How do you calculate the size of a drywell?

To calculate the size of a drywell, first determine the volume of runoff water it needs to handle, which can be estimated based on the area of the surface draining into it and the expected rainfall. Next, calculate the drywell's storage capacity by considering its dimensions (depth and diameter) and the porosity of the surrounding soil, which affects how quickly water can infiltrate. Use the formula for the volume of a cylinder (V = πr²h) to find the drywell's volume and ensure it meets or exceeds the runoff volume requirement. Lastly, consider local regulations and soil conditions, which may influence the final design.