The volume change component of deformation is often ignored or assumed to be zero in tectonic studies of metamorphic belts. However, when estimating original geometries of deformed regions, volume change is just as important as the other two components of deformation, finite strain and rotation. Major permanent volume change in metamorphic rocks is accomplished by solution transfer facilitated by flow of H₂O-rich fluids. Therefore, estimates of volume change can be combined with solubilities to estimate volumes of fluid flow. Previously applied methods for estimating rock volume change are based on estimates of absolute stretch, or changes in whole-rock chemical compositions. Estimates based on these approaches give large discrepancies even when applied to the same region. In this study, we develop a largely unexplored method for estimating volume change using the direction and deformation type of deformed mineral veins. The assumptions in this method are few and appropriate uncertainties can be estimated. Application of the new method to the metagreywacke in the Del Puerto Canyon of the Franciscan belt constrains the syn-metamorphic volume change to be greater than 7%, contrasting with previous proposals for large volume-loss in the same region. The results of previous studies can be modified taking into account grain rigid body rotation and grain boundary sliding. The final result of our approach yields a volume change of 7–21% vol.% and implies large amounts of water-rich fluid must have passed through the rock.