One tesla (T) equals exactly 10 000 gauss (G). This exact relationship follows from the SI and CGS-Gaussian unit definitions:

• 1 T = 10 000 G
• 1 mT = 10 G
• 1 µT = 0.01 G
• 1 G = 0.0001 T = 0.1 mT = 100 µT

To convert T to G, multiply by 10 000. To convert G to T, divide by 10 000.

The tesla (T) is the SI unit of magnetic flux density (also called magnetic induction), defined as one weber per square metre (Wb/m²). It measures the strength of a magnetic field at a given point — specifically, how much magnetic flux passes through a unit area perpendicular to the field.

The unit is named after inventor and physicist Nikola Tesla. One tesla is a relatively strong field: a typical refrigerator magnet produces about 5–10 mT (0.005–0.01 T), while clinical MRI machines operate at 1.5–3 T.

Most clinical MRI scanners operate at 1.5 T or 3 T (15 000 G or 30 000 G). Higher-field research systems reach 7 T at leading centres, with the record whole-body human MRI at 11.7 T (the Iseult magnet at CEA NeuroSpin, France). The field must be extremely uniform across the imaging volume to produce clear images.

For comparison:

• Earth's surface field: ~25–65 µT (0.25–0.65 G)
• Fridge magnet: ~5–10 mT (50–100 G)
• Neodymium magnet: 1–1.4 T (10 000–14 000 G)
• Clinical MRI: 1.5–3 T (15 000–30 000 G)
• Research MRI: 7–11.7 T (70 000–117 000 G)

Earth's magnetic field varies with location, but typical values at the surface range from about 25 µT near the equator to about 65 µT near the poles:

Geophysicists and geomagnetists historically used nanotesla (nT), sometimes called gamma (γ), where 1 γ = 1 nT = 0.00001 G. The field at Earth's surface is roughly 25 000–65 000 nT.