The root-mean-square (RMS) stacking velocity for a horizontally layered sequence is calculated using the Dix formula, Vrms = sqrt[ sum(Vi^2 x ti) / sum(ti) ], where Vi is the P-wave velocity of each layer and ti is the one-way vertical travel time through that layer (ti = layer thickness / Vi); this formula weights each layer's velocity contribution by the proportion of total travel time spent within that layer.