Gardner's relation, or Gardner's equation, named after G. H. F. Gardner and L. W. Gardner, is an empirically derived equation that relates seismic P-wave velocity to the bulk density of the lithology in which the wave travels. The equation reads: where is bulk density given in g/cm3, is P-wave velocity given in ft/s, and and are empirically derived constants that depend on the geology. Gardner et al. proposed that one can obtain a good fit by taking and . Assuming this, the equation is reduced to: where the unit of is feet/s. If is measured in m/s, and the equation is: