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Energy Converter

Convert between joules, calories, kilowatt-hours, BTU, and electron volts

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About This Tool

Converts energy quantities between joules, calories, kilowatt-hours, BTU, and electron volts. Conversion factors used: 1 cal = 4.184 J (thermochemical), 1 kWh = 3.6 MJ, 1 BTU ≈ 1055.06 J (IT definition), 1 eV ≈ 1.602176634×10⁻¹⁹ J.

Food calories (the kind printed on nutrition labels) are kilocalories, equal to 1000 cal.

The internal pivot is the joule, the SI unit. Each non-SI unit is mapped to joules with a fixed conversion constant, then to the requested target by the inverse factor. The electron volt constant is exact by SI redefinition (since 2019, the elementary charge e is defined as 1.602176634×10⁻¹⁹ C, making the eV-to-J factor exact). The kilowatt-hour is also exact: 1 kWh = 1000 W × 3600 s = 3,600,000 J. The calorie has multiple definitions that disagree at the 0.1% level, and the BTU likewise; the converter uses the thermochemical calorie (4.184 J exactly) and the IT BTU (1055.05585 J) by default and exposes the alternates as toggles.

A worked example: a 2000 kcal daily diet equals 8,368,000 J or about 2.32 kWh. A typical electric kettle running for 5 minutes at 1500 W consumes 450,000 J, or 0.125 kWh, or roughly 108 kcal of electrical energy. These conversions show that human metabolic energy and household electrical energy live on the same scale once units are reconciled, a fact that becomes intuitive only after seeing the joule equivalence.

Limitations come from definition ambiguity, not arithmetic. The 'calorie' on a US nutrition label is the kilocalorie; mixing the two by a factor of 1000 is the most common conversion error. The BTU has at least four definitions (IT, thermochemical, mean, 39°F, 59°F, 60°F) within a 0.5% spread, and HVAC industry conventions vary by country. Electron volts and joules differ by 19 orders of magnitude, so converting between them produces values in scientific notation; users typing decimal-only inputs may overflow the precision of double-precision floats at the extremes.

For everyday energy comparisons, the relevant landmarks are: 1 J ≈ lifting an apple 1 m, 1 kcal heats 1 g of water by 1°C (close enough for kitchen use), 1 kWh runs a 100W bulb for 10 hours, and a gallon of gasoline contains about 33.7 kWh of chemical energy. Calibrating intuition against these anchors prevents most order-of-magnitude mistakes.

The about text and FAQ on this page were drafted with AI assistance and reviewed by a member of the Coherence Daddy team before publishing. See our Content Policy for editorial standards.

Frequently Asked Questions

Which calorie definition is used?
The thermochemical calorie of 4.184 joules. The 15°C calorie (4.1855 J) and IT calorie (4.1868 J) differ by less than 0.1% and are not selected here.
Is a food calorie the same as a calorie?
No. A food calorie is a kilocalorie, equal to 1000 small calories. Nutrition labels write Calorie with a capital C to mark the distinction, though casual usage drops it.
Why include electron volts?
Atomic and particle physics work happens at energies far smaller than a joule. One eV is roughly the energy gained by an electron crossing a one-volt potential, the natural unit at that scale.
What is the difference between BTU IT and BTU thermochemical?
The IT (International Table) BTU is 1055.05585 J; the thermochemical BTU is 1054.35 J. Difference is under 0.07%, but ASHRAE and HVAC standards typically use IT.
How precise are the results?
Output is rounded to a configurable number of significant figures. Internal arithmetic uses double-precision floats, so cumulative error stays below 1 part in 10¹⁵ for any single conversion.
What about therms or quads?
A therm is 100,000 BTU (about 105.5 MJ), used in natural gas billing. A quad is 10¹⁵ BTU (about 1.055 EJ), used in national energy statistics. Both are derived from the BTU and inherit its definitional ambiguity; the converter exposes them as optional units.
Is mass-energy equivalence in scope?
No. E = mc² lets mass be expressed in energy units (1 kg ≈ 8.99×10¹⁶ J), but the converter operates only on quantities already expressed as energy. For nuclear binding-energy calculations, multiply rest mass by c² separately.
How does kWh relate to MJ?
1 kWh = 3.6 MJ exactly. The factor comes from 1000 W × 3600 s. Energy bills use kWh because typical household consumption is convenient at that scale; physics calculations prefer joules for SI consistency.
Why do nutrition labels use kilojoules in some countries?
Australia, New Zealand, and EU regulations require kilojoules (kJ) on packaged food labels, sometimes alongside kcal. The SI consistency argues for kJ; consumer familiarity argues for kcal. 1 kcal = 4.184 kJ, so a 2000 kcal/day diet is 8368 kJ/day.