + 21.6 x lean mass in kg. Third, TDEE = BMR x the same activity factors used everywhere else, from 1.2 to 1.9.

With the default values, 70 kg at 20 percent body fat, fat mass is 14 kg and lean mass is 56 kg. BMR = 370 + 21.6 x 56 = 1579.6 kcal, and moderately active (x1.55) gives a TDEE of about 2448 kcal/day.

For comparison, Mifflin-St Jeor gives a 30-year-old man of the same weight and 175 cm a BMR of 1649 and a TDEE of 2556, about 108 kcal more. At 15 percent body fat the two equations meet: lean mass is 59.5 kg and Katch-McArdle gives 1655. For that person, any leaner and Katch-McArdle comes out higher; any fatter and it comes out lower.

Katch-McArdle or Mifflin-St Jeor?

Mifflin-St Jeor only needs weight, height, age and sex, which makes it the safer default. Katch-McArdle drops height, age and sex and works from lean mass, which is your weight minus the fat your body fat percentage implies. That makes it reward an accurate body fat number and punish a guessed one.

Use Katch-McArdle when you have a measured body fat percentage, especially if you are noticeably leaner or heavier than average. Use Mifflin-St Jeor when body fat is a guess.

Where to get a body fat number

For a TDEE calculator, body fat from a DEXA scan or hydrostatic weighing is the accurate option. Calipers and bioimpedance scales are cheaper but drift, so keep using the same method rather than mixing them. A guessed percentage makes this equation less accurate than Mifflin-St Jeor, not more.

When lean mass changes the answer

Lean, muscular people are where Mifflin-St Jeor most often falls short, because it assumes average composition for your weight. Two 90 kg men, one at 12 percent body fat and one at 30 percent, get the same Mifflin-St Jeor number. Katch-McArdle gives the leaner one about 350 kcal more BMR.

At higher body fat the opposite happens: total weight overstates the tissue that actually burns energy, so Katch-McArdle tends to come in lower. It also has no age or sex term. Differences between men and women, and between younger and older adults, are carried entirely by lean mass - a reasonable approximation, but not a perfect one.

How accurate is a TDEE calculator? Body fat error in numbers

In any TDEE calculator, body fat percentage is the input most likely to be wrong. At 70 kg, each percentage point is 0.7 kg of lean mass, which is about 15 kcal of BMR and about 23 kcal of TDEE at a moderate activity level.

A DEXA scan is usually within 1 to 3 points. Bioimpedance scales and calipers can be off by 3 to 5 points or more, depending on hydration, the skinfold sites and who takes the measurement. A 5-point error moves BMR by about 76 kcal and TDEE by about 117 kcal: smaller than the gap between two activity levels, but worth knowing about.

Estimating from a mirror or a chart of photos is less reliable still. If that is all you have, Mifflin-St Jeor is usually the safer choice.