A customer would like to tie his gutters into a drywell. How do I measure how big of a pit I need to dig? The soil is sandy so it drains well. I was going to do a 4'x4 pit filled with 1"-2" gravel. I'm sure it has to do with roof sq ft and amount of rain but the only formulas I found had to do with soil analysis. Looking for a quick real world answer. Is one of the basins they sell at supply yards that much better than a gravel filled pit?