Magnetic Field Converter

Magnetic field strength, more precisely the magnetic flux density, measures how concentrated a magnetic field is at a point. The SI unit is the tesla (T). It’s the partner of magnetic flux: flux (in webers) is the total field passing through an area, while the tesla tells you how tightly that field is packed.

Metric prefixes:

• 1 T = 1,000 mT (millitesla)
• 1 T = 1,000,000 µT (microtesla)

CGS unit (gauss):

• 1 T = 10,000 gauss (G)
• 1 kG = 1,000 G = 0.1 T

Fields you can picture:

• The magnetic field of a thought, measured over the scalp: about 1 pT
• The heart, measured at the chest: roughly 50 pT
• Earth’s field at the surface: 25-65 µT (0.25-0.65 G, or 25,000-65,000 nT)
• A fridge magnet: about 5 mT (50 G)
• A neodymium magnet at its pole face: 0.3-0.5 T
• A hospital MRI scanner: 1.5-3 T

The gauss refuses to retire because a tesla is enormous for everyday magnets. Quoting a fridge magnet as “50 gauss” feels more natural than “5 millitesla,” so magnet datasheets still reach for it. Geophysics went the other way and standardised on the nanotesla: Earth’s field is quoted as roughly 50,000 nT, and a magnetic survey map is contoured in nT because the anomalies being hunted are a few tens of nanotesla against that background.

At the bottom of the scale sits biomagnetism, and the numbers there are absurd. A magnetoencephalogram reads brain activity at around one picotesla, which is fifty million times weaker than the Earth’s field the machine is sitting in. That is why those instruments live inside magnetically shielded rooms and use SQUID sensors cooled with liquid helium: the signal is not faint compared to the noise, it is faint compared to the planet.

The MRI number puts the scale in perspective. A 3 T scanner runs about 50,000 times stronger than Earth’s field, which is exactly why a loose wrench becomes a projectile in that room. Field strength also drops off fast with distance, roughly with the cube of distance for a small magnet, so a magnet that grabs hard on contact does almost nothing an inch away.