Kilocalories to kilojoules converter

Nutrition labels often use Calories to mean kilocalories. Converting the stated energy into kilojoules allows comparison with another label's energy column. Check that both amounts refer to the same serving size; a per-serving figure is not a per-100-gram figure.

One kilocalorie is exactly 4.184 kilojoules. The kilocalorie used on food labels is the thermochemical kilocalorie, exactly 4.184 kJ.

Multiply the number of kilocalories by 4.184. For example, 2000 kcal × 4.184 = 8,368 kJ. To go the other way, divide by 4.184.

Multiply by 4.2. That is 0.4% high of the exact factor 4.184, so 2000 kcal comes out as 8,400 kJ instead of 8,368.

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 3 worked examples whose answers come from independent sources; for example, “2000 kcal to kJ” is checked against Exact-fraction computation from the legal unit definitions (scripts/gen-pairs.py, Python fractions).

NIST SP 811 (2008), Guide for the Use of the International System of Units, Appendix B; NIST Handbook 44, Appendix C: General tables of units of measurement; BIPM, The International System of Units (SI Brochure), 9th edition.