Energy Converter
This converter expresses one energy quantity in joules (J), kilojoules, megajoules, gigajoules, watt-hours, kilowatt-hours, thermochemical calories, International Table kilocalories, International Table British thermal units, or electronvolts.
Select the source and target units, enter a numerical value, and calculate. The tool changes units only; it does not convert energy to power unless a time interval is supplied elsewhere.
Method used
Every supported unit has a joule factor. SoupCalc multiplies the input by the source factor, then divides by the target factor:
target value = input × source joules per unit ÷ target joules per unit
The registry uses these definitions:
| Unit | Joule factor |
|---|---|
1 Wh |
3,600 J |
1 kWh |
3,600,000 J |
1 cal |
4.184 J (thermochemical calorie) |
1 kcal (IT) |
4,186.8 J |
1 Btu (IT) |
1,055.05585262 J |
1 eV |
1.602176634 × 10⁻¹⁹ J |
SI prefixes are decimal: 1 kJ = 1,000 J, 1 MJ = 10⁶ J, and 1 GJ = 10⁹ J.
Worked example
Convert 2.5 kWh to megajoules:
2.5 × 3,600,000 = 9,000,000 J;9,000,000 ÷ 1,000,000 = 9 MJ.
SoupCalc therefore displays 9 MJ apart from display formatting. The same energy could also be written as 9,000 kJ.
Using the result
The result represents the same energy quantity in another unit. Watt-hours include power multiplied by time: one watt-hour is one watt sustained for one hour. A kilowatt (kW) by itself is power, not energy, and cannot be converted to kWh without duration.
Names such as calorie and Btu have multiple historical definitions. Preserve the qualifier. On this page, unqualified cal is the thermochemical calorie, while kcal (IT) and Btu (IT) use the International Table definitions.
Unit definitions and limits
The displayed precision cannot exceed the source value's accuracy. Energy meters, batteries, fuels, food labels, heat-transfer calculations, and laboratory measurements each have domain-specific conventions, tolerances, and losses that a unit conversion does not model.
Food Calories are commonly kilocalories, often written with a capital C, but labeling rules vary by jurisdiction. Do not assume the tool's small cal option represents a food Calorie. Likewise, battery nameplate energy and usable delivered energy can differ with voltage, temperature, discharge rate, and system efficiency.
The electronvolt factor follows the current SI definition through the exact elementary charge. For regulated billing, engineering design, nutrition, or safety work, apply the standards and rounding rules required in that field.
Sources
- BIPM — The International System of Units (SI Brochure)
- NIST Guide to the SI, Appendix B — Conversion Factors
- NIST Guide to the SI — Notes on International Table and thermochemical energy units
Editorial record
Written and manually reviewed by the SoupCalc Editorial Team. Joule factors, the distinction between calorie definitions, and the worked example were checked against the current unit registry and automated tests. Last reviewed: August 10, 2026. This page remains unavailable for indexing and advertising until all approved localizations pass review.