kJ ⇄ cal Converter for Chemistry
Accepts decimals and scientific notation, e.g. 2.75e4 or 4.5 x 10^-1.
1 cal = 4.184 J (thermochemical calorie), so 1 kJ = 239.006 cal.
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KJ To cal: The Conversion For Science Problems
In chemistry and physics, “calories (cal)” usually means the small calorie: the energy to warm 1 gram of water by 1 °C. It is 1,000 times smaller than a food Calorie (kcal).
cal = kJ × 1,000 ÷ 4.184 (1 kJ = 239.006 cal)
kJ = cal × 4.184 ÷ 1,000
The converter above follows the usual textbook rules: it keeps the same number of significant figures as your input (or lets you choose), and accepts scientific notation like 2.75e4 or 4.5 x 10^-1.
cal vs Cal vs kcal: a lower-case cal is a small calorie. A capital-C Calorie on food labels is a kilocalorie (kcal) = 1,000 cal. Always check which one your question asks for. For food labels, use the kJ to calories converter instead.
Worked Examples
Convert 23.4 kJ To Calories (cal)
- Convert kJ to J: 23.4 kJ × 1,000 = 23,400 J
- Convert J to cal: 23,400 J ÷ 4.184 J/cal = 5,592.7 cal
- Round to 3 significant figures (23.4 has three): 5.59 × 10³ cal (5,590 cal)
Convert 15.00 kJ To Calories (cal)
15.00 kJ × 1,000 ÷ 4.184 = 3,585.09 cal. The input has 4 significant figures, so the answer is 3,585 cal (3.585 × 10³ cal).
458 kJ Is Equivalent To How Many Calories?
458 × 1,000 ÷ 4.184 = 109,464.6 cal → 1.09 × 10⁵ cal (3 sig figs), or 109 kcal.
Convert 2.75 × 10⁴ kJ To Calories
2.75 × 10⁴ kJ = 2.75 × 10⁷ J. Divide by 4.184: 6.57 × 10⁶ cal (3 sig figs), which is 6,570 kcal.
Convert 125 cal To Kilojoules
125 cal × 4.184 J/cal = 523 J = 0.523 kJ.
Bomb Calorimeter: Energy In A Candy Bar
A 1.0 g sample of a candy bar is burned in a bomb calorimeter with a total heat capacity of 3.0 kJ/°C, and the temperature rises by 7.0 °C. How much energy is in a 60 g bar?
- Heat released by 1.0 g: q = C × ΔT = 3.0 kJ/°C × 7.0 °C = 21 kJ
- For 60 g: 21 kJ/g × 60 g = 1,260 kJ → 1.3 × 10³ kJ (2 sig figs)
- In food Calories: 1,260 ÷ 4.184 = 301 kcal → 3.0 × 10² Calories (2 sig figs)
Quick Reference: kJ To cal
| Kilojoules | Calories (cal) | Kilocalories (kcal) |
|---|---|---|
| 0.45 kJ | 110 cal (2 s.f.) | 0.108 kcal |
| 2.5 kJ | 6.0 × 10² cal | 0.598 kcal |
| 12.5 kJ | 2.99 × 10³ cal | 2.99 kcal |
| 23.4 kJ | 5.59 × 10³ cal | 5.59 kcal |
| 43.3 kJ | 1.03 × 10⁴ cal | 10.3 kcal |
| 393.5 kJ | 9.405 × 10⁴ cal | 94.05 kcal |
| 458 kJ | 1.09 × 10⁵ cal | 109 kcal |
Significant Figures In Energy Conversions
- The conversion factor 4.184 J/cal is exact (a definition), and so is 1,000 J/kJ, so they don’t limit significant figures.
- Your answer keeps the same number of significant figures as the measured value.
- Trailing zeros after a decimal point count: 15.00 kJ has 4 significant figures, 15 kJ has 2.
- For very large or small answers, use scientific notation so the significant figures are clear.
Why Do Some Textbooks Use 4.18 Or 4.186?
4.184 J/cal is the thermochemical calorie, the standard definition. Some older textbooks round to 4.18, and some use 4.186 (close to the specific heat of water at 15 °C). If your course specifies a value, use that one; the differences are well under 0.1%.
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FAQ
How many calories (cal) are in 1 kJ?
1 kJ = 239.006 cal, because 1 cal = 4.184 J exactly.
What is 23.4 kJ in calories?
23.4 kJ = 5,592.7 cal, which rounds to 5.59 × 10³ cal with three significant figures.
Is a kilojoule bigger than a calorie?
Yes. 1 kJ is about 239 small calories, but only about 0.239 food Calories (kcal).
How do I convert cal to kJ?
Multiply by 4.184 to get joules, then divide by 1,000. For example, 125 cal = 0.523 kJ.