Magnetic unit conversions: gauss, tesla, and field strength

Magnetism is the one area on UnitSnap where two full unit systems compete for daily use: the SI system, built around the tesla, and the older CGS (centimeter-gram-second) system, built around the gauss, which persists stubbornly in magnet datasheets, hobbyist electronics, and some geophysics literature. This guide covers the four categories in UnitSnap's Magnetism group, magnetomotive force, magnetic field strength, magnetic flux, and magnetic flux density, and focuses on the gauss-to-tesla conversion that causes the most confusion.

Magnetic flux density: gauss versus tesla

Magnetic flux density, often just called magnetic field in casual use, is measured in tesla (T) in SI and gauss (G) in CGS, with a conversion factor of exactly 10,000 gauss per tesla, making it one of the easier unit conversions once the factor is memorized: divide gauss by 10,000 to get tesla, or multiply tesla by 10,000 to get gauss. Earth's magnetic field at the surface is roughly 0.25 to 0.65 gauss (25 to 65 microtesla) depending on location, a refrigerator magnet is typically 50 to 100 gauss at its surface, a neodymium magnet can exceed 5,000 gauss (0.5 tesla) at its surface, and an MRI machine runs 1.5 to 3 tesla, 15,000 to 30,000 gauss, which is why the room around one is treated as a serious safety zone for anything ferromagnetic.

Magnet datasheets from manufacturers frequently quote gauss because it produces larger, more marketable-looking numbers and because the convention predates widespread SI adoption in the permanent-magnet industry, so converting to tesla is often necessary before plugging a magnet's rating into an SI-based engineering formula.

Magnetic field strength: a related but distinct quantity

Magnetic field strength, in amperes per meter (A/m) in SI or oersted (Oe) in CGS, is closely related to flux density but is not the same quantity: field strength describes the magnetizing force applied, independent of the material's response, while flux density describes the resulting field including how the material responds to it. In free space the two are proportional through a fixed constant, but inside a magnetic material they diverge, which is the entire basis of how transformer cores and permanent magnets are characterized. The CGS-to-SI conversion here is 1 oersted equals roughly 79.58 A/m.

Magnetic flux: the total field through a surface

Magnetic flux, in webers (Wb) in SI or maxwells in CGS, measures the total magnetic field passing through a given area, rather than the field's intensity at a point, and it is the quantity behind Faraday's law of induction, the physics that makes generators, transformers, and induction charging work. The conversion is 1 weber equals 100,000,000 (10⁸) maxwells, a large factor that reflects how much smaller the CGS unit is.

Magnetomotive force: the magnetic analog of voltage

Magnetomotive force, in ampere-turns (A·t) in SI or gilberts in CGS, plays a role in magnetic circuits analogous to voltage in electrical circuits: it is the driving force that pushes magnetic flux through a magnetic path, such as a transformer core, and it depends on both the current through a coil and the number of turns in that coil. This quantity mostly appears in transformer and electromagnet design rather than in consumer contexts, but the same gauss-and-tesla instinct applies: check which system a spec sheet is using before comparing it to a design calculation done in the other one.

Where these units actually show up

Speaker magnets are one of the most common places a consumer encounters a gauss rating directly: a speaker datasheet listing magnet flux density in gauss is describing the same quantity an MRI datasheet describes in tesla, just at a vastly smaller scale, typically hundreds to a few thousand gauss rather than tens of thousands. A stronger magnet in a speaker generally means tighter control over the cone's motion and better bass response, which is why magnet flux density is treated as a meaningful spec rather than a marketing afterthought.

Electric motors and generators depend on magnetic flux density in the air gap between rotor and stator, typically in the range of 0.5 to 1.5 tesla for common industrial designs, a figure engineers work with directly in SI rather than converting from gauss, since motor design calculations are built on SI formulas from the start. Hall-effect sensors, used in everything from a car's crankshaft position sensor to a smartphone's compass, are calibrated in either unit depending on the manufacturer, which is precisely why a datasheet's stated unit needs to be checked before comparing two sensors' sensitivity figures, and a design that assumes one convention while the component was characterized in the other will misjudge its actual sensitivity by a factor of 10,000.

Geophysics and compass work sit at the small end of this scale entirely, measuring Earth's field and its local variations in nanotesla or gamma (an older CGS unit numerically equal to nanotesla) rather than gauss or tesla directly, since the whole field of interest is a tiny fraction of a single gauss. A magnetometer survey reporting a 50 nanotesla anomaly is describing a real, measurable, but extremely small deviation from the roughly 50,000 nanotesla (0.5 gauss) background field most locations on Earth's surface experience.

Working confidently with magnetic units

The gauss-tesla factor of 10,000 is worth memorizing outright since it recurs in every calculation in this group, and the safest habit is converting any CGS figure to SI immediately on reading it, before it gets combined with other quantities, rather than trying to track two unit systems through a multi-step calculation. When a magnet's rating looks implausibly large, four or five digits, it is very likely quoted in gauss rather than tesla, and dividing by 10,000 will usually bring it back to a sensible range. Keeping a short list of anchors close at hand, Earth's field, a fridge magnet, a neodymium magnet, an MRI scanner, turns a converted number from an abstract figure into something that can be checked at a glance, the same habit that makes every unit family in this guide easier to trust on the first read rather than on the third careful check.