The magnetic flux density (B) is the quotient of the magnetic flux Phi (`phi`) and the area "A" through which it passes. It is derived from the force on moving charges. The flux density is like the field strength (H) a directed quantity.
In vacuum and in air, the flux density (B) corresponds to the product of the magnetic field constant µ0 and the magnetic field strength (H). The ratio of magnetic flux density to magnetic field strength in empty space is the magnetic field constant µ0.
Strong permanent magnets, e.g. holding magnets with a breakaway force of 1,000 Newton (N), reach a magnetic flux density of 0.5 Tesla (T) to 1 Tesla. The earth's magnetic field is approximately 0.05 Tesla. The flux density of a coil also depends on whether or not iron is inside the coil. Iron increases the magnetic flux density of a current-carrying coil. The reason for this are the Weiss' districts.