Energy Converter
Convert energy units — joules, calories and watt-hours.
1 J =
0.001 kJ
How to use
- Type the value to convert — decimals are fine.
- Pick the current unit on the left and the target unit on the right. The middle button swaps direction.
- The result updates as you type. Check the conversion table and formula below, too.
Frequently asked questions
How accurate are the results?+
Calculations use the internationally standardized (SI) definitions; only the display is rounded to about six significant digits. That's plenty for everyday, school and work use.
What's the difference between kcal and kJ on food labels?+
They express the same energy: 1 kcal = 4.184 kJ. US and Korean labels use kcal; European and Australian labels use kJ. A 2,000 kJ snack is about 478 kcal.
Are diet 'calories' cal or kcal?+
Everyday 'calories' are actually kilocalories. The 2,000-calorie daily guideline = 2,000 kcal = 8,368 kJ.
Does it need a fast connection?+
No. The math runs instantly inside your browser with no server round-trip — it even works offline once the page is open.
Is it really free?+
Yes. No sign-up, no install, no usage limits. The site is ad-supported.
Unit guide
Calories, joules and watt-hours
Food energy is quoted in kcal or kJ, electrical energy in Wh and kWh. Anchors: a bowl of rice ≈ 300 kcal, a chocolate bar ≈ 250 kcal ≈ 1,046 kJ, a phone battery ≈ 15 Wh, and 1 kWh on your power bill = 860 kcal.
How this converter calculates
Every unit is converted to kilojoules first, then divided into the target unit, using the thermochemical calorie (1 cal = 4.184 J).