Energy Converter

Energy converter that switches between joules, calories, kWh, BTU, foot-pounds, electronvolts, and more instantly. Free, runs in your browser, no signup.

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Last verified July 2026

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Direct answer

What does this converter do?

Energy converter that switches between joules, calories, kWh, BTU, foot-pounds, electronvolts, and more instantly. Free, runs in your browser, no signup.

  • 1 BTU = 1.055 kilojoule · 1 kilocalorie = 4.184 kilojoule
  • All conversions route through the joule as the base unit, so every pair of units agrees regardless of which two you enter.
  • Standard factors are shown in the formula panel at the precision the calculator actually uses, so the arithmetic and the panel agree.
Bar chart of 2 kWh expressed in MJ, kcal, and BTU units
2 kWh equals 7.2 million joules of energy

Reference factors (energy)

Exact factors in terms of the joule, from NIST standards:

  • 1 joule = 1 joule
  • 1 calorie = 4.184 joule
  • 1 kilojoule = 1000 joule
  • 1 BTU = 1055.06 joule
  • 1 kilocalorie = 4184 joule
  • 1 kWh = 3.6e+06 joule
Formula & evidence

How this calculator works

Reviewed 2026-09-20 · BoringToolsKit Editorial Team

Formula

Energy = value x (joules per source unit), then joules / (joules per target unit). Factors: 1 kWh = 3600000 J, 1 cal = 4.184 J, 1 kcal = 4184 J, 1 Wh = 3600 J, 1 BTU = 1055.06 J.

Worked example

1 BTU = 1.055 kilojoule · 1 kilocalorie = 4.184 kilojoule

Sources

Sources support the method, terminology, or planning context. Editable assumptions remain your responsibility.

Assumptions to verify

  • Inputs are treated as exact decimal values.
  • Standard (not historical or survey) unit definitions are used.
  • The calculation uses the factors at the precision shown in the formula panel; rounding beyond that is display-only and adds no accuracy.

Frequently asked questions

Why convert through joule instead of directly?

Routing every unit through a single base (joule) means each unit needs only one exact factor against the standard, and any pair combines cleanly. Direct pair factors drift apart over time in hand-maintained tables; a base-unit graph cannot.

Where do these exact factors come from?

From published standards (NIST: SP 811 energy conversions). Units like the inch are defined exactly (1 in = 2.54 cm since 1959), so conversions between standard units are exact, not approximate.

How precise is the converted energy value?

The conversion applies the standard factors shown in the formula panel. The practical limit is your input: the result only regroups the number you entered, so digits beyond your original reading do not add accuracy. Note that 1 kWh = 3.6 MJ exactly, while thermal and electrical conversions carry their own efficiency assumptions.

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Cite this tool

BoringToolsKit. “Energy Converter.” boringtoolskit.com/energy-converter/ (reviewed 2026-09-20). Free to reference in articles, syllabi, and answer posts with a link.

Privacy: Inputs and results stay in this browser. Conversion runs locally in your browser; review the output before relying on it.

Worked example: 2 kWh to every unit

Take 2 kWh as the input. The converter multiplies by 3,600,000 because one kilowatt-hour is 1,000 watts running for 3,600 seconds: 2 x 3,600,000 = 7,200,000 J, displayed as 7,200,000 J. Dividing by 1,000,000 gives the megajoule reading, 7.2 MJ, which is the form you will see on EU appliance labels.

The calorie columns come from the same joule value. 7,200,000 / 4,184 = 1,720.84 kcal (food Calories), or 1,720,841.3 small calories if you divide by 4.184 instead. For BTU, 7,200,000 / 1,055.06 = 6,824.26 BTU, close to the 6,826 BTU you get from the rough 3,412 BTU per kWh rule because 3,412 is rounded. To go the other way, a 10,000 BTU air conditioner delivers 10,000 x 1,055.06 = 10,550,600 J, or about 2.93 kWh per hour of runtime. At a 15 cent per kWh tariff, one hour of cooling costs roughly 44 cents.

Where energy conversions go wrong in practice

The biggest trap is the word calorie. This tool treats 1 cal = 4.184 J exactly, the thermochemical definition. The International Table calorie used in older engineering tables is 4.1868 J, about 0.07 percent larger, and the 15 C calorie is 4.1855 J. Over a 2,000 kcal daily diet those differences are under 2 kcal, but on a 10 million joule lab budget they add up to thousands of calories of discrepancy.

Second, BTU itself has variants. The 1,055.06 J factor here is the IT British thermal unit; the thermochemical BTU is 1,054.35 J, so a 100,000 BTU furnace rating shifts by roughly 67,000 J between definitions. Third, watt-hours and watt-seconds are easy to mix up: a 60 W bulb left on for one day uses 60 x 24 = 1,440 Wh = 1.44 kWh = 5,184,000 J, not 60 J. Finally, rounding to whole joules in the hero display means sub-joule inputs like 0.5 cal (2.092 J) round to 2 J, so check the decimal readouts for small values.