How this instrument works
CGS stands for centimetre-gram-second, a coherent system of units built on those three base units the way SI is built on the metre, kilogram, and second. It was formalized in 1874 by the British Association for the Advancement of Science as a practical framework for electrical and mechanical measurement, decades before the modern SI (originally called MKS, for metre-kilogram-second) was adopted internationally. The erg, CGS's unit of energy, and the dyne, its unit of force, both derive from that base — one erg is the work done by a force of one dyne acting over a distance of one centimetre.
Because a gram is a fixed decimal fraction of a kilogram and a centimetre is a fixed decimal fraction of a metre, the erg-to-joule relationship is exact by construction rather than an experimentally measured ratio: 1 erg = 1 g·cm²/s², and expanding that in kilograms and metres gives exactly 1×10⁻⁷ J. There is no rounding anywhere in the 1×10⁻⁷ factor — it falls straight out of the definitions of gram and centimetre relative to kilogram and metre.
SI has replaced CGS in almost all engineering and everyday physics, but the erg hasn't fully disappeared. Astrophysics papers routinely quote the energy released by a supernova or a gamma-ray burst in ergs, because the joule figure for the same event would run to unwieldy powers of ten in the other direction — a habit that persists in the literature even as the rest of physics teaching moved to SI decades ago. Some older textbooks, and fields like plasma physics and spectroscopy that grew up around Gaussian-CGS electromagnetic units, still carry the erg forward as well.
- Enter an energy value into the Ergs field — it opens at 50,000,000, a round CGS-scale figure.
- Read the converted value in Joules (J) beneath it; it recalculates the instant the erg figure changes.
- Use it to translate an energy figure from an astrophysics paper or older physics text (quoted in ergs) into the SI joules used in most modern engineering work.
- Try a small value like 1 erg to see how tiny the CGS energy unit is next to the SI joule — the result comes out in scientific notation.
- Negative erg values are rejected — energy in this context is a magnitude, so the field only accepts zero or positive numbers.
Worked example — 50,000,000 ergs, a typical CGS-text energy figure
A physics text working in CGS units states an energy of 50,000,000 ergs — a round figure of the kind that appears often in older mechanics problems. Enter 50000000 into Ergs and Joules (J) reads 5 — five joules exactly, since 50,000,000 × 1×10⁻⁷ = 5 with nothing left to round because the conversion factor itself carries no uncertainty.
At the small end, a single erg — the CGS base energy unit — converts to just 1×10⁻⁷ joules, roughly the kinetic energy of a mosquito in flight, which is part of why CGS quantities in everyday physics look so much larger numerically than their joule equivalents. At the large end, 1 billion ergs converts to a clean 100 joules, the same exact-factor relationship scaled up.
Questions
What is the CGS system, and is it still used?
CGS (centimetre-gram-second) is a coherent unit system formalized in 1874, predating the modern SI. It has been largely superseded by SI in engineering and mainstream physics teaching, but it persists in specific pockets — astrophysics papers quoting stellar or explosive energies in ergs, and some plasma physics and spectroscopy literature that still uses Gaussian-CGS electromagnetic units.
How is the erg defined?
One erg is the work done by a force of one dyne acting through a distance of one centimetre, which expands to 1 g·cm²/s² in CGS base units. It's the direct CGS analogue of the SI joule, which is 1 kg·m²/s² — same structure, different base units.
Why is the erg-to-joule factor exactly 1×10⁻⁷, not an approximation?
Because it follows purely from how the gram and centimetre relate to the kilogram and metre — a gram is exactly 10⁻³ kg and a centimetre is exactly 10⁻² m. Cubing and squaring those relationships inside the erg's g·cm²/s² definition produces 1×10⁻⁷ exactly, with no experimental measurement or rounding involved anywhere in the derivation.
How much energy is one erg in everyday terms?
About 1×10⁻⁷ joules — roughly the kinetic energy of a mosquito in flight. It's a genuinely tiny unit next to the joule, which is part of why CGS-based physics problems often work with energy figures in the millions or billions of ergs to describe quantities a joule-based calculation would state in single digits.
Where does the erg still show up today?
Most visibly in astrophysics: papers on supernovae, gamma-ray bursts, and other high-energy astronomical events routinely quote the energy released in ergs rather than joules, a convention that has outlasted CGS's use elsewhere in physics simply because it keeps the numbers in a more familiar range for that literature.
What's the difference between CGS and SI (MKS)?
Both are coherent unit systems built the same way, just on different base units — CGS on the centimetre, gram, and second; SI (historically called MKS) on the metre, kilogram, and second. Every CGS derived unit, including the erg for energy and the dyne for force, has a direct SI counterpart (joule and newton) related by an exact power-of-ten factor, since the base units themselves differ by exact powers of ten.