What is the Katch–McArdle formula and why is it different?

Katch–McArdle (1996) calculates BMR using only lean body mass (LBM): BMR = 370 + 21.6 × LBM(kg). Because body fat is metabolically near-inert, removing it from the equation makes the formula independent of sex and height — the two variables that most confound other equations. A 70 kg man at 10% body fat has 63 kg of LBM; a 70 kg woman at 30% body fat has 49 kg. Their Mifflin BMRs differ modestly, but their Katch–McArdle BMRs differ by ~302 kcal — a gap that standard equations hide inside crude sex coefficients.

What is the Cunningham formula?

The Cunningham (1980) equation is an earlier lean-mass formula: BMR = 500 + 22 × LBM(kg). It was derived primarily from athletic populations and tends to run 50–150 kcal/day higher than Katch–McArdle. Strength athletes and bodybuilders often find Cunningham closer to their measured RMR because muscle at high training volumes has a somewhat elevated metabolic cost per kilogram. Both formulas are shown side by side so you can bracket the likely range.

How accurate is Katch–McArdle versus Mifflin–St Jeor?

In most validation studies, Katch–McArdle outperforms Mifflin–St Jeor for people with a known body fat percentage, with mean errors typically under 5% versus 7–10% for Mifflin. The catch is that you need an accurate body fat reading — a 5 percentage point error in BF% translates directly into a ~54 kcal/day BMR error for a 50 kg LBM. If you only have height and weight, Mifflin–St Jeor is still the better choice than guessing your body fat.

How do I measure my body fat percentage accurately?

In descending order of accuracy: DEXA scan (±1–2%, gold standard), BodPod air displacement plethysmography (±2–3%), hydrostatic weighing (±2–3%), multi-site skinfold callipers by a trained technician (±3–4%), bioelectrical impedance (±3–5% under ideal hydration conditions). Consumer smart scales using BIA are convenient but can be off by 5–8% depending on hydration. Even an estimated figure from a visual body fat chart moves Katch–McArdle much closer to reality than omitting it entirely.

What does the "solve for body fat" mode do?

It inverts the Katch–McArdle equation. Given your target TDEE and activity level, the calculator derives the BMR you need, then works back through BMR = 370 + 21.6 × LBM to find the required lean body mass, and finally divides by your total body weight to give the required body fat percentage. This tells you, concretely, how lean you need to be to reach a specific metabolic rate — useful when setting fat loss targets or planning a body recomposition.

Why does body fat percentage matter so much for TDEE?

Fat tissue burns roughly 4–5 kcal per kg per day, while lean tissue (muscle, organs, bone) burns around 20–30 kcal per kg per day. Two people with the same body weight but different body compositions can easily differ by 200–400 kcal/day in resting metabolic rate. This is why a lean 80 kg athlete eats far more than a sedentary 80 kg person at the same apparent weight — their TDEE is genuinely higher, not just because of exercise output but because their resting engine runs hotter.

TDEE Calculator — Katch–McArdle & Cunningham

This TDEE calculator uses the Katch–McArdle formula — the lean-mass-based BMR equation that skips height and sex entirely and works directly from the tissue that actually drives your metabolism. Enter your total body weight and body fat percentage, pick an activity multiplier, and you get three BMR estimates side by side (Katch–McArdle, Cunningham, and Mifflin–St Jeor for comparison), your full TDEE, a calorie goal table for every deficit and surplus, a macro split, and a 12-week weight projection chart.

There is also a unique solve-for-body-fat mode: type a target TDEE and the calculator works backwards to tell you exactly what body fat percentage you need to achieve that metabolic rate at your current body weight — useful for body recomposition planning.

How it works

All two lean-mass formulas share the same input: lean body mass (LBM) in kilograms.

LBM = total weight × (1 − body fat% ÷ 100)

The Katch–McArdle (1996) formula then gives resting BMR as:

BMR = 370 + 21.6 × LBM (kg)

The Cunningham (1980) variant, calibrated to athletic subjects, uses:

BMR = 500 + 22 × LBM (kg)

Both equations are independent of sex and height. The reasoning is physiological: adipose tissue is nearly metabolically inert (it contributes roughly 4–5 kcal/kg/day vs. roughly 20–30 kcal/kg/day for lean tissue), so once you know how much metabolically active tissue you carry, sex and height become redundant predictors. Katch–McArdle and Cunningham effectively strip out the “dead weight” that inflates or deflates predictions in the Mifflin and Harris-Benedict equations.

TDEE is then:

TDEE = BMR × activity factor

Activity factors range from 1.2 (sedentary — desk job, no exercise) to 1.9 (extra active — physical job plus twice-daily training). The activity multiplier is the single biggest lever outside body composition itself: moving from sedentary to moderately active at the same body composition adds roughly 35% to your daily energy requirement.

Worked example

A 30-year-old weighing 80 kg at 18% body fat, moderately active (×1.55):

LBM = 80 × (1 − 0.18) = 65.6 kg
BMR (Katch–McArdle) = 370 + 21.6 × 65.6 = 370 + 1,417 = 1,787 kcal/day
BMR (Cunningham) = 500 + 22 × 65.6 = 500 + 1,443 = 1,943 kcal/day
TDEE (K–M) = 1,787 × 1.55 = ~2,770 kcal/day

Compare the Mifflin–St Jeor result for the same man (178 cm tall): BMR = 10 × 80 + 6.25 × 178 − 5 × 30 + 5 = 1,780 kcal/day — close in this case, but the gap widens significantly for very lean or very muscular subjects.

Body fat %	LBM	Katch–McArdle BMR	TDEE (×1.55)
10% (lean)	72.0 kg	1,925 kcal	2,984 kcal
18% (average)	65.6 kg	1,787 kcal	2,770 kcal
25% (higher)	60.0 kg	1,666 kcal	2,582 kcal
32% (high)	54.4 kg	1,545 kcal	2,395 kcal

The table illustrates why two 80 kg people can have a 590 kcal/day difference in TDEE — not from activity but purely from body composition. It also shows why most people who have dieted and regained weight find maintenance harder: both fat gain and muscle loss reduce LBM and therefore BMR.

Formula note

The coefficient 21.6 in Katch–McArdle and 22 in Cunningham represent the average metabolic cost of lean tissue in kcal per kilogram per day at rest. The constant term (370 or 500) accounts for the small but non-zero metabolic activity of fat tissue and the fixed overhead of vital organ function. Cunningham’s higher constant reflects the fact that his original sample was drawn from trained athletes with proportionally more high-metabolic-rate organ tissue (heart, liver, kidneys) per unit body mass.

Neither formula adjusts for age because aging reduces BMR primarily through the loss of lean mass rather than any intrinsic reduction in lean-tissue metabolism — if LBM is already measured, the age adjustment is redundant.