TDEE Calculator AU + US: The Formulas Apps Get Wrong

TDEE is your Total Daily Energy Expenditure — the kilojoules or kilocalories your body burns in a day. The most evidence-backed formula for everyday adults is Mifflin-St Jeor (1990, n=498), which predicts resting metabolic rate within 10 percent of measured values for 82 percent of community-living adults according to a 2013 Clinical Nutrition validation study. Multiply Mifflin-St Jeor BMR by an activity factor between 1.2 and 1.9 to get TDEE in kcal/day, then multiply by 4.184 to convert to kJ — the unit on every Australian food label.

If you have ever typed your stats into three different calorie apps and seen three different daily targets, you are not imagining things. Apps choose different BMR equations, different activity multipliers, and different rounding rules under the hood, and you only see the final number. As of May 2026, the four most commonly used equations can spread the same person's TDEE by 300-500 kcal a day — easily the difference between losing fat and gaining it.

This guide walks through TDEE step by step in both Australian (kilograms, centimetres, kilojoules) and US (pounds, inches, kilocalories) units, shows the math for three real worked examples, and pulls back the curtain on the four errors most calorie apps make when they calculate your number.

How do you calculate your TDEE in AU and US units?

You calculate TDEE in two steps: estimate your basal metabolic rate (BMR) with a validated equation such as Mifflin-St Jeor, then multiply by a Physical Activity Level (PAL) factor between 1.2 (sedentary) and 1.9 (extra active). The output is kcal/day; multiply by 4.184 to convert to kJ/day, the standard unit on Australian food labels and menu boards.

The Mifflin-St Jeor formula, published in the American Journal of Clinical Nutrition in 1990 from a sample of 498 adults aged 19-78, is the equation most commonly cited by the Academy of Nutrition and Dietetics. It takes weight, height, age, and sex.

The activity multipliers most apps use trace back to a Harris-Benedict-era convention and broadly map to NHMRC's Physical Activity Level (PAL) ranges. PAL is the multiple of BMR you spend each day, and NHMRC notes adult PAL "ranges from 1.2 (bed rest) to 2.2 (very active or heavy occupational work)" — with values of 1.75 and above consistent with good health.

Which TDEE formula is most accurate?

Mifflin-St Jeor is the most accurate single equation for the general adult population — research suggests it predicts resting metabolic rate within 10 percent of measured values for 82 percent of community-living adults and 71-80 percent of women across BMI categories. Katch-McArdle, which uses lean body mass, is preferred when your body fat percentage has been measured recently. Harris-Benedict (1919) is widely cited but tends to overestimate by 5-15 percent in modern populations.

The Frankenfield 2005 systematic review in the Journal of the American Dietetic Association compared four equations — Harris-Benedict, Mifflin-St Jeor, Owen, and WHO/FAO/UNU — against measured resting metabolic rate in healthy non-obese and obese adults. The authors concluded Mifflin-St Jeor "was the most reliable, predicting RMR within 10 percent of measured in more nonobese and obese individuals than any other equation, and it also had the narrowest error range."

A 2013 follow-up study in Clinical Nutrition tested Mifflin-St Jeor in community-living adults and found it "unbiased (95 percent confidence interval -26 to +8 kcal/day)," with 82 percent of estimates falling within 10 percent of measured RMR. Accuracy dipped to 75 percent in obese participants and rose to 87 percent in non-obese ones. A 2020 Journal of Nutritional Science study of 125 women across BMI 17-44 found 71 percent accuracy overall, climbing to 80 percent for women with obesity but falling to 55 percent for underweight women.

Katch-McArdle, developed by Frank Katch and William McArdle in 1975, calculates BMR from lean body mass alone:

BMR (kcal/day) = 370 + (21.6 × lean body mass in kg)

Because muscle tissue is more metabolically active than fat, the Katch-McArdle equation tends to perform better than Mifflin-St Jeor for athletes, lifters, and anyone with body fat noticeably above or below population averages — provided your lean body mass figure comes from a DXA scan, BIA, or skinfold measurement rather than a guess. Without an accurate body fat percentage, Katch-McArdle is no better than Mifflin-St Jeor and is often worse. See our Mifflin-St Jeor vs Katch-McArdle deep dive for the side-by-side decision tree.

How do worked TDEE examples compare in AU and US units?

The same person can produce TDEE estimates 100-200 kcal/day apart depending on which equation is used, even before activity multiplier choice. A 35-year-old, 70 kg, 168 cm moderately active woman comes out at 2,192 kcal (9,170 kJ) on Mifflin-St Jeor and 2,354 kcal (9,849 kJ) on Katch-McArdle — a daily gap of 162 kcal, or roughly a slice of wholemeal bread with butter.

Worked example 1 — Sarah, 35, woman, 70 kg / 154 lb, 168 cm / 66.1 in, body fat 24 percent, lightly active (multiplier 1.375):

Why do most calorie apps get TDEE wrong?

Most calorie apps get TDEE wrong in four predictable ways: they use a single fixed activity multiplier that does not match your week, they ignore the cooked-versus-raw drift in the food side of the equation, they rely on a single formula without offering Katch-McArdle as an option for trained users, and they fail to update your number as you lose or gain weight. Each of these alone can shift your daily target by 100-300 kcal in a direction you cannot see.

The four common errors:

Compounded, these four errors are why two friends can run identical diets, hit identical app targets, and see completely different results on the scale. Research suggests the average self-reported food intake is underreported by 20-30 percent even before the app's TDEE estimate is layered on top.

How should you adjust your TDEE for weight goals?

For sustainable weight loss, subtract 10-20 percent from your maintenance TDEE — typically 300-500 kcal/day, or 1,250-2,090 kJ/day. To gain weight or build muscle, add 10-15 percent. Re-check the number every 4-6 weeks, because your body adapts to lower intakes and your real-world activity rarely stays constant. See our calorie deficit vs surplus guide for the full goal-setting framework.

A reasonable AU-units adjustment for the three worked examples above:

• Sarah (Mifflin TDEE 8,134 kJ): a 15 percent deficit gives a target of 6,914 kJ/day, or about 1,653 kcal.
• James (Mifflin TDEE 11,464 kJ): a 15 percent deficit gives a target of 9,744 kJ/day, or about 2,329 kcal.
• Mary (Mifflin TDEE 6,158 kJ): a 10 percent deficit gives a target of 5,542 kJ/day, or about 1,325 kcal. The smaller deficit reflects the harder time older, lower-TDEE adults have sustaining steep cuts.

For AU readers who eat out often, our chain restaurant calorie guide pairs well with this calculator: it explains how to use the legally required kJ labels on menu boards to stay inside your daily target. Pair the TDEE math here with a barcode-scanning workflow and an AU-prioritised food database, and the gap between calculated target and real-world result shrinks.

Frequently Asked Questions

Why does my TDEE keep dropping as I lose weight?

Your BMR is roughly proportional to weight, so every kilogram you lose subtracts 8-12 kcal/day from BMR and 12-20 kcal/day from TDEE. On top of that, the body adapts to sustained energy deficits through a process called adaptive thermogenesis, which can lower TDEE by an additional 5-10 percent. Recalculate every 4-6 weeks during a cut.

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

BMR (Basal Metabolic Rate) is the energy your body uses at complete rest, measured under strict laboratory conditions. RMR (Resting Metabolic Rate) is measured under more relaxed conditions and is roughly 10 percent higher than true BMR. TDEE adds everything else — daily activity, exercise, the thermic effect of food, and non-exercise activity thermogenesis (NEAT).

Why is my calculated TDEE so different from what my smartwatch shows?

Smartwatches estimate energy expenditure from heart rate and accelerometer data, which research suggests can be off by 20-50 percent at the individual level depending on activity type. Equation-based TDEE and watch-based TDEE measure different things; treat them as two independent estimates and trust the four-week scale trend over either.

Should I use my goal weight or my current weight in the formula?

Always use your current weight. Calculating TDEE for your goal weight gives you the maintenance calories of a person you are not yet, which can produce an aggressive deficit that backfires. Recalculate at your new weight every 4-6 weeks as you lose, and adjust the deficit accordingly.

Does TDEE differ for women on a menstrual cycle?

Research suggests BMR rises 2-12 percent in the luteal phase (after ovulation) compared with the follicular phase, equivalent to roughly 80-150 extra kcal/day during the second half of the cycle. Most calculators ignore this. If you track carefully through a full cycle, you may see slightly easier weight loss and more visible water-weight fluctuations between the two phases.

Can I just use the 8,700 kJ figure on Australian menu boards as my target?

No. The 8,700 kJ value is a Food Standards Code reference for an "average adult" used to calculate percentage daily intake on labels and menus. It is not a personalised TDEE. Your real number could be anywhere from 6,000 kJ for a small, sedentary older adult to 14,000+ kJ for a tall, very active young adult. Use the formulas above instead.

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