Magnetic Field Units & Measurement
Two Systems of Units
Magnetic field measurements use two parallel systems of units, which is a common source of confusion:
- SI (International System) — Uses the tesla (T) as the unit of magnetic flux density. This is the modern standard used in science and engineering.
- CGS (Centimeter-Gram-Second) — Uses the gauss (G). This older system is still widely used in industry, especially in the United States.
The conversion is straightforward: 1 tesla = 10,000 gauss.
Tesla (T) — The SI Standard
Named after the inventor and electrical engineer Nikola Tesla, the tesla is the SI unit of magnetic flux density (also called magnetic induction or B-field).
One tesla is actually a very strong field — much stronger than most fields you'll encounter in everyday life. That's why we typically use smaller prefixed units:
- millitesla (mT) = 0.001 T — Strong permanent magnets at their surface
- microtesla (µT) = 0.000001 T — Earth's field, household items
- nanotesla (nT) = 10-9 T — Geophysical surveys, distant anomalies
- picotesla (pT) = 10-12 T — Biomagnetic fields (heart, brain)
- femtotesla (fT) = 10-15 T — Neuromagnetic signals, SQUID territory
Gauss (G) — The CGS Unit
Named after the mathematician and physicist Carl Friedrich Gauss, who made the first systematic measurements of Earth's magnetic field in the 1830s.
The gauss is widely used in industrial contexts, especially for permanent magnets, magnetic shielding, and electromagnetic compatibility (EMC) testing.
- 1 gauss = 100 microtesla
- 1 milligauss (mG) = 0.1 microtesla
Earth's field is about 0.25-0.65 gauss (or 250-650 milligauss), depending on location.
You'll often see the milligauss unit used in EMF (electromagnetic field) surveys and paranormal investigation equipment. A typical reading of Earth's field is 250-650 mG, while a running appliance might add 1-10 mG at close range.
Other Units: Oersted, A/m, Gamma
Oersted (Oe)
The oersted measures the magnetic field strength (H-field) in the CGS system — not the flux density (B-field) that tesla and gauss measure. In a vacuum, 1 oersted produces a flux density of exactly 1 gauss, so the two are numerically equivalent in free space. In magnetic materials, the relationship is more complex.
Ampere per Meter (A/m)
The SI unit for magnetic field strength (H-field). The relationship to oersted: 1 Oe = 79.577 A/m.
Gamma (γ)
An older unit still seen in geophysics: 1 gamma = 1 nanotesla (nT). Simple conversion, just a different name. You may encounter it in older geological survey data.
Interactive Unit Converter
Magnetic Field Strengths Compared
To give you a sense of scale, here are some common magnetic field strengths from the weakest detectable signals to the strongest fields in the universe:
Scale is logarithmic — each bar step represents many orders of magnitude. The full range spans over 25 orders of magnitude.
Magnetars — a type of neutron star — produce the strongest magnetic fields in the universe: around 1011 tesla (100 billion tesla). At that strength, the field would rearrange your atoms at the molecular level from thousands of kilometers away. Fortunately, the nearest known magnetar is about 9,000 light-years from Earth.
Common Prefixes
Here's a quick reference for SI prefixes as they apply to magnetic field measurements:
| Prefix | Symbol | Factor | Example |
|---|---|---|---|
| femto | fT | 10-15 T | Brain magnetic fields |
| pico | pT | 10-12 T | Heart magnetic fields |
| nano | nT | 10-9 T | Geophysical anomalies |
| micro | µT | 10-6 T | Earth's field, smartphone readings |
| milli | mT | 10-3 T | Small permanent magnets |
| (base) | T | 1 T | Strong magnets, MRI |
Magnetometer X lets you switch between microtesla (µT), gauss (G), and milligauss (mG) in the app settings. Microtesla is the default and the most commonly used unit for smartphone magnetometer readings.