Units to quantify it include the tesla (SI)
and the gauss (CGS).
Another measure of magnetism is the magnetic field strength (H). The physical quantities
(B and H) are equivalent for the portion of a magnetic
field in a vacuum, but differ where the magnetic field is passing
through some material.
As a density (per unit volume), both B and H can be
small though the magnetic field is spatially large,
and large magnetic fields generally have smaller such densities:
the density of Earth's magnetic field (as we measure it
at the surface) is tiny compared to that within a
refrigerator magnet, but is large compared to that
of the surrounding solar system, which is large
compared to that typical of the entire galaxy.