Magnetic Flux Density Converter (tesla, gauss)

Understanding the magnetic flux density converter — concept and principle

Magnetic flux density, usually written B and often simply called the magnetic field, describes how strong a magnetic field is at a point. You meet it in magnet specifications, MRI scanner ratings (commonly 1.5 T or 3 T), geophysics and compass work (the Earth's field), and EMF meters that read in milligauss. This converter switches between the SI tesla and its prefixed forms, the CGS gauss and milligauss, and the nanotesla (called the gamma in geophysics).

How it is calculated

Each unit is converted to tesla and then to the target: result = value × (T per "from" unit) ÷ (T per "to" unit). All factors are exact:

• 1 mT = 10⁻³ T, 1 µT = 10⁻⁶ T, 1 nT (gamma) = 10⁻⁹ T
• 1 G = 10⁻⁴ T, so 1 T = 10,000 G
• 1 mG = 10⁻⁷ T = 0.1 µT

The principle

The tesla is defined through the force a field exerts on a moving charge or a current. A wire 1 m long carrying 1 A at right angles to a uniform field of 1 T feels a force of 1 N (F = B × I × L), so 1 T = 1 N/(A·m), which equals 1 Wb/m². The gauss belongs to the older centimetre-gram-second (CGS) system; because a gauss is one maxwell per square centimetre, and 1 Wb = 10⁸ Mx while 1 m² = 10⁴ cm², the ratio is exactly 10⁴. Both units measure the same physical quantity, so conversion is a pure scale change.

Worked example

1. Inputs: value = 1, from tesla, to millitesla.
2. 1 T × 1 ÷ 0.001 = 1,000 mT.
3. The same field in other units: 1,000,000 µT; 10,000 G; 10,000,000 mG; 1,000,000,000 nT.

How to read the result

Typical magnitudes help you judge a number. The Earth's field at the surface is about 25–65 µT (0.25–0.65 G). Clinical MRI scanners commonly run at 1.5 T or 3 T. Everyday household EMF readings are usually stated in milligauss, where 1 mG = 0.1 µT. Field strength falls quickly with distance from a magnet or current, so readings depend strongly on where the probe is held.

Common mistakes

• Mixing up gauss (flux density B) with oersted (field strength H). In air they are numerically similar, but inside magnetic materials they differ.
• Dropping a factor of ten between mG and µT; remember 10 mG = 1 µT.
• Comparing a "surface field" figure on a magnet listing with a field measured centimetres away.

Frequently asked questions

How many gauss is one tesla?

Exactly 10,000 G.

How do I convert milligauss to microtesla?

Divide by 10. For example, 4 mG = 0.4 µT.

Is a nanotesla the same as a gamma?

Yes. Geophysicists call 1 nT one gamma (γ) when describing small variations of the Earth's field.

Sources: BIPM, The International System of Units (SI Brochure), 9th edition, 2019 · NIST Special Publication 811 (2008), Appendix B · D. J. Griffiths, Introduction to Electrodynamics, Cambridge University Press