Most sump pump buying guides stop at "1/3 HP for most homes, 1/2 HP for tough spots" — a rule of thumb that ignores the two things that actually decide whether a pump can keep a basement dry: how much water is really coming in during a storm, and how much head the pump has to fight to get it out. This calculator runs the full math instead of the shortcut.
Why the drawdown test beats guessing
The most reliable way to know your sump pump's real workload is to measure it directly. Shut the pump off, let the basin refill, and time how many inches the water rises in one minute — that's your drawdown test. Multiplied by the basin's cross-sectional area, it converts directly to gallons per minute of actual inflow. For homes without a basin to test yet, or for sizing a pump before a storm hits, the drainage-area method estimates peak runoff from the catchment area and a rainfall rate instead — useful for new construction or worst-case storm planning.
Total dynamic head is where most sizing goes wrong
A pump's flow rate isn't a single number — it falls as the head it has to overcome rises, following its pump curve down to zero at the shutoff head. Total dynamic head is the vertical lift from the basin to the discharge point plus the friction lost pushing water through the discharge pipe, which grows quickly with a longer or narrower pipe and shrinks quickly with a wider one. Ignoring friction loss is the single biggest reason an on-paper-adequate pump underperforms once it's installed with a long run to daylight.
Short-cycling: the failure mode nobody warns you about
A pump that's too powerful for its basin isn't automatically better — it can drain the basin's usable switch-travel volume in seconds, then sit idle until the next tiny refill, cycling on and off dozens of times an hour. That rapid cycling burns out motors and float switches far faster than steady, well-matched operation. If this calculator flags short-cycling, the fix is usually a larger basin or a wider on/off differential, not a smaller pump — pump sizes below 1/3 HP aren't a standard commercial option.
What this calculator doesn't do
Pump curves here are representative editorial approximations of typical 1/3, 1/2, and 3/4 HP submersible sump pumps — real curves vary by manufacturer and model, so always cross-check the recommendation against your actual pump's published performance curve before buying. Friction-loss figures assume PVC discharge pipe and don't model unusual fitting counts beyond what you enter as extra pipe length. Battery-backup runtime assumes a dedicated 12V DC backup pump at a representative wattage; some backup systems use an inverter to run a full-size AC pump instead, which draws differently. This tool is for planning, not a substitute for a licensed plumber — confirm sizing, check-valve placement, and code requirements with a professional before installing.