The CGS system takes the centimetre, the gram and the second as its base units, where SI takes the metre, the kilogram and the second. That single change of base propagates into every derived unit, which is why an erg is not a joule, a dyne is not a newton, and a poise is not a pascal second. The converter above handles the whole family in both directions and shows you the factor it used.
Arb Digital builds free reference tools that cover a subject completely rather than one slice of it. That matters here, because CGS is not a single system: the mechanical units are unambiguous, while the electrical and magnetic units come in Gaussian, electrostatic and electromagnetic flavours that assign different dimensions to charge. The tool keeps those separate rather than pretending they agree.
What This CGS to SI Converter Does
You choose a quantity, enter a value, and pick a direction. The hero shows the converted number with its unit, and the grid shows four things at once: the factor that converts one CGS unit into SI, the reciprocal factor going the other way, and your own value expressed in both systems simultaneously. Seeing both representations side by side is the fastest way to catch a direction error.
The coverage runs from length and mass through force, energy, power, pressure, dynamic and kinematic viscosity and acceleration, then into the electromagnetic units: gauss, oersted, maxwell, statcoulomb, abcoulomb, abampere, statvolt, statfarad and statohm. Every one of them still appears in current literature, in instrument specifications, or in older but still-cited experimental work.
This page spans the system. Where you need to move between many units of one electromagnetic quantity — tesla to millitesla to gauss to kilogauss, say — the magnetic field converter is the deeper tool, because it separates flux density, field strength and flux and offers every multiple of each. This converter gives you the one canonical CGS-to-SI step for each quantity and covers far more quantities.
How to Use It
- Pick the quantity, not the unit name. Gauss, oersted and maxwell all look like magnetic units and are three different physical quantities. The dropdown labels each pairing explicitly for that reason.
- Set the direction before you read the answer. The hero label and both grid captions change with it, so a mis-set direction is visible rather than silent.
- Check the factor in the grid. If it is not a power of ten, you are in the electromagnetic part of the system and the conversion depends on which CGS variant your source used.
- Raise the decimal places for the electrical units. Statcoulomb and statfarad factors need several significant figures before they mean anything.
- Read the reciprocal. It is often the number you actually want to remember: one tesla is ten thousand gauss, which is easier to carry around than the forward factor.
The Formula: How CGS Units Relate to SI
Mechanical CGS units follow directly from the base-unit change. A dyne is a gram centimetre per second squared, and since a gram is 10−3 kg and a centimetre is 10−2 m, one dyne is 10−5 N exactly. An erg is a dyne centimetre, so it is 10−7 J. A barye is a dyne per square centimetre, which works out at 0.1 Pa. A poise is a barye second, so it is 0.1 Pa·s, and a stokes is a square centimetre per second, or 10−4 m²/s.
Work the default. Ten million ergs converted at 10−7 J per erg gives exactly 1 J, and the reciprocal tells you that one joule is ten million ergs. That is the whole mechanical story: every factor is an exact power of ten, so nothing is ever lost to rounding.
The electromagnetic units are a different matter. Gaussian CGS defines charge so that Coulomb's law carries no explicit constant, which makes charge a mechanical quantity with dimensions of grams to the half power. The consequence is that the statcoulomb-to-coulomb factor contains the speed of light: one statcoulomb is 10 ÷ c coulombs, with c in centimetres per second, giving 3.335641 × 10−10 C. One statvolt is c ÷ 108 volts, or 299.792458 V exactly, since the speed of light is now a defined constant.
Magnetic units split three ways by quantity. One gauss is 10−4 T for flux density, one maxwell is 10−8 Wb for flux, and one oersted is 1,000 ÷ 4π = 79.577 A/m for field strength. The NIST Guide for the Use of the International System of Units, Special Publication 811, publishes these conversion factors, and the BIPM SI Brochure defines the SI units they convert into.
Why There Is No Single CGS System
Mechanics in CGS is unambiguous. Electromagnetism is not, and this trips up more readers than any other aspect of the subject. There are at least three CGS variants in common historical use. The electrostatic system, esu, defines charge from Coulomb's law and gives the statcoulomb. The electromagnetic system, emu, defines current from the force between parallel wires and gives the abampere and the abcoulomb. The Gaussian system mixes the two, taking electric quantities from esu and magnetic quantities from emu.
Because they start from different definitions, the same symbol can mean different things and the same equation can carry different constants. Maxwell's equations in Gaussian units contain explicit factors of the speed of light where the SI forms contain permittivity and permeability. Neither is more correct; they are different bookkeeping conventions for the same physics.
The practical consequence for anybody reading older literature is that you must establish which variant a source is using before you convert anything. An unlabelled electrical quantity in CGS is genuinely ambiguous. The tool lists the esu and emu charge entries separately precisely so that you have to make that choice consciously rather than by accident.
Why CGS Still Appears at All
SI has been the international standard for decades, so it is reasonable to ask why anyone still meets an erg or a gauss. Three reasons. First, entire literatures were written in CGS and continue to be cited: astrophysics in particular still quotes luminosities in ergs per second and magnetic fields in gauss, and converting the whole corpus is not worth the disruption.
Second, some CGS units are conveniently sized for their field. A gauss is roughly the scale of the Earth's magnetic field, which makes it a natural human unit in a way that the tesla is not. A poise sits close to the viscosity of common liquids. Working in units where everyday values land between one and a thousand is a real practical benefit.
Third, theoretical electromagnetism is often cleaner in Gaussian units, where electric and magnetic fields share the same dimensions and the equations become visibly symmetric. Many graduate texts still use Gaussian for exactly that reason, which means every physicist eventually needs to be fluent in both and to convert between them without ceremony.
Where This Sits Among the Other Converters
This page is organised by system rather than by quantity. If you know your quantity and simply want many unit options within it, the specialised converters are quicker: the energy converter, the force converter, the pressure converter, the power converter, the viscosity converter and the length converter all cover far more units of their own quantity than this one does. The electric charge converter and the capacitance converter do the same on the electrical side. For everything at once across ordinary metric and imperial units, the general unit converter is the starting point. This page exists for the one case those tools do not serve: moving a value between two entire systems of units and understanding why the factor is what it is.
Arb Digital builds free tools like this one because useful pages earn attention. If you want tools, calculators or content built for your own audience, we can help.
Browse All Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Treating gauss and oersted as the same unit — one measures flux density and the other field strength. They happen to be numerically equal in vacuum in Gaussian units, which is exactly why the confusion persists.
- Converting an electrical CGS value without knowing the variant — esu and emu charge units differ by a factor of about 3 × 1010. Guessing wrong is not a rounding error.
- Assuming every factor is a power of ten — true for the mechanical units, false for oersted and for everything involving the speed of light.
- Reading the direction selector after the answer — check it first. Both grid captions and the hero label follow it, so it is easy to verify before you trust a number.
- Rounding electrical conversions too early — a statcoulomb factor truncated to three figures will not reconcile with a published result quoted to six.
Related Free Tools From Arb Digital
For depth within a single quantity, use the energy converter, the force converter, the pressure converter, the power converter or the length converter. On the fluid side the viscosity converter covers poise and stokes among many others. For electromagnetism, the magnetic field converter, the electric charge converter and the capacitance converter go further within their quantities than this page does. The general unit converter handles everyday metric and imperial work. Everything Arb Digital publishes is listed on the free online tools hub.
Frequently Asked Questions
They use different base units. CGS builds on the centimetre, gram and second; SI builds on the metre, kilogram and second. Every derived unit inherits that difference, which is why a dyne is a hundred-thousandth of a newton and an erg is a ten-millionth of a joule.
Exactly ten million. An erg is a dyne centimetre, and since a dyne is ten to the minus five newtons and a centimetre is ten to the minus two metres, an erg is ten to the minus seven joules. The relationship is exact, with no rounding involved.
Divide by ten thousand. One tesla is ten thousand gauss, so a 15,000 gauss field is 1.5 tesla. The factor is exact because both units measure magnetic flux density and the relationship comes straight from the base-unit change.
Because SI and CGS rationalise Maxwell's equations differently. SI is rationalised, meaning factors of four pi appear in the constants rather than in the equations; Gaussian CGS is not. One oersted is a thousand divided by four pi amperes per metre, which is about 79.577.
Because Gaussian and electrostatic CGS define charge from Coulomb's law without a separate constant, which makes charge a mechanical quantity. Reconciling that with the SI definition, where current is a base quantity, introduces the speed of light into the factor.
No. The electrostatic and electromagnetic systems define their electrical quantities from different starting laws, and the charge units differ by roughly three times ten to the tenth. You must know which system a source used before converting anything electrical from CGS.
Not in practice. Astrophysics still quotes energies in ergs and magnetic fields in gauss, theoretical electromagnetism is often written in Gaussian units for their symmetry, and a large body of older literature remains in active use. SI is the standard, but fluency in both is still required.
This tool is provided for educational and reference use. Conversion factors follow published national metrology guidance, but where a historical source does not state which CGS variant it used, an electrical or magnetic value may be genuinely ambiguous and should be checked against the original definition rather than converted on assumption.
