Sump Pump Sizing Calculator

This calculator sizes a sump pump from five inputs, but not every input touches every output. Required Pump Capacity is Water Inflow Rate times a fixed 1.5 safety factor and nothing else, so it is driven entirely by that one field. Total Dynamic Head adds Discharge Height to an estimated friction loss built from Discharge Pipe Length and the required capacity, so Pit Diameter and Pit Depth never factor into it. Recommended Pump HP then comes from capacity times head, rounded up to the nearest of four stocked motor sizes (1/3, 1/2, 3/4, or 1 HP).

Pit Diameter and Pit Depth do real work, just on a single output: they set the pit's working volume, which determines how much water the pump draws down each cycle, so a bigger pit means the pump runs less often -- Cycles per Hour is the only result that responds to pit size, while Discharge Height and Discharge Pipe Length are inert to it. Battery Backup Duration divides a fixed 480 Wh backup battery's capacity by the running pump's wattage, and because Required Pump Capacity is always exactly 1.5 times Water Inflow Rate, the duty-cycle term that scales this calculation is a constant regardless of how much water is actually coming in -- it responds only to which HP bracket the pump lands in. Pit dimensions have no effect on Battery Backup Duration at all. This does not size for check-valve losses, multiple pumps, or intermittent high-inflow surges beyond the steady rate entered.