CGS System of Units Converter
Convert between CGS (centimetre-gram-second) units and their SI (International System of Units) equivalents for 10 physical quantities. Enter a value in either system and the calculator instantly applies the exact conversion factor, bidirectionally. Choose the quantity, type your number in CGS or SI, and both results update in real time.
What is the CGS system of units?
The CGS (centimetre-gram-second) system is a coherent metric system built on three base units: the centimetre for length, the gram for mass, and the second for time. It was formalised in 1874 by a committee of the British Association for the Advancement of Science and became the dominant system in physics and chemistry through the late 19th and early 20th centuries. Today the International System of Units (SI) is the global standard for science and commerce, but CGS persists in several specialised fields. Astrophysicists commonly express luminosity and energy in ergs, geophysicists measure gravitational anomalies in milligals, and laboratory magnetism literature still quotes field strengths in gauss and oersted.
CGS mechanical units - force, energy, and pressure
The three most widely encountered CGS mechanical units each have distinct named forms. The dyne is the unit of force: one dyne accelerates one gram at one centimetre per second squared, making it exactly 10^-5 newtons. The erg is the unit of energy or work: one erg is the work done by one dyne acting over one centimetre, equal to exactly 10^-7 joules. The barye (sometimes spelled barie) is the unit of pressure: one dyne per square centimetre, equal to 0.1 pascals. Because these mechanical factors are all exact powers of ten, conversion is straightforward arithmetic.
CGS viscosity units - poise and stokes
Dynamic viscosity in CGS is measured in poise (symbol P), named after Jean Poiseuille. One poise equals 0.1 pascal-seconds. The centipoise (cP) is especially practical: pure water at 20 degrees Celsius has a dynamic viscosity of almost exactly 1 cP, making the unit intuitive for aqueous solutions. Kinematic viscosity (dynamic viscosity divided by fluid density) uses the stokes (symbol St), equal to 10^-4 m squared per second. Again the centistoke (cSt) is the practical subunit: water at 20 degrees C has a kinematic viscosity of about 1 cSt. Viscosity standards in food science, petrochemicals and lubricants are still routinely quoted in centipoise and centistokes.
Gaussian CGS electromagnetic units
Electromagnetic units in CGS are more complex than the mechanical ones because there are actually three competing CGS electromagnetic subsystems: Gaussian (the most common), electrostatic (ESU), and electromagnetic (EMU). This converter uses Gaussian CGS, which is the standard in theoretical physics and astrophysics. In Gaussian CGS, the speed of light c appears explicitly in electromagnetic relations, leading to conversion factors that include c or its inverse. The franklin (statcoulomb) is c times smaller than the SI coulomb (1 Fr = 3.336 x 10^-10 C). The statvolt is c times larger than the volt (1 statV = 299.79 V). The gauss (magnetic flux density) converts exactly at 1 G = 10^-4 T, and the oersted (magnetic field intensity) converts at 1 Oe = 1000/(4 x pi) A/m, approximately 79.58 A/m.
CGS named units and their SI equivalents
| Quantity | CGS Unit | Symbol | SI Unit | Exact Factor |
|---|---|---|---|---|
| Acceleration | gal (galileo) | Gal | m/s² | 1 × 10^-2 |
| Force | dyne | dyn | N | 1 × 10^-5 |
| Energy | erg | erg | J | 1 × 10^-7 |
| Pressure | barye | Ba | Pa | 1 × 10^-1 |
| Dynamic viscosity | poise | P | Pa·s | 1 × 10^-1 |
| Kinematic viscosity | stokes | St | m²/s | 1 × 10^-4 |
| Electric charge | franklin/statcoul | Fr | C | 3.336 × 10^-10 |
| Voltage | statvolt | statV | V | 2.998 × 10^2 |
| Magnetic B-field | gauss | G | T | 1 × 10^-4 |
| Magnetic H-field | oersted | Oe | A/m | 79.577 |
Exact conversion factors defined by NIST and BIPM. Electromagnetic entries use the Gaussian CGS subsystem.
Frequently asked questions
How many dynes equal one newton?
Exactly 100,000 dynes (10^5 dyn) equal one newton. This follows directly from the base-unit definitions: 1 N = 1 kg m/s^2 and 1 dyn = 1 g cm/s^2, so the ratio is (10^3 g / 1 g) x (10^2 cm / 1 cm) = 10^5.
Why is the erg so small?
An erg equals 10^-7 joules, meaning 10 million ergs make up one joule. The unit is small because it is built on the centimetre and gram rather than the metre and kilogram. One erg is roughly the kinetic energy of a flying mosquito. It is a natural unit for astrophysical energies, where individual photon or particle energies are expressed in ergs without unwieldy exponents.
What is the difference between gauss and tesla?
Both measure magnetic flux density (the B-field), but 1 tesla = 10,000 gauss (10^4 G). An MRI machine operates at 1.5 to 3 T (15,000 to 30,000 G). Earth's magnetic field at the surface is roughly 0.25 to 0.65 G (25 to 65 microtesla). Gauss remains the preferred unit in many laboratory instruments, permanent magnet datasheets, and astrophysics publications.
Why do electromagnetic CGS conversions involve the speed of light?
In Gaussian CGS, electrostatic and magnetic units are unified through Coulomb's law, written without a proportionality constant (k = 1). This forces the speed of light c into the relationship between electric and magnetic quantities. The statvolt is about 300 V and the franklin is about 3.3 x 10^-10 C precisely because c is approximately 3 x 10^10 cm/s. SI avoids this by introducing the permittivity of free space as an independent constant.
Is CGS still used in science today?
Yes, in several areas. Astrophysics papers routinely use ergs, gauss, and centimetres. Geophysics uses the milligal for gravitational anomalies. Particle physics uses Gaussian CGS for electromagnetic fields. Rheology and the petroleum industry widely use centipoise and centistokes for viscosity. Engineering almost universally uses SI, but any student reading older literature or working in these specialised fields will encounter CGS regularly.
What is a barye and how does it relate to the bar?
The barye (Ba) is the CGS unit of pressure, equal to 1 dyne per square centimetre (0.1 Pa). It is not the same as the bar: one bar equals 10^6 barye (one megabarye). Atmospheric pressure is about 1,013,250 barye or 1.01325 bar. The barye is rarely encountered in modern usage, but it appears in older physical chemistry references and some geophysical literature.