This calculator converts the erg, the CGS (centimetre-gram-second) unit of energy, into the units you actually work with day to day, and back again. Ergs are vanishingly small, about the kinetic energy of a mosquito in flight, so you will mostly meet them in older physics texts, astrophysics papers quoting stellar output in erg per second, or particle physics literature that still mixes CGS and SI units. Enter any value in the input unit dropdown, choosing from erg, joule, kilojoule, calorie, kilocalorie, BTU, kilowatt-hour, electron volt, foot-pound or watt-hour, and the calculator instantly returns the equivalent in ergs, joules, kilojoules and calories in the four result tiles, plus a full conversion table showing every unit alongside its exact multiplication factor from one erg. A quick reference dropdown lets you jump straight to common values, such as one joule, one kilocalorie or one electron volt, without typing anything. Every figure updates live as you type, so you can check a result or compare several units side by side in seconds. The underlying conversions are fixed by definition, since 1 erg equals exactly 10 to the power of negative 7 joules, and use standard reference values for calories, BTU, electron volts and foot-pounds, so results are exact rather than estimated, though you should confirm which calorie definition a source uses before quoting a figure in scientific work.
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Electron volts = 1 J / 1.60218 x 10-19 = 6.2415 x 1018 eV
What Is an Erg?
The erg is the unit of energy (or work) in the CGS (centimetre-gram-second) system of units. It is defined as the work done by a force of one dyne acting through a distance of one centimetre. The name comes from the Greek word ergon, meaning work.
One erg is an extremely small amount of energy. To put it in perspective: one joule equals exactly 10,000,000 ergs (107 erg), and one erg equals 0.0000001 joules (10-7 J). A single erg is roughly the kinetic energy of a flying mosquito.
Erg Conversion Formula
All erg conversions are based on the exact relationship between the erg and the joule:
1 erg = 10-7 J (exactly)
From this, all other conversions follow by applying the standard SI and non-SI energy conversion factors:
To convert from ergs to...
Multiply ergs by
Example: 1 erg =
Joules (J)
1 x 10-7
1 x 10-7 J
Kilojoules (kJ)
1 x 10-10
1 x 10-10 kJ
Calories, thermochemical (cal)
2.39006 x 10-8
2.39006 x 10-8 cal
Kilocalories (kcal)
2.39006 x 10-11
2.39006 x 10-11 kcal
BTU (British Thermal Unit)
9.47817 x 10-11
9.47817 x 10-11 BTU
Watt-hours (Wh)
2.77778 x 10-11
2.77778 x 10-11 Wh
Kilowatt-hours (kWh)
2.77778 x 10-14
2.77778 x 10-14 kWh
Electron volts (eV)
6.24151 x 1011
6.24151 x 1011 eV
Foot-pounds (ft-lb)
7.37562 x 10-8
7.37562 x 10-8 ft-lb
Where Is the Erg Used?
The CGS system (and the erg) was the dominant scientific unit system before the SI (International System) was adopted globally. Today, the erg appears most frequently in:
Astrophysics: stellar luminosity and energy output are traditionally quoted in erg/s. The Sun's luminosity is approximately 3.83 x 1033 erg/s. Gamma-ray bursts, among the most energetic events in the universe, release 1047 to 1053 ergs.
Particle physics: older papers use ergs alongside electron volts for energy comparisons.
Historical scientific texts: any pre-1970s physics or chemistry literature using CGS units will state energies in ergs.
Geophysics: earthquake energy was historically measured in ergs before the moment magnitude scale became standard.
In modern physics, engineering, and everyday science, the joule (J) is the preferred unit of energy under the SI system.
Erg vs Joule: Key Comparison
The joule is 10 million times larger than the erg. Both measure the same physical quantity (energy or work), but they arise from different base unit systems:
Erg (CGS): based on centimetres, grams, and seconds. 1 erg = 1 g cm2 s-2.
Joule (SI): based on metres, kilograms, and seconds. 1 J = 1 kg m2 s-2.
Because 1 metre = 100 cm and 1 kilogram = 1,000 g, the ratio works out as: 1 J = 1 kg m2 s-2 = 1,000 g x (100 cm)2 s-2 = 1,000 x 10,000 g cm2 s-2 = 107 erg.
Sources and method: Conversion factor 1 erg = 10-7 J is exact by SI definition (BIPM). Calorie conversion uses the thermochemical calorie: 1 cal = 4.184 J exactly (ISO 31-4). BTU conversion: 1 BTU = 1,055.05585262 J (ISO 31-4). Electron volt: 1 eV = 1.602176634 x 10-19 J (CODATA 2018 exact value). Foot-pound: 1 ft-lb = 1.3558179483314 J.
This calculator provides exact conversions based on defined SI relationships. For scientific publications, always state which calorie definition you are using (thermochemical, international table, or 15-degree), as they differ slightly.
