Pick up almost any packet of food in an Australian supermarket and the energy is listed in kilojoules. Open a fitness app or a US recipe site, though, and suddenly it's calories. No wonder so many people end up doing rough mental maths in the cereal aisle, wondering if 1,200 kJ for breakfast is a lot or not much at all.
Here's the short version: kilojoules and calories measure the exact same thing — energy. Australia just uses kJ as the official unit on food labels, while a lot of the fitness world (especially anything coming out of the US) still runs on calories. Once you've got the conversion sorted, working out your daily targets gets a lot easier.
Calories to KJ — The Quick Conversion
Multiply by 4.184 and a calorie becomes a kilojoule. Most people round that to 4.2, which is close enough for everyday use — divide instead of multiply to go the other way, from kJ back to calories.
A few common numbers, side by side:
| Calories | Kilojoules (Approx.) |
|---|---|
| 100 cal | 420 kJ |
| 200 cal | 840 kJ |
| 500 cal | 2,100 kJ |
| 1,000 cal | 4,200 kJ |
| 1,500 cal | 6,300 kJ |
| 2,000 cal | 8,400 kJ |
| 2,500 cal | 10,500 kJ |
Why does Australia use kJ at all? It comes down to food labelling law — Food Standards Australia New Zealand requires energy content to be listed in kilojoules on packaged food. Calories aren't banned, but they're not the legal standard, so most local brands skip them entirely. If you've ever wondered why your protein bar says "980 kJ" with no calorie number in sight, that's the reason.
How Many Kilojoules Should You Eat Per Day?
8,700 kJ gets thrown around a lot as the "average" figure for an Australian adult — Healthdirect cites it as the daily energy requirement to maintain a healthy weight, and it's also the reference value behind %DI labels on packaged food. Worth remembering it's a population average, not a number tailored to you specifically.
Converted to calories, that's roughly 2,080.
Age, sex, body size, and activity level all shift the number from there. Roughly speaking:
| Activity Level | Women (kJ/Day) | Men (kJ/Day) |
|---|---|---|
| Sedentary (desk job, minimal movement) | 6,300–7,500 | 7,500–9,200 |
| Lightly Active (some walking, 1–3 workouts/week) | 7,500–8,800 | 9,200–10,900 |
| Moderately Active (regular training, active daily life) | 8,800–10,500 | 10,500–12,600 |
| Very Active (hard training most days or physical job) | 10,500–13,400 | 12,600–15,900 |
A couple of things worth knowing before you lock in a number. Most people sit a category lower than they think — three workouts a week plus a desk job usually lands you in lightly active, not moderate. And genetics, muscle mass, and how much you naturally move during the day can swing your real number by a thousand kJ or more either way. The table's a starting point. Where you actually land takes a few weeks to figure out.
How Many KJ Per Day to Lose Weight?
The basic idea hasn't changed just because the units have: eat less energy than you burn, and your body taps into stored fat to make up the difference.
A commonly used target is a 2,000 kJ daily deficit — roughly equivalent to the 500-calorie deficit you'll see in international guides. Eat for Health, the Australian government's nutrition site, uses a similar figure: cutting around 2,000 kJ a day is generally enough to lose about half a kilogram of fat per week.
So if your maintenance level is 9,200 kJ a day, a weight-loss target would sit somewhere around 7,000–7,200 kJ. That deficit doesn't all have to come from eating less — a brisk walk, a gym session, or just a more active day at work all chip away at it too.
| Goal | Daily Energy Target |
|---|---|
| Lose Weight | ~2,000 kJ below maintenance (≈500 cal) |
| Maintain Weight | Match your maintenance level |
| Build Muscle | Slight surplus (~800–1,200 kJ above maintenance) |
Going much further than a 3,000–4,000 kJ daily deficit tends to backfire — energy crashes, irritability, and a metabolism that slows down to compensate. Slow and steady wins here, even if it doesn't feel like it some weeks.
How Many Calories (or KJ) Should You Burn a Day?
Most fitness trackers and apps still display "calories burned," even here in Australia — so it helps to know what that number actually represents, and how it relates to the kJ figures on your food.
Your total daily energy burn comes from four sources:
- Resting energy (the biggest chunk). Just keeping your body running — heartbeat, breathing, digestion, cell repair — accounts for 60–70% of your total burn. For most adults, that's somewhere between 5,800 and 8,000 kJ (1,400–1,900 calories) a day, depending on body size and sex.
- Everyday movement. Walking to the car, doing the washing, standing at your desk — none of it counts as "exercise," but it adds up. Research from the American Journal of Physiology found this kind of incidental movement can vary by up to 8,400 kJ (2,000 calories) a day between two people living otherwise similar lives. If your weight's stuck and nothing else has changed, this is usually the first place to look.
- Exercise itself. A solid 45-minute strength session burns roughly 800–1,500 kJ (200–350 calories). An hour of moderate cardio sits around 1,700–2,500 kJ (400–600 calories). Lower than most apps tell you, if we're honest — but exercise is still worth doing for plenty of reasons beyond the energy burned.
- Digesting your food. Yes, eating burns energy too. Protein is the most "expensive" to digest — your body uses up roughly 20–30% of its energy just processing it.
Don't Just Do Cardio — Add Strength Training
Cardio burns energy while you're doing it, and then mostly stops. A 45-minute run is done working for you the moment you're back home.
Strength training plays a longer game. Building muscle raises how much energy your body burns at rest, day in and day out. A study published in the European Journal of Applied Physiology found that resistance training can keep metabolism elevated for up to 38 hours afterwards — a process called EPOC, where your body keeps burning extra fuel while it repairs muscle tissue. A run doesn't do that.
Cardio's fine. But if total energy burn is the goal, get some lifting into the mix.
Cable Training Burns More Than You'd Expect
Worth knowing if you train at home. With free weights, tension tends to drop off at certain points in a movement — the top of a curl, the lockout on a press. Cable machines don't have that problem. The resistance stays on the muscle the whole way through the rep.
That sustained load pulls in more muscle fibres, which is part of why compound cable movements — rows, presses, squats, crossovers — tend to feel harder than they look. Depending on intensity, that can work out to 1,250–2,100+ kJ (300–500+ calories) per hour. Over time, the muscle built from consistent training keeps nudging your resting burn higher too. For a home setup, a Smith Machine or Power Rack with cable system covers a lot of ground.
Reading Food Labels — Understanding kJ Per Serving
Australian food labels show energy two ways: per 100g (or 100mL) and per serving. Both numbers matter, but they tell you different things.
The per-100g figure is there so you can compare products fairly — a muesli bar and a chocolate bar might look similar per serving, but per 100g can reveal which one is actually more energy-dense. The per-serving figure tells you what you're getting if you eat the amount the manufacturer suggests — which, especially for snack foods, is often smaller than what bodybuilding people actually eat.
Then there's %DI — Percentage Daily Intake. This shows what proportion of an average 8,700 kJ day that serving represents. A muesli bar at 700 kJ and "8% DI" means roughly 8% of an average day's energy in one bar. Handy for a quick comparison, less useful if your own target is quite different from 8,700 kJ — in which case, the raw kJ number matters more than the percentage.
One thing that catches people out: serving sizes on Australian labels aren't standardised the way some other countries' are. A "serving" of one brand's crackers might be 4 crackers; another brand might call 6 crackers a serving. Worth a glance before assuming the numbers are comparable.
Frequently Asked Questions
1. How many kJ per day for a woman to lose weight?
Depends on the starting point, but most women aiming to lose weight do well somewhere around 6,300–7,500 kJ a day. Work backwards from your maintenance number — say it's 8,500 kJ, knock off about 2,000 kJ and you're at 6,500, which lines up with the half-a-kilo-a-week pace Eat for Health talks about. Smaller frame or less active during the day? Closer to 6,000 kJ is more realistic.
2. How to convert kJ to calories?
Divide by 4.2. So 2,000 kJ is roughly 476 calories, and the often-quoted 8,700 kJ figure works out to about 2,070 calories. Other direction — calories to kJ — just times by 4.2 instead. Some apps use 4.184 if you want to be properly precise, but honestly, 4.2 does the job for everyday stuff.
3. Is burning 2000 kJ a day good?
Yeah, that's actually the figure Australian health guidelines point to for shedding about half a kilo a week. Doesn't need to come purely from a workout either — a brisk walk plus eating a slightly smaller dinner gets most people there without it feeling like a big deal.
4. How many kJ do 10,000 steps burn?
Roughly 1,250–2,100 kJ, give or take, depending on how much you weigh and how quickly you're walking. Bigger person, faster pace — closer to the top end. The real value of 10,000 steps isn't the kJ number anyway, it's that it keeps you ticking over throughout the day instead of one gym session followed by hours on the couch.
5. What is a good daily kJ goal?
Comes down to what you're after. For general health, matching your maintenance level — somewhere in the 7,500–10,500 kJ range for most adults — keeps things steady. For weight loss, knock off about 2,000 kJ from that. For muscle gain, add a bit instead, maybe 800–1,200 kJ above maintenance. The 8,700 kJ figure on food labels is a population average — useful as a reference point, but your actual number will likely sit higher or lower than that.
Conclusion
Calories and kilojoules are just two ways of measuring the same thing — multiply calories by 4.2 to get kJ, or divide kJ by 4.2 to get calories back. For day-to-day eating, 8,700 kJ is the population-wide reference Australia uses, but your own number will sit higher or lower depending on your size, sex, and activity level — often by a couple of thousand kJ either way.
For weight loss, a deficit of around 2,000 kJ a day is a sensible, well-supported target — achievable through food, movement, or a combination of both. And if you're working on the "energy out" side of the equation, what you train with matters. A Smith machine or power rack at home means full-body, muscle-building sessions are always within reach — no commute, no waiting for equipment, just consistent training that adds up over weeks and months.
References
1. Eat For Health – Frequently Asked Questions: Australian government nutrition guidance explaining that one calorie equals 4.2 kilojoules, that 8,700 kJ is used as the average reference figure for food labelling, and that a roughly 2,000 kJ daily deficit corresponds to losing about half a kilogram of fat per week.
2. Food Standards Australia New Zealand – Nutrition Information Panels: Explains the mandatory requirements under Standard 1.2.8 of the Food Standards Code, including that energy must be declared in kilojoules (or kilojoules and kilocalories) per serving and per 100g/100mL, alongside protein, fat, carbohydrate, sugars, and sodium.
3. Healthdirect – Kilojoules: Australian government health information stating that the average adult needs about 8,700 kilojoules a day to maintain a healthy weight, and that one kilocalorie equals 4.2 kilojoules.
4. European Journal of Applied Physiology – Excess post-exercise oxygen consumption following heavy resistance exercise: A study found that resistance training elevated metabolism for up to 38 hours post-session, supporting strength training's role in long-term energy expenditure beyond the workout itself.
5. American Journal of Physiology: Endocrinology and Metabolism – Nonexercise activity thermogenesis (NEAT): environment and biology: Research found that non-exercise daily movement can vary by up to 2,000 calories (around 8,400 kJ) per day between individuals of similar body weight.
