TDEE Calculator With Body Fat: The Katch-McArdle Formula

A TDEE calculator with body fat swaps the usual weight-based equation for Katch–McArdle: resting energy = 370 + 21.6 × lean body mass in kg. An 80 kg man at 20% body fat has 64 kg of lean mass, so the formula gives 1,752 kcal at rest. Multiply by an activity band and you have a daily total. The catch is that your body fat figure adds its own error.

How does a TDEE calculator with body fat work?

The only input beyond weight is lean mass, your weight times one minus your body fat fraction. A BMR calculator with body fat stops at resting energy. A TDEE calculator scales that up for the movement in your day with an activity band, and BMR vs TDEE spells out the difference.

NASEM’s definition is the one I prefer for the scaling. The 2023 report defines the physical activity level as total expenditure divided by basal rate and splits adults into four bands. Here they are on the 1,752 kcal example.

Activity level	Activity band (× basal rate)	Total daily energy (kcal)
Inactive	below 1.53	up to 2,681
Low active	1.53–1.68	2,681–2,943
Active	1.68–1.85	2,943–3,241
Very active	1.85–2.50	3,241–4,380

The same man (35 years old, 178 cm, 80 kg) gets 2,659, 2,875, 3,059 and 3,410 kcal from NASEM’s weight-based equations. Each one falls inside the matching band. For him, the two routes agree. Katch–McArdle gives resting energy and NASEM’s bands are multiples of basal rate, so treat the match as a guide.

A second case shows the same thing. A 35-year-old woman of 165 cm and 65 kg with 30% body fat has 45.5 kg of lean mass. Katch–McArdle gives 370 + (21.6 × 45.5) = 1,353 kcal at rest. Mifflin–St Jeor gives 1,345 kcal. The gap is 8 kcal, too small to matter for either formula.

Choose the band that fits a typical week, start from the middle of it, and ignore your best day. If you want the population averages first, how many calories per day you need has them.

The calorie calculator covers the weight-based route. It takes your age, height, weight and activity, applies the NASEM 2023 equations and doesn’t ask for body fat.

How much does body fat change the answer?

A lot, and it separates people a weight-based formula treats as identical. I took the same 80 kg man at three body fat levels and ran three formulas.

Body fat	Lean mass (kg)	Katch–McArdle (kcal)	Cunningham (kcal)	Mifflin–St Jeor (kcal)
12%	70.4	1,891	2,049	1,743
20%	64.0	1,752	1,908	1,743
30%	56.0	1,580	1,732	1,743

Mifflin–St Jeor never sees body fat, so its 1,743 kcal is the same for all three men. Katch–McArdle spreads them by 311 kcal. Cunningham, which is 500 + 22 × lean mass, spreads them by 317 kcal. A lean man at 12% body fat gets 148 kcal more than the weight-based formula gives, and a man at 30% gets 163 kcal less. That spread is what body fat buys you. The weight-based equation can’t tell these three men apart.

Is Katch-McArdle more accurate than Mifflin-St Jeor?

Slightly more informative, but not reliably more accurate. The evidence is thin and mixed. Four studies follow. The weight-based rival has its own piece, the Mifflin-St Jeor formula.

• In the 498 people behind the Mifflin–St Jeor equation, fat-free mass explained more of resting energy (R² 0.64) than weight did (0.56). The full equation scored 0.71.
• In 620 Chilean women measured by indirect calorimetry, Katch–McArdle had the highest share within 10% of the measured value among 12 equations, and that share was only 39.5%.
• In 90 recreational athletes, the Cunningham equation did well, while Mifflin–St Jeor and four other common equations were under 50% accurate.
• In 30 professional weightlifters, all eight equations tested, Katch–McArdle included, underestimated resting energy. The closest, Cunningham, was still 375 kcal low on average.

My reading is that lean mass helps most in athletes, where weight-based equations did worst. For everyone else the gain looks small.

How accurate is the body fat number I put in?

Not very, and the formula passes your error straight into your calories. A 2026 systematic review (Oliver and colleagues) compared impedance devices with a four-compartment reference method in healthy adults. Across 12 studies, average bias in body fat percentage ran from −3.5 to +4.4 points. The limits of agreement typically spanned 15 to 20 points. For fat-free mass they often exceeded ±6 kg.

Put that into the formula. A 6 kg lean mass error is 21.6 × 6 = 130 kcal. Even a 5-point miss on body fat for an 80 kg man shifts lean mass by 4 kg and the answer by 86 kcal. Consumer scales are the likeliest weak spot. A 2025 review notes that single-frequency consumer devices often lack raw data access, clinical validation and regulatory certification.

I’d measure at the same time of day, in the same state, on the same device, and average three mornings. Switching devices midway gives you a trend that is really a change of instrument.

When is a TDEE calculator with body fat worth using?

Use it if you have a repeatable body fat measurement and your build is far from average, such as regular heavy lifting. Run both versions. NASEM’s standard error for its own equations is 241 kcal for women and 342 kcal for men. A gap of about 150 kcal between two methods is smaller than that, so it is noise. Take the average and move on. If they differ by more, I trust neither until three weeks of weigh-ins break the tie. That 150 kcal line is my own rule of thumb.

Skip it if your body fat figure comes from a guess or a different device each month. The weight-based equation will serve you about as well, and how far off TDEE calculators still are explains why neither is exact. Both equations are for adults and don’t cover pregnancy.

Calsa reads body fat from your scale through Apple Health or Health Connect, and shows weight as a smoothed trend instead of daily jumps.