The total volume of extrusive volcanism on the Moon provides a basic thermal and geologic constraint, and accurate volume assessments are contingent upon constraining lava flow depths. Here, employing UV-VIS data from the Clementine mission, we estimate mare thickness values in the Imbrium Basin by analyzing ejecta from large (>10 km diameter) impact craters that penetrate (or failed to penetrate) through the mare. Mare thickness values are found to range from at least ∼2 km at the basin center to 1.5–2.0 km in the buried ring shelf annulus. This corresponds to a basalt volume of ∼1.3 × 106 km3, almost a factor of two greater than the volume estimated from partially filled craters alone. Our results indicate that thickness measurements from penetrating craters, combined with minimum estimates from partially filled and non-penetrating craters, provide a more complete picture of the spatial variation of basalt thickness values than could previously be obtained.