Convert energy (joule, kWh, calorie, BTU)

The value is converted into joules, then into the target unit. A kilowatt-hour is not a power: it is one kilowatt sustained for one hour, so 1,000 W × 3,600 s. The calorie used here is the thermochemical calorie; the “Calorie” with a capital C on food labels is in fact a kilocalorie.

E(J) = x × kfrom ; y = E(J) / kto

1 kWh = 1000 × 3600 = 3.6 × 10⁶ J

1 kcal = 4184 J

The joule is the SI unit of energy, work and quantity of heat. The kilowatt-hour is used to bill electricity. The BTU is still common in English-speaking heating and air conditioning. European food labels print kJ and kcal side by side precisely because both units coexist.

Value

The amount of energy alone, without the unit.

From

Starting unit. The default is the kilowatt-hour, the one on electricity bills.

To

Target unit. The default is the kilojoule, the one on food labels.

Assumptions

• 1 kWh = 3.6 × 10⁶ J exactly.
• 1 cal = 4.184 J (thermochemical calorie).
• 1 BTU ≈ 1,055.06 J (international BTU).

Limits

• Energy is not power: a “2,000 W” appliance does not consume 2,000 Wh.
• Conversion efficiency is not modelled: burning 1 kWh of gas does not deliver 1 kWh of useful heat.
• The metabolisable energy of food differs from its heat of combustion.

An electricity bill counts kilowatt-hours, so it counts energy. Multiplying an appliance's power by how long it runs gives that energy. On the nutrition side, 2,000 kcal a day is about 8,368 kJ: the two figures describe the same intake.

• Confusing kW with kWh.

  The kW is instantaneous power; the kWh is energy accumulated over time.

• Reading “250 calories” on a package as 250 cal.

  It means 250 kcal, that is 250,000 calories in the physical sense.

• Adding gas and electricity kWh as if they cost the same.

  The energy content is comparable; the price and the efficiency are not.

• Power converterEvery power routes through the watt. Metric horsepower is 735.49875 W and mechanical horsepower 745.7 W: two different units sharing one everyday name.
• Pressure converterEvery pressure routes through the pascal, the SI unit equal to one newton per square metre. The bar is exactly 100,000 Pa and the standard atmosphere 101,325 Pa.
• Data storage converterThe maths routes through the bit. Decimal prefixes (1 kB = 1,000 bytes) and binary ones (1 KiB = 1,024 bytes) are handled separately, as IEC 80000-13 specifies.
• Calorie calculatorThree steps: Mifflin-St Jeor resting burn, times a lump activity factor, then a goal offset. The result is an estimate, not a diet.
• Angle converterEvery angle routes through the radian, the SI unit. A half turn is π radians, 180 degrees or 200 gradians: three ways of describing the same opening.
• Length converterEvery value goes through the meter, then to the target unit. The inch is 25.4 mm exactly since the 1959 agreement, as in the NIST SI conversion factors.

Written by Rédaction HexaCalc (editorial team). Content last updated: August 25, 2026. No third-party medical or financial review is claimed.

• NIST Guide to the SI, Appendix B: Conversion Factors — National Institute of Standards and Technology
• The International System of Units (SI) — Bureau international des poids et mesures (BIPM)