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Kilocalories to Kilojoules Converter
↔ Convert kJ to kcal instead

Common Conversions

kcal kJ
0.1 0.418
0.5 2.092
1 4.184
5 20.92
10 41.84
25 104.6
50 209.2
100 418.4
250 1046
500 2092
1000 4184

Why this conversion matters in chemistry

Biochemistry leans on kcal: ATP hydrolysis under standard biochemical conditions is conventionally written ΔG°' = −7.3 kcal/mol, equivalent to −30.5 kJ/mol with the exact 4.184 factor. The actual cellular ΔG sits closer to −50 to −65 kJ/mol because the [ATP]/[ADP][Pi] ratio is far from unity and the reaction quotient Q pulls ΔG well below the standard value. Working in kcal or kJ is mostly a matter of which textbook generation you grew up on; the conversion factor is exact, so neither choice introduces rounding error of its own.

Formula

kJ = kcal × 4.184

Worked Examples

1 kcal = 4.184 kJ

The defining identity of the thermochemical calorie.

100 kcal = 418.4 kJ

About the food-energy content of a small apple — the same unit on a US nutrition label and a European one, just written differently.

57.8 kcal = 241.8 kJ

The standard enthalpy of formation of gaseous water — magnitude only; the actual value carries a negative sign.

0.5 kcal = 2.092 kJ

A small reaction energy — about the heat released by a few millimoles of a typical exothermic acid-base step.

Frequently Asked Questions

How do I convert kcal to kJ?
Multiply by 4.184. So 100 kcal becomes 418.4 kJ. The factor is exact through the thermochemical-calorie definition.
Is the thermochemical calorie the same as the food Calorie?
Yes. The Calorie with a capital C on US food labels equals one kilocalorie. So 1 Cal = 1 kcal = 4.184 kJ — the same energy, three different ways to write it.
Why do chemists keep both kcal and kJ in circulation?
Older biochemistry and physical-chemistry literature reports energies in kcal/mol; modern IUPAC standards use kJ/mol. Combining values across that generational gap requires the conversion to keep the numbers consistent.
What does the combustion of methane look like in both units?
Standard enthalpy of combustion of methane is −890.4 kJ/mol, equivalently −212.8 kcal/mol. The two numbers describe the same physical process; the unit shift is just notation.