A magnetic material's response to an external field is measured by its magnetic susceptibility ($\chi$). For a diamagnetic material, $\chi$ is:
The text states that 'χ is small and negative for materials, which are termed diamagnetic. In the latter case M and H are opposite in direction.' Also, 'For diamagnetic materials χ = –10$^{-5}$ whereas χ = +10$^{-5}$ for paramagnetic materials.'