From the StormTech product line
The StormTech SC-740 chamber optimizes storage volume in a small footprint. By providing 2.2 ft3/ft2 (0.67 m3/m2) [minimum] of storage, the SC-740 chambers can minimize excavation, backfill and associated costs.
3 Product Variations
SC-740 Cumulative Storage Volumes Per Chamber. Assumes 40% Stone Porosity. Calculations are based Upon a 6” (150 mm) Stone Base Under Chambers.