🍎 Kilojoules to calories — what the packet is telling you
← Health & FitnessEvery nutrition information panel in Australia prints the energy twice: kilojoules, which the Food Standards Code requires, and kilocalories — the "calories" everyone counts — which is allowed alongside it. They are the same amount of energy in two units, and one food calorie is 4.184 kilojoules exactly. Type either figure in and this page answers in the other one, then says what share of a day it is, because a number on a pack means very little until it is put beside the day it comes out of.
The answer
The amounts people look up most
| Kilojoules | Calories |
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A conversion, not health advice. One food calorie is 4.184 kilojoules — the thermochemical calorie, defined as 1 cal = 4.184 J, multiplied by the thousand that makes a food Calorie — so converting is that one figure and nothing else is rounded. The day this page measures against is 8,700 kJ, or 2,079 kcal: the average adult intake the Australian Dietary Guidelines build their serve sizes on. It is a reference figure for an average adult rather than a target, and what anyone actually needs depends on their size, age, sex and how much they move. The panel figure is per serve, and a serve is not always the pack — read the serve size above the panel, and where the energy is given per 100 g, multiply both figures by the grams in your serve.
Why use this calculator?
Almost nobody needs help multiplying by 4.184. What people need help with is the packet. An Australian panel prints kilojoules first and calories second, and the two figures are far enough apart that it is easy to take the wrong one for the one you are counting — or to read "calories" as though it meant the small ones and land a thousand times out. And a figure on its own says very little: 870 kJ means something once you know it is a tenth of a day. So this page answers in the other unit, and then puts the answer beside the day.
Where the kilojoule and the calorie come from
Two units, one amount of energy, and a history that explains most of the confusion:
- The joule is the physicist's unit: one watt for one second, or the energy of an apple falling about a metre. It is named after James Prescott Joule, a Salford brewer's son who spent the 1840s measuring heat and work with paddle wheels and falling weights, and showed that heat is not a substance but a form of motion. The unit carrying his name only arrived in 1948, when the international system adopted it.
- The calorie is the older one, and it was a laboratory measure before it was a food one: the heat needed to raise a gram of water by one degree. It is a small unit — a cup of tea holds thousands of them — so food has always been counted in kilocalories, a thousand at a time. Labels dodge the awkwardness by capitalising it: a Calorie with a capital C is a kilocalorie, which is why the "200 calories" on a packet is 836 kilojoules rather than 0.8 of one.
- Australia prints kilojoules because of metrication: the country went metric through the 1970s, and the Food Standards Code now requires a nutrition information panel to give its energy in kilojoules, with kilocalories permitted alongside. Europe does the same, while the United States prints Calories only — so one muesli bar can carry "836 kJ / 200 kcal" in Sydney and "200 calories" in Chicago.
- The figure is worked out, not burnt. Nobody sets fire to a biscuit for the label. The energy is calculated from what the food contains, using agreed factors per gram: protein 17 kJ, carbohydrate 17 kJ, fat 37 kJ, dietary fibre 8 kJ, alcohol 29 kJ. Fat is the dense one and alcohol is nearly as dense, which is the whole reason a beer and a pastry land where they do.
How does this calculator work?
There is one number in it, and it is a definition rather than a measurement — one food calorie is 4.184 kilojoules, exactly:
kilojoules = kilocalories × 4.184
kilocalories = kilojoules ÷ 4.184
- The factor is exact. A calorie has been defined as 4.184 joules since the thermochemical calorie was agreed, and a food Calorie is a thousand of those. Nothing is rounded on the way through; the card prints to the nearest kilojoule or calorie, which is as precise as a label ever is.
- Both directions from one box. The unit select says which figure was typed in, so the same box answers either question — the kilojoule figure to get calories, or the calorie figure to get kilojoules.
- The day is 8,700 kJ, or 2,079 kcal. That is the average adult intake the Australian Dietary Guidelines work their serve sizes from, so the share of it shown here is the same arithmetic the guide does. It is a reference for an average adult rather than a target: the guide's own advice is built around serve ranges that change with age, sex and activity.
- How many a day matters more than the label. Multiplying by the number eaten is where a small figure turns into a real one — one biscuit at 340 kJ is not much, and four of them is 1,360 kJ, a sixth of the day before any meal is counted.
Frequently asked questions
Is a calorie the same as a kilocalorie?
On a food label, yes. The calorie a nutrition panel means is a kilocalorie — a thousand of the small laboratory calories — which is why the two figures on a packet are 4.184 apart rather than 0.004 apart. A diet "calorie" and a chemist's calorie are two sizes of the same unit, and the label is always the big one.
How do I convert kilojoules to calories in my head?
Divide by four. It is not exact — dividing by 4.184 is — but it is close enough to check a label in a shop: 870 kJ is about 218 that way against the exact 208, which is a few per cent out and never wrong by a factor of ten. Going the other way, multiply by four. Two anchors are worth remembering because they turn up constantly: 100 kcal is 418 kJ, and 1,000 kJ is 239 kcal.
Why does the label lead with kilojoules?
Because the Food Standards Code requires it, and because Australia went metric in the 1970s — the kilojoule is the SI unit, and the calorie is the older measure that stuck in everyday speech. So the panel gives kilojoules first and kilocalories second, while every conversation about food uses the second one.
Where do the kilojoules on a panel actually come from?
From the food's composition rather than from burning it. Protein and carbohydrate are counted at 17 kJ a gram, fat at 37, dietary fibre at 8 and alcohol at 29, and the energy on the label is the sum of those. The factors came out of calorimetry — food really was burnt in a bomb calorimeter, once, to work out what each gram is worth — but what gets printed is arithmetic done on the ingredients.
Is the figure on the packet for the whole packet?
Usually not. Australian panels give the energy per serve, and a serve is whatever the maker decides it is — a whole small pack, or a quarter of a big one — with the serves per pack printed above the panel. Where the panel also gives figures per 100 g, those are for comparing one product with another rather than for eating, since 100 g of something can be most of the pack. Eating the lot means multiplying the serve figure by the serves per pack.
Should I count kilojoules or calories?
It makes no difference to your body, which only sees energy: the two figures are one amount written twice. Count whichever you think in, and use this page when a label gives the other. The figure that matters is the one for the portion actually eaten, which is why the "how many a day" box does more work here than the conversion does.
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