Short answer
A properly sized sump pump is not based on basement square footage. It is sized to your local water table, peak hydrostatic pressure, pit volume, and total dynamic head (vertical lift plus pipe resistance). Most residential basements need a ⅓ HP or ½ HP pump, but high-water-table sites in Western Washington often need engineered ¾ HP systems or dual-pump setups.
Dive deep
The deciding number is gallons per minute (GPM) at peak head height: how fast water fills the basin in a severe storm versus how fast the pump can discharge through your actual pipe length and elevation rise. On site we measure static and friction head (basin floor to discharge exit, pipe diameter, and fittings), basin volume so cycles are long enough to protect the motor, peak inflow when soils are saturated, dedicated electrical capacity for startup draw, and whether the check valve or exterior discharge line is choking flow.
Buying “the biggest HP available” is a common mistake. An oversized pump can empty the pit too fast, short-cycle, overheat, wear out the switch, and hammer the discharge line. Sizing is precision work, not maximum power.
When it applies
- New active drainage / sump installation
- Replacing an aged or failing pump
- Very short cycle times in rain (on/off every few seconds)
- Standing water near the basin during heavy storms
When it does not
- Passive dampness or humidity without an active collection path
- Surface runoff from bad grading toward the house
- Roof/downspout discharge dumping next to the foundation
- Those need exterior diversion first — pump size alone will not fix them
What we do next
If your pump runs constantly in heavy rain or cannot keep the basin down, get an engineered flow-rate assessment before the switch fails. See our Seattle sump pump replacement and sump pump services for how we size and install the system correctly.
