Energy Converter
Convert energy between joules, kilojoules, calories, kilocalories, watt-hours, kilowatt-hours, BTU, and electron volts.
How to use this tool
- Enter energy and from unit in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your kilojoules and the full breakdown beneath it.
Formula
All units convert to joules (J) first, then to the target unit:
J = input × factorfrom
output = J ÷ factorto
Key factors: J = 1 | kJ = 1000 J | cal = 4.184 J | kcal = 4184 J | Wh = 3600 J | kWh = 3,600,000 J | BTU = 1055.06 J | eV = 1.602 × 10−19 J
How it works
The joule (kg·m²/s²) is the SI unit of energy. Each input unit has an exact or defined joule-equivalent factor; the converter multiplies to get joules, then divides to get each output. The thermochemical calorie (4.184 J) used here is the standard scientific calorie; the kilocalorie (food calorie) is exactly 1000 of these.
A very common mistake is confusing the calorie (cal, 4.184 J) with the kilocalorie (kcal, 4184 J): food nutrition labels express energy in kilocalories but often label them simply as "Calories" (capital C). Treating a food Calorie as a small calorie understates energy content by a factor of 1000.
Worked example
Convert 1 kilocalorie to all energy units
- Input: 1 kcal (from_unit = kcal), so base in joules = 1 × 4184 = 4184 J.
- kJ: 4184 / 1000 = 4.184 kJ. Small calories: 4184 / 4.184 = 1000 cal.
- Watt-hours: 4184 / 3600 ≈ 1.1622 Wh. Kilowatt-hours: 4184 / 3,600,000 ≈ 0.001162 kWh.
- BTU: 4184 / 1055.06 ≈ 3.9657 BTU.
4184 J | 4.184 kJ | 1000 cal | 1 kcal | 1.1622 Wh | 0.001162 kWh | 3.9657 BTU
Common mistakes to avoid
- Confusing food calories (kilocalories, kcal) with physics calories (cal) -- a nutrition label listing 250 Calories means 250 kcal = 250,000 cal, not 250 cal.
- Mixing up watt-hours (Wh) and joules without the time factor -- 1 Wh = 3,600 J because watts are joules per second and an hour has 3,600 seconds.
- Using BTU and BTU/hr interchangeably -- BTU is energy while BTU/hr is power (rate of energy); air conditioner ratings in BTU/hr are often loosely called BTU.
Key terms
- Joule (J)
- The SI unit of energy, equal to the work done by a force of one newton over one metre. Also equals one watt-second.
- Kilocalorie (kcal)
- Equal to 4184 joules. This is the unit used in food nutrition; the 'Calorie' on food labels is a kilocalorie.
- BTU (British Thermal Unit)
- An imperial energy unit equal to 1055.06 J, defined as the heat needed to raise one pound of water by 1 °F. Used in HVAC and heating.
- Kilowatt-hour (kWh)
- A practical unit of electrical energy equal to 3,600,000 joules (3.6 MJ). The unit used on household electricity bills.
Frequently asked questions
- Why does a food calorie equal 1,000 physics calories?
- Nutritional Calories (with a capital C) are actually kilocalories. The kilocalorie was the practical unit used in early nutrition science, and the convention stuck even as the lowercase calorie remained the physics unit.
- How many joules are in a kilowatt-hour?
- 1 kWh = 3,600,000 J (3.6 MJ). This comes from 1,000 watts multiplied by 3,600 seconds in an hour.
- What is a BTU used for?
- British Thermal Units measure heat energy in HVAC, refrigeration, and fossil fuel contexts in the US. One BTU is the energy needed to raise one pound of water by one degree Fahrenheit -- roughly the energy in a burning match.