Mifflin-St Jeor vs Katch-McArdle: Which TDEE Formula Wins?

Mifflin-St Jeor wins for the general population, predicting resting metabolic rate within 10 percent of measured values in more adults than any other simple equation; Katch-McArdle wins for trained athletes and lean individuals with a recent body composition scan. The two equations ask different questions. Mifflin needs only height, weight, age and sex. Katch-McArdle needs lean body mass. As of May 2026, the Academy of Nutrition and Dietetics still recommends Mifflin-St Jeor as the default for clinical practice.

Calorie tracking apps quietly choose one of these formulas for you and call the output your TDEE. Most pick Mifflin-St Jeor because it is the most validated. A handful, especially those aimed at lifters, default to Katch-McArdle. The choice matters: the same person can see TDEE estimates 150-200 kcal apart depending on which equation runs in the background.

This guide compares both formulas, shows the math step-by-step in Australian (kg, cm, kJ) and US (lb, in, kcal) units, summarises what research suggests about accuracy, and explains which to use for your body. We will not skip the algebra most apps hide.

What is the difference between Mifflin-St Jeor and Katch-McArdle?

Mifflin-St Jeor estimates basal metabolic rate (BMR) from total body weight, height, age and sex. Katch-McArdle estimates it from lean body mass alone. Mifflin treats every kilogram the same. Katch-McArdle assumes fat tissue is almost metabolically silent and only credits your fat-free mass.

The two equations come from different research programmes. Mifflin-St Jeor was published in The American Journal of Clinical Nutrition in 1990 by Mifflin, St Jeor, and colleagues, who measured resting energy expenditure by indirect calorimetry in 498 healthy adults aged 19 to 78 and built a regression model with R² = 0.71. Katch-McArdle traces back to lean-mass research by Frank Katch and William McArdle and is mathematically identical to the older Cunningham formula.

Here are the two formulas side by side in both metric and US units.

The big practical difference: Mifflin-St Jeor does not need a body fat reading. Katch-McArdle does. If your only body fat estimate comes from a smart scale or a guess, Katch-McArdle inherits that error directly into your BMR.

How accurate is each TDEE formula in 2026?

A 2005 systematic review in the Journal of the American Dietetic Association found Mifflin-St Jeor predicted resting metabolic rate within 10 percent of measured values more often than any competing equation, and with the narrowest error range. A 2023 athlete meta-analysis found lean-mass-based equations like Katch-McArdle and Cunningham performed better in low-body-fat populations.

The Frankenfield 2005 review evaluated Mifflin-St Jeor against Harris-Benedict, Owen, and WHO/FAO/UNU equations across multiple studies of nonobese and obese adults. Mifflin was the most reliable in both groups. The Academy of Nutrition and Dietetics adopted this as their official recommendation for clinical practice and reaffirmed it in their adult weight management guideline.

That picture flips in athletes. A 2023 systematic review and meta-analysis in Sports Medicine pooled 23 studies and 1,058 athletes. Mifflin-St Jeor systematically underestimated RMR in this population, while Cunningham (mathematically identical to Katch-McArdle), Ten-Haaf, and a few other lean-mass-based formulas tracked measured values without significant bias. The Ten-Haaf equation hit ±10 percent of measured RMR in 80.2 percent of athletes, far above the 40-64 percent that Mifflin-St Jeor achieved.

Accuracy also drops in specific populations. A 2022 validation study in older adults with obesity (mean age 73, BMI ≥30) found Mifflin-St Jeor under-predicted measured RMR by 548 kcal on average. That study favoured WHO/FAO/UNU and Harris-Benedict for that population, though research suggests no simple equation handles older obese adults well.

Here is a rough accuracy snapshot:

How do you calculate TDEE step-by-step in AU and US units?

You calculate BMR first, then multiply by an activity factor between 1.2 (sedentary) and 1.9 (very heavy training). Convert the kcal result to kilojoules for AU labels by multiplying by 4.184. Most calorie apps hide both the activity factor and the BMR step. Doing the math by hand once is the best way to spot where your app is wrong.

The standard activity multipliers, popularised after the original Katch & McArdle textbook and now used across the calorie-tracking industry, are:

Most apps overestimate this factor. If you commute by car, sit at a desk, and lift three times a week, you are probably between 1.375 and 1.55, not 1.725. The math compounds: misreading by one tier inflates daily calories by 250-400 kcal, which is enough to stall a calorie deficit entirely.

Worked example: AU units (kilojoules and centimetres)

Imagine you are a 35-year-old woman, 165 cm, 70 kg, moderately active (PAL ≈ 1.55), with a DEXA body fat reading of 28 percent.

Mifflin-St Jeor:

• BMR = (10 × 70) + (6.25 × 165) − (5 × 35) − 161
• BMR = 700 + 1,031.25 − 175 − 161 = 1,395 kcal/day
• TDEE = 1,395 × 1.55 ≈ 2,162 kcal/day
• In kilojoules: 2,162 × 4.184 ≈ 9,046 kJ/day

• Lean body mass = 70 × (1 − 0.28) = 50.4 kg
• BMR = 370 + (21.6 × 50.4) = 370 + 1,088.6 = 1,458 kcal/day
• TDEE = 1,458 × 1.55 ≈ 2,260 kcal/day
• In kilojoules: 2,260 × 4.184 ≈ 9,456 kJ/day

Worked example: US units (kcal, pounds, inches)

Now consider a 28-year-old man, 5 ft 10 in (178 cm, 70 in), 175 lb (79.4 kg), training hard six days a week (PAL ≈ 1.725), with a body fat reading of 10 percent from a hydrostatic scan.

Mifflin-St Jeor (metric form):

• BMR = (10 × 79.4) + (6.25 × 178) − (5 × 28) + 5
• BMR = 794 + 1,112.5 − 140 + 5 = 1,772 kcal/day
• TDEE = 1,772 × 1.725 ≈ 3,057 kcal/day

• Lean body mass = 79.4 × (1 − 0.10) = 71.5 kg
• BMR = 370 + (21.6 × 71.5) = 370 + 1,544.4 = 1,914 kcal/day
• TDEE = 1,914 × 1.725 ≈ 3,302 kcal/day

If you do not have a DEXA, hydrostatic, or BodPod reading, smart-scale body fat is too noisy to feed into Katch-McArdle reliably. Skinfold calipers used by a trained practitioner are an acceptable middle ground.

Which TDEE formula should you use for your body?

Use Mifflin-St Jeor if you do not have a recent and accurate body fat percentage, which covers most people. Use Katch-McArdle if your body fat is below roughly 20 percent and you have a DEXA, hydrostatic, BodPod, or trained-skinfold reading from the last six months.

Five practical decision rules:

These are the same rules registered dietitians apply, with one twist: most apps do not ask which formula you want. KCALM defaults to Mifflin-St Jeor and lets you adjust based on tracked outcomes rather than swapping equations.

Why do calorie apps disagree about your TDEE?

Two apps using the same Mifflin-St Jeor equation can produce TDEE estimates 200-400 kcal apart for the same person because they pick different activity multipliers, round inputs differently, or apply hidden adjustments for goal weight. The formula is the easy part. The activity factor is where most of the error lives.

Some apps cap activity at 1.55 even if you select "very active". Others bake a 200 kcal deficit into "maintenance" by default. A few use older Harris-Benedict revisions instead of Mifflin and never tell you. If your app shows you a single number with no working, you cannot tell whether the disagreement is the equation, the activity factor, or a hidden goal adjustment.

The fix is simple: calculate your BMR by hand once, decide your activity multiplier honestly, and treat the app's number as a starting estimate. Track for two weeks at the resulting calorie target and compare actual weight change against the predicted change. If your weight is flat at a supposed 500 kcal deficit, your real maintenance is probably 500 kcal lower than the app says. This calibration matters more than the formula choice, especially during a sustained cut, where metabolic adaptation can pull TDEE down further.

For the deeper science behind the Mifflin equation specifically, see our Mifflin-St Jeor explainer. For practical macro splits once your TDEE is set, see the body recomposition guide.

Frequently Asked Questions

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

Not for most people. Research suggests Katch-McArdle wins in trained athletes and lean individuals because Mifflin penalises every kilogram of bodyweight equally, including muscle. In the general population, Mifflin-St Jeor predicts resting metabolic rate within 10 percent of measured values more often than Katch-McArdle, and the Academy of Nutrition and Dietetics adopted Mifflin as the default recommendation in 2014. Katch-McArdle only outperforms when your body fat percentage is measured accurately, usually via DEXA, hydrostatic weighing, or BodPod.

Do Australian dietitians use Mifflin-St Jeor or NHMRC reference values?

Most use both. The NHMRC Estimated Energy Requirements published in the Nutrient Reference Values for Australia and New Zealand are population-level guidance derived from FAO/WHO methodology and reported in kilojoules. For individual clients, Australian Accredited Practising Dietitians commonly use Mifflin-St Jeor for BMR and then apply a physical activity level (typically 1.4 to 1.75) consistent with NHMRC guidance, before converting the final number to kilojoules.

What is the difference between BMR, RMR, and TDEE?

BMR is the energy your body burns at complete rest, measured under tightly controlled conditions including overnight fasting and a 24-hour rest. RMR is a slightly looser version measured after a few hours rest and a short fast; it is typically 5-10 percent higher than true BMR. TDEE adds the calories you burn from movement, exercise, and the thermic effect of food on top. In practice, calorie apps treat BMR and RMR as interchangeable, then multiply by an activity factor to estimate TDEE.

How accurate is a smart scale's body fat reading for Katch-McArdle?

Not accurate enough to rely on. Bioelectrical impedance smart scales typically show errors of 4 to 8 percentage points compared to DEXA, and the error increases at very low or very high body fat. Feeding a wrong body fat percentage into Katch-McArdle propagates that error directly into BMR. If your only reading comes from a household scale, Mifflin-St Jeor is the safer default.

How often should you recalculate your TDEE?

Recalculate any time your weight changes by more than 3-5 kg, or every 8-12 weeks during an active diet phase. Both Mifflin-St Jeor and Katch-McArdle use weight as a direct input, so a lighter bodyweight produces a lower predicted BMR. Recalculating also surfaces metabolic adaptation, where measured TDEE falls below the formula's prediction. For people coming off a long deficit, see our reverse dieting guide for how to raise calories without rapid weight regain.

Can KCALM switch between Mifflin-St Jeor and Katch-McArdle?

KCALM uses Mifflin-St Jeor by default to match what registered dietitians recommend for the general population. The app calibrates your maintenance calories from your actual logged intake and weight trend over the first 14 days, which corrects for activity-factor errors regardless of which formula generated the starting estimate. If you have a recent DEXA reading and want to start from Katch-McArdle instead, the TDEE calculation guide shows the math.

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