Energy Conversion Calculator — J, kWh, Btu and Calories

Background.

Energy appears under different units because electrical billing, mechanics, heat transfer and nutrition developed their own practical scales. A utility bill uses kilowatt-hours, laboratory work often uses joules, fuel and heating data may use British thermal units, and nutrition uses Calories. This calculator puts a stated energy amount on one common joule scale and converts it to every supported representation.

The joule is the SI derived unit of energy and work. One joule is one newton-metre of work. Power is energy per time, so one watt is one joule per second. Holding one watt for one hour means 3,600 seconds of energy transfer, which makes one watt-hour exactly 3,600 joules and one kilowatt-hour exactly 3.6 million joules.

Prefixes follow powers of ten: one kilojoule is 1,000 joules and one megajoule is one million. That makes electrical conversions transparent. One kWh is 3.6 MJ, while 500 Wh is 0.5 kWh and 1.8 MJ. The page shows all of these together so a factor-of-one-thousand error stands out.

Calories require a label. This page uses the thermochemical calorie, exactly 4.184 joules, and its kilocalorie of 4,184 joules. On a food label, the capital-C word Calorie normally means one kilocalorie, not the small calorie. Enter a nutrition value in kcal rather than cal or the result will differ by one thousand.

The British thermal unit also has variants. The supported factor is the International Table Btu, 1,055.05585262 joules as published by NIST. The selector names it `Btu (IT)` rather than presenting an unqualified historical unit. A source using a thermochemical or mean Btu should be identified before conversion.

Foot-pound-force is mechanical work: one pound-force acting through one foot. Its factor follows from the exact international pound, standard gravity and international foot, giving 1.3558179483314004 joules. It should not be confused with pound-foot torque merely because both can be written as force times distance. Context determines whether the quantity is work or a rotational moment.

Conversion first multiplies by joules per source unit and then divides by joules per target unit. Decimal.js preserves the stated factors, and every output rounds only once at ten decimal places. The simultaneous breakdown comes from that same unrounded canonical value, so pairwise selectors cannot drift from the supporting outputs.

Signed energy is allowed. Work done on a system may be positive while work done by it is negative under one convention, and heat transfer or change in stored energy is often signed. The calculator preserves the sign without choosing the convention. A non-finite value or unknown unit is rejected.

Equal energy does not imply equal usefulness or cost. A heater can turn electrical energy into heat efficiently, while an engine converts only part of fuel energy into shaft work. This page applies no efficiency, coefficient of performance, price or time. Use a power calculator when the rate is the unknown and an application-specific calculator when losses matter.

The worked example converts one kWh to exactly 3.6 MJ and approximately 860.4206500956 thermochemical kcal. That is a mathematical equivalence, not a dietary comparison or an estimate of how much useful work a person or machine will obtain from the energy.

What is energy conversion calculator?

An energy converter preserves an amount of work, heat or stored energy while changing the unit used to express it. Joule is the canonical SI unit. Watt-hours combine power and elapsed time, calories and Btu are historical heat units, and foot-pound-force is a customary mechanical work unit.

How to use this calculator.

1. 01Enter the energy and identify the definition printed with its source unit.
2. 02Choose the target energy unit.
3. 03Read the primary conversion and use joules or megajoules as a scale check.
4. 04For food labels, select kcal for capital-C Calories.
5. 05Apply efficiency or price separately; unit conversion alone assumes neither.

The formula.

Each unit has a joules-per-unit factor. Multiply the input by its source factor to create one canonical energy, then divide by the target factor. Prefix and watt-hour factors are exact; the named NIST definitions govern calorie, Btu and foot-pound-force.

For 1 kWh, multiply by 3,600,000 J/kWh to obtain 3,600,000 J or 3.6 MJ. Dividing by 4,184 J/kcal yields 860.4206500956 kcal at the output boundary. All other displayed units use that same canonical value.

A worked example.

Frequently asked questions.

References& sources.

1. [1]NIST SP 811 Appendix B.8 supplies energy/work conversion factors including Btu (IT), thermochemical calorie, watt-hour and foot-pound-force.
2. [2]BIPM SI Brochure defines joule, watt and the SI prefix system.
3. [3]NIST SP 330 independently lists joule and watt in the SI derived-unit table.
4. [4]FDA Nutrition Facts guidance supplies the scope for capital-C dietary Calories.
5. [5]NIST Handbook 44 Appendix C provides a customary/SI legal-metrology cross-check.