Energy Converter
Convert between joules, kilojoules, calories, kilocalories, watt-hours, kilowatt-hours, BTU, electronvolts, foot-pounds, ergs, and more — instantly.
All Unit ConvertersAll conversions from 1 joule
| Unit | Symbol | Value |
|---|
How the energy converter reconciles joules, calories, BTUs and electronvolts in one table
Energy shows up in more incompatible units than almost any other physical quantity, because every field that measures it grew its own convention before anyone agreed on a standard. Physicists use joules and electronvolts. Nutrition labels use kilocalories. American utility bills use BTUs and kilowatt-hours. Explosive yield gets measured in tons of TNT. This converter treats the joule, the SI coherent unit for energy, as the hub and routes all nineteen other units through it.
Every multiplication happens client side against a fixed array of factors. There is no lookup service and no external API call involved, so the tool works offline once the page has loaded.
The conversion path, and the surprising factor for calories
Full unit table, factor relative to one joule
| Unit | Symbol | Factor (J) |
|---|---|---|
| Joule | J | 1 (base unit) |
| Kilojoule | kJ | 1,000 |
| Megajoule | MJ | 1,000,000 |
| Gigajoule | GJ | 1,000,000,000 |
| Calorie (thermochemical) | cal | 4.184 |
| Calorie (15°C) | cal₁₅ | 4.18580 |
| Kilocalorie (dietary) | kcal | 4,184 |
| Watt-hour | Wh | 3,600 |
| Kilowatt-hour | kWh | 3,600,000 |
| Megawatt-hour | MWh | 3,600,000,000 |
| BTU (IT) | BTU | 1,055.05585 |
| Therm (US) | thm | 105,480,400 |
| Foot-pound force | ft·lbf | 1.35581795 |
| Inch-pound force | in·lbf | 0.112984829 |
| Erg | erg | 0.0000001 |
| Electronvolt | eV | 1.602176634e-19 |
| Kiloelectronvolt | keV | 1.602176634e-16 |
| Megaelectronvolt | MeV | 1.602176634e-13 |
| Hartree | Eₕ | 4.3597447222e-18 |
| Ton of TNT | tTNT | 4,184,000,000 |
Two unit families worth understanding
Electronvolt scale, from eV to hartree
The electronvolt and its multiples describe energy at the scale of individual particles and chemical bonds. The hartree, used in computational chemistry, is roughly 27.2 eV and represents the electric potential energy between two electrons separated by one Bohr radius.
Utility and heating units
The BTU and therm dominate US natural gas and HVAC billing. A therm is defined as exactly 100,000 BTU, and utility invoices routinely quote gas usage in therms while equipment ratings quote BTU per hour.
Where the definitions come from
The joule itself is named for James Prescott Joule, whose 19th century experiments established the mechanical equivalent of heat. The electronvolt’s factor became an exact defined number rather than a measured one after the 2019 redefinition of the SI base units fixed the elementary charge at precisely 1.602176634×10⁻¹⁹ coulombs. The ton of TNT as an energy unit dates to nuclear weapons yield reporting during and after the Manhattan Project, standardized at 4.184 gigajoules per ton by convention rather than by measuring an actual ton of explosive, since real TNT detonations vary.
Energy unit references
- BIPM SI Brochure defines the joule and documents the 2019 fixing of the elementary charge used for the electronvolt.
- NIST Units and Systems of Measurement covers accepted non-SI energy units including the BTU and calorie variants.
- US Energy Information Administration, Units and Calculators explains therms and BTUs as used on real utility bills.
- FDA Nutrition Facts Label guidance confirms the dietary Calorie is a kilocalorie in scientific terms.
Energy figures across disciplines
Converting a food label’s Calorie count into kilojoules for a diet tracker that uses metric units, translating a US utility bill from therms into kilowatt-hours to compare gas and electric heating costs, working through a physics or chemistry problem set that mixes joules and electronvolts, sizing HVAC equipment rated in BTU per hour against a European appliance rated in watts, and putting historical explosive yield figures quoted in tons of TNT into joules for a classroom or research comparison.
Common Questions
FAQ: Energy Converter
One kilocalorie (kcal) equals exactly 4184 joules. This is the “dietary calorie” you see on food packaging. The thermochemical calorie is slightly different at 4.184 joules (singular), meaning one kcal = 1000 thermochemical calories.
One BTU (British Thermal Unit, IT definition) equals 1055.05585 joules. The BTU is still widely used in the US for air conditioning, heating, and natural gas measurement. A BTU is roughly the energy released burning a single wooden match.
One kilowatt-hour equals 3,600,000 joules (3.6 MJ). This is because 1 kW = 1000 J/s, and 1 hour = 3600 seconds, so 1 kWh = 1000 × 3600 = 3,600,000 J.
Technically, one kilocalorie equals 1000 calories. However, food nutrition labelling uses “Calorie” (with a capital C) to mean kilocalorie. So when a food label says 200 Calories, it means 200 kcal = 836,800 joules.
From the blog
Reading around units
Where measurement systems came from and why two of them still argue.
