Energy units are one of the quiet causes of feeding errors. A daily need is calculated in kilocalories, the food label gives kilojoules or megajoules, and the cat food bag in a shop says “Calories”. They all describe the same thing, but they cannot be divided into each other until they are in the same unit. This guide gives the three factors you need, shows the conversion both ways with a dog and a food as examples, explains why the capital-C Calorie on an American label is a kilocalorie and gives a table you can keep next to the bag. The Pet Food Energy Converter does the arithmetic.
The three factors
| From | To | Multiply by |
|---|---|---|
| kcal | kJ | 4.184 |
| kJ | kcal | 0.239006 (÷ 4.184) |
| MJ | kcal | 239.006 |
| kcal | MJ | 0.004184 |
| kJ | MJ | 0.001 |
The factor 4.184 is the thermochemical calorie in joules, and it is exact by definition. A kilocalorie is 1,000 calories, so 1 kcal = 4,184 J = 4.184 kJ. A megajoule is 1,000 kJ, which is 239.006 kcal.
A day’s energy in three units
Take a neutered 20 kg adult dog. Its daily energy is 1.6 × 70 × 20^0.75 = 1,059 kcal (see the Dog Calorie Calculator).
| Unit | Working | Daily need |
|---|---|---|
| kcal | 1,059 | |
| kJ | 1,059 × 4.184 | 4,431 |
| MJ | 4,431 ÷ 1,000 | 4.43 |
A 4 kg neutered cat needs 238 kcal, which is 996 kJ, or 0.996 MJ. The numbers look different, but the animal’s need is the same.
A food’s energy in three units
Foods are labelled per kilogram, per 100 g or per serving. Take a dry food at 3,600 kcal/kg:
| Unit | Working | Per kg |
|---|---|---|
| kcal/kg | 3,600 | |
| kJ/kg | 3,600 × 4.184 | 15,062 |
| MJ/kg | 15,062 ÷ 1,000 | 15.06 |
Per 100 g, divide by ten: 360 kcal, 1,506 kJ. If a label says “ME 15 MJ/kg”, convert it: 15 × 239.006 = 3,585 kcal/kg. A food of 14 MJ/kg is 3,346 kcal/kg, and one of 16 MJ/kg is 3,824 kcal/kg.
How much to feed, in either unit
The number of grams is the daily need divided by the energy per gram, with both in the same unit.
| Method | Working | Grams a day |
|---|---|---|
| In kcal | 1,059 ÷ 3.6 | 294 |
| In kJ | 4,431 ÷ 15.06 | 294 |
| In MJ | 4.431 ÷ 0.01506 | 294 |
The answer is the same because the factor cancels. The mistake is mixing units: dividing 1,059 kcal by 15 MJ/kg gives a meaningless number. Always write the unit next to each figure, and convert one of the two so that both match.
Why the label says “Calories”
On American labels, the word Calorie with a capital C means a kilocalorie. Nutrition labelling for people and pets uses this convention. A small calorie (lowercase) is a thousand times smaller and is used in physics. For pet food, treat Calorie, Cal, kcal and “calories” on a feeding guide as the same thing, unless the label says kJ. The only time to pause is when a number looks a thousand times out: 3,600 on a bag is kcal per kg, 3.6 is kcal per gram.
| What you see | What it means |
|---|---|
| 360 kcal/100 g | 3,600 kcal/kg |
| 3.6 kcal/g | 3,600 kcal/kg |
| 3,600 Cal/kg | 3,600 kcal/kg |
| 15.06 MJ/kg | 3,600 kcal/kg |
| 1,506 kJ/100 g | 3,600 kcal/kg |
Our guide to kcal per kg, per 100 g and per cup goes further into the per-cup problem, which is an issue of volume rather than units.
Treats and small pack sizes
Treats often show energy per treat in kcal or kJ. A treat at 17.5 kcal is 73 kJ (17.5 × 4.184 = 73.2). A dog with a 10% treat allowance of 106 kcal can have about six such treats. If a treat packet lists kJ only, divide by 4.184 first to compare with the daily need. See dog treat calories for how to count them.
Where the energy value comes from
The energy on a label is either measured in feeding trials, calculated from the analysis with modified Atwater factors (3.5 kcal/g protein, 8.5 for fat, 3.5 for carbohydrate) or declared by the maker from their own method. The unit conversion does not tell you how reliable the value is. A figure stated to the last kilojoule may still be an estimate that is off by several percent. The conversion is exact; the label value is not. For how to estimate calories from the analysis, see how to calculate calories in dog food from the label.
Rounding
The factors are precise, and you should round only at the end. Rounding 4.184 to 4.2 makes a 1,059 kcal need 4,448 kJ instead of 4,431, a 0.4% difference. That is far smaller than the uncertainty in the food’s energy value, so use 4.2 for a quick mental sum, and 4.184 in a calculator.
| Need (kcal) | × 4.184 (kJ) | × 4.2 (kJ) | Difference |
|---|---|---|---|
| 238 (4 kg cat) | 996 | 1,000 | 0.4% |
| 1,059 (20 kg dog) | 4,431 | 4,448 | 0.4% |
| 1,781 (40 kg dog) | 7,452 | 7,480 | 0.4% |
A worked example
Ravi has moved to a country where pet food is labelled in kJ. His dog needs 1,059 kcal a day. The new bag says 1,500 kJ per 100 g.
| Step | Working | Result |
|---|---|---|
| Energy per 100 g in kcal | 1,500 ÷ 4.184 | 358.5 kcal |
| Energy per kg | × 10 | 3,585 kcal/kg |
| Energy per gram | 358.5 ÷ 100 | 3.585 kcal/g |
| Daily grams | 1,059 ÷ 3.585 | 295 g |
Ravi’s dog needs 295 g of the new food, close to the 294 g of the old one, because the two foods have almost the same energy. He did the check before changing the bowl, which is the point of converting: it prevents an overfeeding or underfeeding error in the first week of a new bag.
Reading a foreign label step by step
Suppose a bag from another country gives its energy as “ME 1,520 kJ per 100 g”. The steps to use it for a cat that needs 238 kcal a day:
| Step | Working | Result |
|---|---|---|
| kJ per 100 g to kcal per 100 g | 1,520 ÷ 4.184 | 363.3 kcal |
| kcal per kg | 363.3 × 10 | 3,633 kcal/kg |
| kcal per gram | 3,633 ÷ 1,000 | 3.633 |
| Grams a day | 238 ÷ 3.633 | 65.5 g |
Another example: a label of 1,300 kJ per 100 g is 310.7 kcal per 100 g, or 3,107 kcal/kg, and the same cat would need 76.6 g. Do the conversion once, write the kcal per gram on the pack, and use it from then on.
Metabolizable energy and gross energy
Labels that give an energy value usually mean metabolizable energy (ME), which is the energy available to the animal after losses in digestion. Gross energy, the heat released if the food were burned, is higher and is not used for feeding. If a label does not say, assume it is ME, and ask the maker if it matters. ME can be measured in feeding trials or calculated from the analysis with modified Atwater factors, as in how to calculate calories in dog food from the label. A calculated figure is an estimate, even though the conversion into kilojoules is exact.
Keeping a conversion card
A small card next to the food container saves repeating the sum. Write the food’s name, the energy in the units the label uses, the energy in kcal per gram, and the daily grams for each pet.
| Pet | Daily need (kcal) | Food (kcal/g) | Grams a day |
|---|---|---|---|
| 20 kg dog | 1,059 | 3.6 | 294 |
| 4 kg cat | 238 | 3.633 | 65.5 |
Update the card when the food or the weight changes. If you are moving countries, do the conversion once for each food you are considering, and compare them on kcal per gram before you look at price.
Common mistakes
- Dividing kcal by kJ without converting.
- Mixing up per kg, per 100 g and per gram.
- Reading 15 MJ/kg as 15 kcal.
- Rounding the factor too early.
- Trusting a precise-looking label value as exact.
- Forgetting the capital-C Calorie is a kilocalorie.
- Not converting treat energies.
Quick summary
- 1 kcal = 4.184 kJ; 1 MJ = 239.006 kcal.
- A 1,059 kcal day is 4,431 kJ or 4.43 MJ; a food at 15 MJ/kg is about 3,585 kcal/kg.
- Convert the need and the food into the same unit before dividing.
- A food Calorie on a US label is a kilocalorie.
- The conversion is exact; the energy value on the label is the weaker number.
- Use the Pet Food Energy Converter.
Frequently asked questions
How do I convert kcal to kJ?
Multiply kilocalories by 4.184. For 1,059 kcal that gives 4,431 kJ.
How do I convert MJ per kg to kcal per kg?
Multiply by 239.006. A food at 15 MJ/kg is about 3,585 kcal/kg.
Is a Calorie the same as a kcal?
Yes. The capital-C Calorie used for food is a kilocalorie, or 1,000 small calories.
Why do some labels use kJ?
Many countries label food energy in kilojoules or megajoules. Pet food sold there often follows the same convention.
Does a unit conversion change how much to feed?
No. It changes the number on the label, not the food. Convert both the pet's need and the food into the same unit before dividing.
How accurate is the conversion?
The factor is exact enough for feeding. The energy value on the label is the larger uncertainty.
Sources
- NIST Guide to the SI (SP 811), Appendix B.8: thermochemical calorie = 4.184 joules exactly
- NRC (2006) Nutrient Requirements of Dogs and Cats, National Academies Press (energy expressed in kcal and kJ)
Related calculators
- Pet Food Energy ConverterConvert pet food energy between kcal per kg, per 100 g, per pound and per ounce.
- Dog Calorie CalculatorSee how many calories your dog needs a day: its resting energy requirement (RER) and daily maintenance energy (MER) for its life stage.
- Cat Calorie CalculatorWork out how many calories your cat needs each day, from its weight and life stage.
Keep reading
- kcal per kg vs per 100 g vs per Cup, ExplainedHow to convert pet food energy between kcal per kg, per 100 g, per gram, per cup and per can, with a conversion table and worked label examples.
- Calculate Calories in Dog Food From the LabelHow to estimate calories in dog food from the guaranteed analysis using modified Atwater factors: dry and wet worked examples, NFE and the limits.
- Dog Treat Calories: Reading Labels and CountingHow to find the calories in dog treats: kcal per treat or per 100 g, weighing a treat, worked label examples, a counting log and how treats fit into the daily total.
- When Ash Is Missing on a Pet Food LabelEstimate calories when ash is not listed: use 8% for dry and 2.5% for wet food. A 4-point ash swing moves a dry food about 4% (336 to 350 kcal per 100 g).