One millitesla is exactly 10 gauss. This is the practical scale of real magnets — fridge magnets, loudspeaker gaps, motor air gaps and magnetic separators all live between a few gauss and a few thousand.
About the units
A millitesla is a thousandth of a tesla, and a gauss is 10⁻⁴ tesla, so the relationship is a clean factor of ten. It is the tidiest conversion in the category and the one worth memorising, since it covers the range most magnetic work actually happens in. Below this scale, microtesla and nanotesla cover the Earth's field and biomagnetic signals; above it, tesla covers MRI and research magnets. The millitesla and the gauss between them span from a weak stray field to the surface of a strong permanent magnet.
The exact factor
The factor is exactly 10. Reference values: Earth's field 0.05 mT or 0.5 G; a fridge magnet 5 mT or 50 G; a loudspeaker magnet gap 1,000 mT or 10,000 G; the threshold for pacemaker interference around 0.5 mT or 5 G; and the five-gauss line around an MRI scanner, which marks the boundary beyond which ferromagnetic objects become projectiles. That last figure is a regulatory boundary in every MRI installation and is always stated in gauss, never in millitesla.
Where you meet this conversion
Magnet selection and magnetic safety. Permanent magnet datasheets give surface field in gauss and pull force in newtons. Hall effect sensors are specified in millitesla, so pairing a sensor to a magnet requires the conversion. Magnetic field exposure limits for workers and for implanted medical devices are published in millitesla, while the warning signage around MRI suites uses the five-gauss line. Magnetic separation in recycling and mineral processing specifies field strength in gauss at a stated working distance.