The conversion factor
All conversions on this site use the thermochemical calorie, defined as exactly 4.184 joules. From that definition:
- 1 kcal = 1,000 cal = 4,184 J = 4.184 kJ
- 1 kJ = 1,000 J = 0.239005736… kcal = 239.005736… cal
Because the factor is exact, no conversion here carries a hidden approximation.
Other calorie definitions
Other calorie definitions exist. The International Table calorie is 4.1868 J, about 0.07% larger than the thermochemical calorie, and the 15 °C calorie is about 4.1855 J. Nutrition sources commonly use 4.184 J, and so do we. For food-energy work the difference is negligible.
How we calculate
Everything is converted through joules using the exact factors above. The pre-built pages and charts are generated with exact decimal arithmetic, so a result such as 478.01 kcal for 2000 kJ is computed to many digits and rounded only for display. The live converter runs in your browser using standard double-precision numbers, which are accurate to about 15 significant digits, far more than we display. Rounding adapts to size: four decimal places below 1, three below 100, two below 10,000 and one above. Trailing zeros are removed.
Why label figures may differ
Food labels do not convert kJ to kcal with 4.184. Manufacturers apply a standard energy factor per nutrient in each unit and round each result separately. For example, fat is counted at 37 kJ and 9 kcal per gram, a ratio of 4.11. A printed pair such as "1046 kJ (250 kcal)" therefore may not match kJ ÷ 4.184 to the decimal. This is expected.
Reference intakes and food values
- Daily reference intakes: 8,700 kJ (Australia and New Zealand average adult diet), 8,400 kJ or 2,000 kcal (EU reference intake), and 2,000 kcal (US Daily Value basis).
- Nutrient energy factors: 17 kJ per gram of carbohydrate and protein, 37 kJ of fat, 29 kJ of alcohol, 8 kJ of dietary fibre.
- Food values are typical figures in line with USDA FoodData Central and are approximate.
Sources
- NIST Guide for the Use of the International System of Units (SP 811), for the definitions of the calorie.
- BIPM, The International System of Units (SI Brochure), for the joule and SI prefixes.
- USDA FoodData Central, for typical food energy values.
- Food Standards Australia New Zealand, for Australian and New Zealand labelling.
- Regulation (EU) No 1169/2011, for EU food information and energy conversion factors.
Quality checks
The conversion code is tested against known values. We also test that the in-browser converter and the pre-built pages and charts agree, and the site is checked for broken internal links whenever content changes.
Corrections
If you find an error, contact us. We correct confirmed mistakes promptly, and each page shows when it was last updated.
This site provides information only and is not medical advice.
Last updated 11 Oct 2026. How we calculate.