Katch-McArdle vs Mifflin-St Jeor comes down to a single question: does your formula know how much muscle you carry, or is it guessing from the outside? Mifflin-St Jeor estimates your basal metabolic rate from total weight, height, age, and sex — the same four inputs almost every calculator asks for. Katch-McArdle throws those out and works from one number instead: your lean body mass, which you get from your body-fat percentage. That difference sounds academic until you run the same person through both and watch the results split apart. This page walks through how each formula works, does the arithmetic on a real example, and tells you which one is actually worth using.
TDEE Calculator
Total Daily Energy Expenditure — your real maintenance calories.
Biological sex
BMR formulas use biological sex, not gender identity.
Leave blank if unsure — we'll fall back to Mifflin-St Jeor (the standard).
Most people overestimate this. When in doubt, drop one tier.
Both formulas are trying to predict the same thing — how many calories your body burns at complete rest over 24 hours. They just look at your body differently.
Mifflin-St Jeor sees your outside: a 68 kg, 165 cm, 30-year-old woman. It has no idea whether that 68 kg is mostly muscle or mostly fat, so it assumes an average body composition baked into its coefficients and gives you a number. For most people that assumption is close enough, which is exactly why it's the default on this site and nearly everywhere else.
Katch-McArdle ignores your height, age, and sex entirely. It only wants to know how many kilograms of you are lean tissue — muscle, bone, organs, water — because that lean mass is what actually burns calories at rest. Fat mass burns very little. So Katch-McArdle asks for your body-fat percentage, strips the fat out of the equation, and calculates from what's left.
That's the whole story. Mifflin guesses your composition; Katch-McArdle measures it. Whether that upgrade helps or hurts depends entirely on whether your body-fat number is honest — and whether your composition is unusual enough for the guess to be wrong.
Katch-McArdle vs Mifflin-St Jeor, side by side
The two equations at a glance. Notice how few inputs Katch-McArdle uses — and that all of its accuracy rides on one of them:
Factor
Mifflin-St Jeor
Katch-McArdle
Core input
Total body weight
Lean body mass (from body-fat %)
Also uses
Height, age, sex
Nothing else
Female formula
10W + 6.25H − 5A − 161
370 + 21.6 × LBM
Male formula
10W + 6.25H − 5A + 5
370 + 21.6 × LBM (same for both sexes)
Needs body-fat %?
No
Yes — required
Accounts for muscle?
No — assumes average
Yes — directly
Separate male/female math
Yes
No — sex is already captured in LBM
Best for
General adults
Lean, muscular, or athletic people who know their body fat
Weak spot
Off for unusual body composition
Only as good as your body-fat estimate
W = weight in kg, H = height in cm, A = age in years, LBM = lean body mass in kg = weight × (1 − body-fat fraction).
How each formula actually works
Mifflin-St Jeor is a plain linear equation. Plug in weight in kilograms, height in centimetres, age in years, add or subtract the sex constant (+5 for men, −161 for women), and you get BMR in kcal per day. It came out of a 1990 study by Mark Mifflin and Sachiko St Jeor that measured resting metabolism in 498 healthy adults, and it's been the clinical default ever since — you can read the original paper through PubMed. If you want the full step-by-step, our how-to-calculate-tdee page walks the arithmetic in detail.
Katch-McArdle takes a detour through your body composition first. Before you can use it, you need lean body mass, which means you need body-fat percentage. Lean body mass is just your weight with the fat subtracted: if you weigh 68 kg at 25% body fat, then 25% of 68 kg (17 kg) is fat, and the remaining 51 kg is lean. Our lean-body-mass-calculator does this step if you'd rather not do it by hand. Once you have LBM in kilograms, the formula is short: BMR = 370 + 21.6 × LBM. That single '21.6 per kilogram of lean tissue' is the entire model — it treats every kilogram of lean mass as burning the same amount, and adds a flat 370 kcal baseline on top.
Because Katch-McArdle already reads sex indirectly (men carry more lean mass at the same weight, so their LBM is higher), it doesn't need a separate male and female version. The one equation covers everyone. That's elegant, but it also means the formula lives or dies on the accuracy of your body-fat number — garbage in, garbage out. If you want to try both against your own stats, the tdee-calculator-with-body-fat page runs the body-fat-aware version directly.
Field note from Sukie
When I ran my own numbers through both
I'm a decent test case for this because my body fat sits a little lower than average for my size, which is exactly the situation where the two formulas disagree. My stats: 30 years old, 68 kg, 165 cm, and a body fat around 25% from a DEXA scan I did a couple of years ago (not a bathroom scale guess — those read all over the place).
Now Katch-McArdle. First my lean body mass: 68 kg × (1 − 0.25) = 68 × 0.75 = 51 kg of lean tissue. Then BMR = 370 + (21.6 × 51) = 370 + 1,101.6 = about 1,472 kcal/day.
So Mifflin says 1,400 and Katch-McArdle says 1,472 — a 72 kcal gap, with Katch reading higher. That direction is the tell. Because 25% body fat is a bit leaner than the 'average' composition Mifflin quietly assumes, Katch-McArdle credits me for carrying more muscle than a typical 68 kg woman, and rewards it with a higher resting burn. If I were carrying more fat at the same weight, the two would have crossed and Katch would have come in lower.
Multiply both by my honest activity factor — I lift three times a week and walk a lot, so 1.375 — and the day-level numbers become 1,925 vs 2,024 TDEE. About a 100 kcal/day difference. Not huge, but over a three-month cut it's the difference between a deficit that feels right and one that stalls. In practice I split the difference, ate around 1,700 to lose, and let two weeks of scale data tell me who was right. (Spoiler: real-world weigh-ins beat both formulas every time.)
— Sukie
How the two formulas diverge as body fat changes
Here's the part most people miss. Keep the same woman fixed — 68 kg, 165 cm, 30 years old, so her Mifflin BMR stays at 1,400 — and only change her body-fat percentage. Watch what Katch-McArdle does:
Body fat %
Lean mass (kg)
Katch-McArdle BMR
vs Mifflin (1,400)
15%
57.8
1,618
+218 (Katch higher)
20%
54.4
1,545
+145 (Katch higher)
25%
51.0
1,472
+72 (Katch higher)
30%
47.6
1,398
−2 (roughly equal)
35%
44.2
1,325
−75 (Katch lower)
40%
40.8
1,251
−149 (Katch lower)
The crossover sits near 30% body fat for this profile. Below it, Katch-McArdle reads higher than Mifflin (you carry more muscle than average); above it, Katch reads lower (you carry more fat). Mifflin can't see any of this — its number never moves, because body fat isn't one of its inputs.
When Katch-McArdle beats Mifflin-St Jeor — and when it doesn't
The formula isn't 'better' in the abstract. It's better in specific situations and actively worse in others:
Use Katch-McArdle if you're lean and muscular. An athlete at 12% body fat carries far more lean mass than Mifflin's average assumption, so Mifflin will underestimate their burn. Katch corrects for it.
Use Katch-McArdle if you have a reliable body-fat number — DEXA, a good BIA scale used consistently, or skinfold calipers done by someone competent. The formula is only as trustworthy as this input.
Use Katch-McArdle if you're recomping. As muscle replaces fat at a stable weight, Mifflin's number won't budge, but Katch will climb as your lean mass rises — which better reflects reality.
Stick with Mifflin-St Jeor if you're guessing your body fat. A visual estimate or a fluctuating bathroom-scale reading can be off by 5–8 points, which swings Katch-McArdle by 150+ kcal. A guessed Katch is worse than an honest Mifflin.
Stick with Mifflin-St Jeor if you're average build. Near the crossover point, the two formulas basically agree, so the extra step buys you nothing.
Stick with Mifflin-St Jeor for very high body fat. At the obese end, Katch-McArdle tends to underestimate because the flat 370 baseline and single per-kilo coefficient stop fitting well — Mifflin was validated across a wider weight range.
Short version: leaner and more muscular than average, with a real measurement → Katch-McArdle. Everyone else → Mifflin-St Jeor.
So which should you use?
For most people, most of the time: Mifflin-St Jeor. It needs no body-fat measurement, it's validated across a broad population, and when your composition is close to average — which is what 'average' means — it lands right on top of Katch-McArdle anyway. There's no prize for using the fancier formula if it just reproduces the simpler one.
Switch to Katch-McArdle only when you have a genuine reason: you're notably lean or muscular, you're tracking a body recomposition, and you have a body-fat number you actually trust. In those cases Mifflin's blindness to composition is a real weakness, and Katch-McArdle's focus on lean mass is a real fix. If that's you, the tdee-calculator-with-body-fat page is built for exactly this, and the bmr-calculator will show you the resting figure on its own.
Either way, remember what these formulas are: educated starting points, not measurements. My own numbers came out 72 kcal apart, and neither one was 'the truth' — the truth was whatever the scale showed after two weeks of eating a fixed amount. Pick the formula that fits your body, use it to set an opening target, then let your own data correct it. If you're still deciding between the classic equations, the mifflin-st-jeor-vs-harris-benedict comparison covers the other rivalry worth knowing. The formula gets you in the right neighbourhood; the eat-and-weigh test finds the house.
Is Katch-McArdle more accurate than Mifflin-St Jeor?+
Only for the right person. For lean, muscular individuals with a reliable body-fat measurement, Katch-McArdle is usually more accurate because it calculates from lean body mass rather than assuming an average composition. For people with average build or a guessed body-fat number, Mifflin-St Jeor is just as good or better.
Do I need to know my body fat % to use Katch-McArdle?+
Yes — it's mandatory. Katch-McArdle works entirely from lean body mass, which you can only get from your body-fat percentage. Without a body-fat number you literally cannot run the formula, which is why calculators fall back to Mifflin-St Jeor when body fat is left blank.
Which formula does calculator.net use?+
Calculator.net's BMR tool defaults to Mifflin-St Jeor and also offers Harris-Benedict and Katch-McArdle as selectable options — but Katch-McArdle only activates once you enter a body-fat percentage. This site works the same way: Mifflin by default, Katch-McArdle when you supply body fat.
Why did Katch-McArdle give me a higher number than Mifflin?+
Because you're leaner than average for your weight. When your body fat is below roughly 28–30%, Katch-McArdle credits you with more lean mass than Mifflin assumes and returns a higher BMR. Above that point the two cross over and Katch reads lower.
Does Katch-McArdle use different math for men and women?+
No — it uses one equation for everyone: 370 + 21.6 × lean body mass in kg. Sex is already reflected in your lean mass (men carry more at the same weight), so there's no separate male and female version the way Mifflin-St Jeor has.
How do I convert body fat percentage into lean body mass?+
Subtract your fat weight from your total weight. Lean body mass = weight × (1 − body-fat fraction). At 68 kg and 25% body fat: 68 × 0.75 = 51 kg of lean mass. Our lean body mass calculator does it for you if you'd rather not do the arithmetic.
Is the Cunningham equation the same as Katch-McArdle?+
Nearly. Cunningham (500 + 22 × LBM) is a close cousin, tuned on an athletic population, and gives slightly higher numbers than Katch-McArdle. Both work from lean body mass; for everyday use they're close enough that the choice rarely matters.
If I don't trust my body-fat measurement, which formula should I pick?+
Mifflin-St Jeor. A body-fat estimate that's off by 5–8 points can swing Katch-McArdle by more than 150 kcal, which defeats the point of the more precise-looking formula. An honest Mifflin beats a Katch built on a bad guess every time.