People often say "magnetic field" as if there were only one quantity, but practical magnetics uses both B and H. Data sheets, textbooks, and instruments can switch between them without much warning. This guide explains what each symbol means and why Tesla and A/m are not just two labels for the same thing.

1. B and H describe different parts of the picture

Magnetic field strength H describes the magnetizing field produced by current, coils, or magnetic circuits. Magnetic flux density B describes the resulting magnetic flux per area in space or inside a material. In vacuum the two are linked by B = μ0H, but inside real materials that relationship changes with permeability.

That is why B and H are related but not identical. When a material responds strongly to magnetization, B can change a lot even if the applied H field changes only modestly.

2. Which units belong to which quantity

B is commonly expressed in Tesla, Gauss, mT, or μT. H is commonly expressed in A/m, kA/m, or Oersted. If a page converts B to H directly, it must also state the permeability assumption; otherwise the conversion is not physically complete.

  • Use B when reading flux density from Hall probes, magnet specs, or field maps.
  • Use H when describing magnetizing force, coils, and drive conditions.
  • Do not treat B and H as pure unit swaps unless the medium is clearly specified.

3. Why materials matter

Air and vacuum respond weakly, so B and H stay nearly proportional there. Ferromagnetic materials are different. Their permeability can be large, nonlinear, and history-dependent, which is why magnetic cores, permanent magnets, and hysteresis curves need more care than simple unit conversion alone.

This is also why the same coil current can produce very different B values depending on whether the path is mostly air, ferrite, or steel.

4. Practical reading habits

When you read a sensor output, magnet data sheet, or research paper, check the symbol first, then the unit, and then the material assumption if B and H are both mentioned. That order prevents many interpretation mistakes.

In day-to-day work, B is often the quantity people measure, while H is often the quantity engineers control.

If you need a quick way to convert Tesla, Gauss, mT, μT, A/m, kA/m, and Oersted, try our Magnetic Field Converter.