Energy Converter: Joules, Calories, kWh, BTU and eV

The energy converter changes a value between ten units: joule (J), kilojoule (kJ), calorie (cal), kilocalorie (kcal), watt-hour (Wh), kilowatt-hour (kWh), British thermal unit (BTU), electronvolt (eV), foot-pound (ft·lbf) and erg (erg).

Updated 2026-09-30Reviewed by CalcX Editorial Team5 sources5 min read

What the energy converter does

Every factor is a definition: 1 kJ = 1,000 J, 1 cal = 4.184 J (thermochemical), 1 kcal = 4,184 J, 1 Wh = 3,600 J, 1 kWh = 3,600,000 J, 1 BTU = 1,055.05585262 J (International Table), 1 eV = 1.602176634e-19 J, 1 ft·lbf = 1.3558179483314004 J and 1 erg = 10⁻⁷ J. Where a unit has several definitions, the dropdown pins one.

Power (energy per second) belongs on the power converter, hours on the time converter, arithmetic on the scientific calculator.

How to use it

Pick the unit you have and the unit you want; both dropdowns list the same ten units. The page opens with Joule (J) on the left and Kilojoule (kJ) on the right.
Type on either side and the other converts live; the middle button swaps the units and carries the value across.
The focused field also takes expressions: 4.184*250 shows = 1,046 as it converts; Enter or = settles it.
An unfinished 3 + shows = ? and keeps the last settled value.
All Units repeats the quantity in every unit; click a row to move it to the focused side. The keypad opens a standard calculator.

Worked examples

Every readout below comes from this converter.

A food label. A label's "250 Calories" is 250 kcal. Put Kilocalorie (kcal) on the left card and Joule (J) on the right, type 250: the Joule row reads 1,046,000 J and the Kilowatt-hour row 0.2905555556 kWh. Type the same 250 on the Calorie (cal) card instead and the Joule card reads 1,046 J.

A meter reading. One kilowatt-hour is 3,600 kJ = 3,600,000 J. A 10 kWh day reads 36,000,000 J, 10,000 Wh and 8,604.20650096 kcal.

A heating figure. 100,000 BTU reads 105,505,585.262 J, 105,505.585262 kJ and 29.3071070172 kWh. One BTU is 1,055.05585262 J, the International Table value.

A particle energy. 13.6 eV — the energy that ionizes atomic hydrogen — reads 2.178960e-18 J and 5.207840e-22 kcal. eV answers are exponential: 1 J is 6.241509e+18 eV.

Where the units come from

The joule is the SI derived unit of energy (1 kg·m²/s²), and kJ, Wh and kWh are plain multiples of it. The calorie began as the heat that warms one gram of water by one degree Celsius; the thermochemical calorie was fixed at exactly 4.184 J, so the food calorie is 4,184 J.

The British thermal unit began as the heat that warms one pound of water by one degree Fahrenheit; its exact value depends on which steam-table calorie it is built from, and NIST SP 811 lists 1 BTU(IT) = 1.055 056 × 10³ J. An electronvolt is the energy one electron gains crossing one volt, exactly 1.602176634 × 10⁻¹⁹ J since 2019. The erg is the old centimetre–gram–second energy unit, 10⁻⁷ J; the foot-pound is one pound-force over one foot, from 0.3048 m, 0.45359237 kg and 9.80665 m/s².

Reading results and limits

Display rule.: Results carry up to 12 significant digits. Between 10⁻¹⁰ and 10¹⁵ they print with comma grouping and at most 10 decimal places; outside that band, exponential form with six decimals (1.602177e-19); zero prints as 0.
Stored precision.: The stored value carries more digits than the screen. Exponential readouts keep six decimals: 1 eV reads 1.602177e-19 J against the exact 1.602176634e-19 J. Decimal readouts stop at 10 places: 1 J reads 0.0002390057 kcal, though 1 kcal = 4,184 J exactly gives 0.000239005736… kcal. Below the threshold, 10⁻¹¹ J reads 1.000000e-11 J. Negative inputs convert normally: −5 kcal is -20,920 J.
Energy only.: It does not convert power: ratings in W or BTU/h belong on the power converter, and hours on the time converter.

Representative readouts:

Input (left card)	Row shown	Readout
1 kcal	Joule	4,184 J
1 BTU	Joule	1,055.05585262 J
1 Wh	Calorie	860.420650096 cal
1 kWh	Kilojoule	3,600 kJ
1 erg	Joule	0.0000001 J
1 eV	Joule	1.602177e-19 J
1 J	Electronvolt	6.241509e+18 eV
1 J	Kilocalorie	0.0002390057 kcal

Common mistakes

"Calories" on a food label are kilocalories. 250 kcal is 1,046,000 J; 250 cal is 1,046 J — they differ by a factor of 1,000.
A kWh is 3,600 kJ, not 3,600 J. One kilowatt-hour is 3.6 million joules (3.6 MJ); three dropped zeros are a 1,000× error.
BTU comes in several tables. This tool uses the International Table BTU, 1,055.05585262 J; the thermochemical BTU is about 1,054.350 J, roughly 0.067% smaller.
Watt-hours are energy; watts are power. 100 Wh is 360,000 J; a 100 W bulb runs 10 hours on 1 kWh. Power ratings in W or BTU/h convert on the power converter, not here.
An electronvolt is tiny. 1 J is 6.241509e+18 eV, so 1 eV prints as 1.602177e-19 J; read the exponent.

Where it is used

Food labels. A "250 Calories" label is 250 kcal = 1,046,000 J, while lower-case 250 cal is 1,046 J — the two differ by a factor of 1,000.
Electricity bills. A 10 kWh day is 36,000,000 J and 8,604.20650096 kcal; the meter's decimal multiples stay exact.
Heating and fuel. BTU figures to joules or kWh: 100,000 BTU is 105,505,585.262 J and 29.3071070172 kWh.
Particle and atomic physics. The 13.6 eV that ionizes hydrogen reads 2.178960e-18 J; eV answers print exponentially.

Sources

BIPM, SI Brochure, 9th edition — the joule and the electronvolt 1.602176634 × 10⁻¹⁹ J (Table 8). bipm.org （访问日期：2026-09-30）
NIST Special Publication 811, Appendix B.8 — thermochemical calorie 4.184 J, International Table Btu 1.055 056 × 10³ J, foot pound-force 1.355 818 J, erg 10⁻⁷ J. nist.gov （访问日期：2026-09-30）
Wikipedia, Calorie — the food calorie = 1 kcal = 1,000 cal, thermochemical calorie = 4.184 J. en.wikipedia.org （访问日期：2026-09-30）
Wikipedia, British thermal unit — Btu variants between 1,054 J and 1,060 J, the International Table definition, Btu/h as power. en.wikipedia.org （访问日期：2026-09-30）
Wikipedia, Electronvolt — 1 eV = 1.602176634 × 10⁻¹⁹ J exact under the 2019 SI, and 13.6 eV to ionize atomic hydrogen. en.wikipedia.org （访问日期：2026-09-30）