CGS units are a system of measurement built on three base units: the centimeter for length, the gram for mass, and the second for time. The name “CGS” is simply an abbreviation of those three foundations. While the International System of Units (SI) has largely replaced CGS in everyday science and engineering, CGS units remain surprisingly common in certain fields, particularly magnetism, astrophysics, and theoretical physics.
Where the CGS System Came From
The CGS system was formally introduced by the British Association for the Advancement of Science in 1874. A group of prominent physicists, including James Clerk Maxwell and William Thomson (later known as Lord Kelvin), argued for basing the system on the centimeter, gram, and second rather than larger units. Their reasoning was practical: these smaller base units produced convenient numbers for the laboratory measurements common at the time. For decades, CGS was the dominant system in physics and engineering before the meter-kilogram-second (MKS) system, and eventually SI, took over as the international standard.
The Base and Mechanical Units
Everything in the CGS system derives from its three base units. Length is measured in centimeters (cm), mass in grams (g), and time in seconds (s). From those three, you can build units for any mechanical quantity.
- Force: the dyne. One dyne is the force needed to accelerate one gram by one centimeter per second squared. It’s a tiny amount of force. One newton (the SI unit) equals 100,000 dynes.
- Energy: the erg. One erg is the work done when a force of one dyne moves an object one centimeter. It takes 10 million ergs to equal a single joule.
- Pressure: the barye. One barye is one dyne of force spread over one square centimeter. Ten baryes equal one pascal.
These numbers highlight why CGS fell out of favor for many applications. The units are so small that real-world quantities produce awkwardly large numbers. A lightbulb using 60 watts, for example, consumes 600 million ergs per second.
Fluid Mechanics: The Poise and Stokes
Two CGS units have held on stubbornly in fluid mechanics. The poise, named after French physiologist Jean Poiseuille, measures dynamic viscosity (how thick or resistant a fluid is to flow). It’s defined as one dyne-second per square centimeter. In practice, you’ll often see the centipoise: water at room temperature has a viscosity of roughly one centipoise, which makes it a conveniently intuitive reference point.
The stokes, named after Irish physicist George Stokes, measures kinematic viscosity, which accounts for how a fluid’s density affects its flow. One stokes equals one square centimeter per second. The centistokes is more commonly used in practice. Both the poise and the stokes remain standard in industries like petroleum, food science, and coatings, where they’ve been entrenched for over a century.
Electromagnetic Units: Where Things Get Complicated
The CGS system’s biggest quirk is how it handles electricity and magnetism. Unlike SI, which uses a single unified set of electromagnetic units, CGS splits into three competing subsystems:
- ESU (electrostatic units) builds electromagnetic quantities from the force between electric charges, setting the electrostatic force constant equal to 1.
- EMU (electromagnetic units) builds from the force between current-carrying wires, setting the magnetic force constant equal to 1.
- Gaussian units combine the two, using ESU for electric quantities and EMU for magnetic ones. This is by far the most common CGS electromagnetic system in textbooks and research papers.
This split is one reason SI eventually won out as the global standard. As one overview in IEEE Magnetics Letters put it, SI “unifies magnetic and electrical units, whereas CGS bifurcates into EMU and electrostatic units.” The Gaussian system was the most internally consistent of the three, but having multiple subsystems created confusion, especially for students moving between textbooks that used different conventions.
CGS Magnetic Units Still in Wide Use
Despite SI being the official standard, CGS magnetic units remain deeply embedded in materials science and magnetism research. A 2022 tutorial in Nature’s Communications Physics noted that while the CGS system “is more prevalent” in magnetism literature, some sources use SI, creating a persistent stumbling block for newcomers.
The three magnetic units you’ll encounter most often are:
- Gauss (G): measures magnetic flux density (how strong a magnetic field is at a given point). One gauss equals 0.0001 tesla in SI. A typical refrigerator magnet produces about 50 gauss; Earth’s magnetic field is roughly 0.5 gauss.
- Oersted (Oe): measures magnetic field strength (the driving force that creates a magnetic field). Converting to SI involves dividing by 4π and multiplying by 1,000, giving units of amperes per meter.
- Maxwell (Mx): measures total magnetic flux (the total amount of magnetic field passing through a surface). One maxwell equals 10⁻⁸ webers in SI.
If you read data sheets for permanent magnets, magnetic recording media, or geophysics papers, you’ll likely see gauss and oersted rather than tesla and amperes per meter. The CGS values simply produce more convenient numbers at the scales these fields work with.
Why Some Physicists Still Prefer CGS
In theoretical physics, particularly electrodynamics, the Gaussian CGS system has a genuine mathematical advantage. All four electromagnetic field quantities (the electric field, the displacement field, the magnetic induction, and the magnetic field strength) share the same dimensions. In SI, they don’t, which means extra constants appear in equations to keep the units consistent.
The tradeoff is that Gaussian units aren’t “rationalized,” so factors of 4π show up inside Maxwell’s equations themselves. SI is rationalized, meaning those 4π factors get absorbed into the constants, making the core equations look cleaner. Which system feels more elegant depends on what you’re solving. For theoretical work where you want to see the physical symmetry between electric and magnetic fields, Gaussian CGS can be more transparent. For engineering calculations, SI’s unified structure is less error-prone.
Converting Between CGS and SI
For mechanical quantities, conversions are straightforward powers of ten:
- 1 dyne = 10⁻⁵ newtons
- 1 erg = 10⁻⁷ joules
- 1 barye = 0.1 pascals
- 1 poise = 0.1 pascal-seconds
- 1 stokes = 10⁻⁴ square meters per second
Electromagnetic conversions are messier because the systems define the quantities differently, not just in different sizes. Converting magnetic susceptibility data from CGS (reported in emu per mole) to SI (cubic meters per mole) requires multiplying by 4π × 10⁻⁶. That factor of 4π appears because CGS and SI distribute geometric constants differently across their equations. If you’re working with published data, always check which system the source uses before plugging numbers into formulas.
Where You’ll Encounter CGS Today
In day-to-day science and education, SI dominates. But CGS units persist in specific niches. Astrophysics papers routinely use CGS for quantities like luminosity (in ergs per second) and radiation pressure. Magnetism research, as noted, still leans heavily on gauss, oersted, and emu. Fluid dynamics and rheology use poise and stokes by convention. And many classic physics textbooks, particularly in electrodynamics, present equations in Gaussian CGS alongside or instead of SI.
If you’re a student encountering CGS for the first time, the key thing to remember is that the system isn’t fundamentally different from SI. It uses the same physical laws, just scaled differently. Most confusion comes not from the mechanical units, which convert neatly, but from the electromagnetic ones, where the underlying definitions diverge. Keeping a conversion table handy and double-checking the unit system of any source you’re referencing will save you from the most common mistakes.