Magnetic viscosity arises when the activation energy E required to produce a change in mangetization is provided thermally. Activation energy functions for single domain particles and materials with ``strong'' and ``weak'' domain-wall pinning are discussed. The magnetic viscosity parameter Sv, given by kT(∂E/∂H)-1T, where kT is the Boltzmann energy and H is the magnetic field, is shown to be proportional to kT/(v Ms), where Ms is the spontaneous magnetization and v is the activation volume swept out as the energy barrier is overcome. For single domain particles and strong domain-wall pinning the constant of proportionality is one while for weak pinning it is 1/2. The generality of this simple relationship is shown to be independent of the details of the activation model.