Energy Converter (J, kJ, cal, kcal)

Understanding the energy converter — concept and principle

This converter changes an amount of energy between joules, kilojoules, calories, kilocalories (food Calories), watt-hours, kilowatt-hours and British thermal units. You need it to compare food labels printed in kcal and kJ, relate food energy to electricity, or translate heating and appliance figures between metric and US units.

How it is calculated

Every unit is converted through joules (J):

• 1 kJ = 1,000 J
• 1 cal (thermochemical) = 4.184 J, so 1 kcal = 4,184 J
• 1 cal (International Table, IT) = 4.1868 J
• 1 Wh = 3,600 J and 1 kWh = 3,600,000 J (1 W = 1 J/s for 3,600 s)
• 1 BTU (IT) ≈ 1,055.056 J

The calculator multiplies your value by the starting unit's size in joules and divides by the target unit's size.

The principle

The joule is the SI unit of energy: the work done by a force of one newton over one metre. The calorie began as the heat needed to warm one gram of water by one degree Celsius, but that amount varies slightly with the starting temperature, so it was later defined directly in joules. Two definitions survive: the thermochemical calorie (exactly 4.184 J), commonly used for food energy, and the International Table calorie (exactly 4.1868 J), used in some engineering tables. The "Calorie" with a capital C on food packaging is a kilocalorie. Watt-hours come from power multiplied by time, which is why electricity bills use kWh.

Worked example

Default input: 100 kcal to kJ.

1. 100 kcal × 4,184 J/kcal = 418,400 J.
2. 418,400 J ÷ 1,000 = 418.4 kJ.
3. Other units: 418,400 ÷ 3,600 = 116.2222 Wh (0.1162222 kWh); 418,400 ÷ 1,055.056 ≈ 396.567 BTU; 418,400 ÷ 4.1868 ≈ 99,933.12 IT calories.

The calculator shows 418.4 kJ, matching these steps.

How to read the result

A useful benchmark: 2,000 kcal, a commonly used reference for daily food energy, is 8,368 kJ, about 2.32 kWh, or an average power of about 97 W spread over 24 hours. The small difference between thermochemical and IT calories (about 0.07 %) matters only in precise scientific work. Note that kWh measures energy, while kW measures power, the rate at which energy is used.

Common mistakes

• Confusing small calories (cal) with food Calories (kcal), a factor of 1,000.
• Mixing up kW and kWh.
• Assuming body exercise converts food energy to work at 100 %; human muscles are far less efficient, and most energy becomes heat.
• Using the BTU and therm interchangeably; one therm is 100,000 BTU.

Frequently asked questions

How many kJ are in one kcal?

4.184 kJ.

Is a food Calorie the same as a kilocalorie?

Yes. One food Calorie (capital C) equals 1 kcal, or 1,000 small calories.

How many kcal is 1 kWh?

About 860 kcal (3,600,000 ÷ 4,184 ≈ 860.4).

Sources: BIPM, The International System of Units (SI Brochure), 9th edition · NIST Special Publication 811, Guide for the Use of the International System of Units (SI), Appendix B