An equation used to calculate the interval velocity within a series of flat, parallel layers, named for American geophysicist C. Hewitt Dix (1905 to 1984). Sheriff (1991) cautions that the equation is misused in situations that do not match Dix's assumptions. The equation is as follows:

where V_{int} = interval velocity t_{1} = traveltime to the first reflector t_{2} = traveltime to the second reflector V_{RMS1} = root-mean-square velocity to the first reflector V_{RMS2} = root-mean-square velocity to the second reflector.