Joule to Kilojoule
Free instant joule to kilojoule converter. 1 J = 0.001 kJ.
Common Joule to Kilojoule values
| Joule (J) | Kilojoule (kJ) |
|---|---|
| 1 | 0.001 |
| 5 | 0.005 |
| 10 | 0.01 |
| 25 | 0.025 |
| 50 | 0.05 |
| 100 | 0.1 |
| 500 | 0.5 |
| 1000 | 1 |
How does Joule to Kilojoule conversion work?
Type a value in the Joule (J) field and the equivalent in Kilojoule (kJ) appears instantly to the right. The math uses the exact formula shown above — no rounding errors, no approximations beyond standard floating-point precision. Use the swap button to flip the units and see the inverse conversion.
Joule. The SI unit of energy. About the energy of a small apple falling from waist height.
Kilojoule. 1,000 joules. Common unit on food labels in metric countries.
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Conversion factors come from the official standard (NIST, BIPM, ISO definitions where applicable). Values are precise to standard floating-point limits.
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How it works — the formula
1 cal (thermochemical) = 4.184 J exactly
1 kcal = 4,184 J = 4.184 kJ
1 kWh = 3,600,000 J = 3.6 MJ
1 BTU (IT) = 1,055.05585262 JThe joule is the SI derived unit of energy: 1 J = 1 N·m = 1 W·s. The "calorie" exists in three flavors — thermochemical (4.184 J exact, used in chemistry), International Table (4.1868 J, used in nutrition tables in some countries), and 15 °C (~4.1855 J). US food labels and the FDA use the thermochemical kilocalorie. The BTU is defined as the energy needed to raise 1 lb of water by 1 °F; the International Table BTU (1,055.05585262 J) is the standard for HVAC and energy markets per ISO 31.
Worked examples
- Inputs:
- 500 kcal = ? J
- Output:
- 500 × 4,184 = 2,092,000 J = 2.092 MJ
- Inputs:
- 1 kWh = ? BTU
- Output:
- 3,600,000 / 1,055.05585262 ≈ 3,412.14 BTU
- Inputs:
- 10,000 BTU/h cooling
- Output:
- 10,000 × 1,055.05585262 / 3,600 ≈ 2,930.7 W ≈ 2.93 kW
Limitations
- "Calorie" without a qualifier is ambiguous — nutrition labels almost always mean kilocalories (1 Calorie = 1 kcal = 1,000 cal).
- BTU comes in five definitions (IT, thermochemical, 39 °F, 59 °F, 60 °F) within ~0.1% — the IT BTU is the most common modern usage.
- Electronvolt (1 eV ≈ 1.602 × 10⁻¹⁹ J) is standard in atomic physics but not used for everyday energy.
- Energy and power are different units (energy = power × time); 1 watt ≠ 1 joule, and 1 kWh ≠ 1 kW.
Conversions are exact for joule-defined units (calorie, kWh, BTU-IT). Food-calorie counts on labels are reported with two-significant-digit precision and may differ from thermochemical equivalents at the third digit.