Sump Pump Size Calculator — GPM for Your Basement
Enter your basin diameter and water rise (or use the storm estimate) to get the gallons per minute your pump must move. No sign-up, no data stored.
Measure the rise during the heaviest rain you get — or use 1 in/min as a storm estimate. Lift is floor-to-discharge height.
Pump recommendation
How sump pump size is determined
- Inflow (GPM) = basin area × water rise: area = (diameter² × 0.785) sq in; each inch of rise in one minute = area × 0.00433 gallons/min.
- Headroom: multiply inflow by 1.5× so the pump isn't running at its limit.
- Total dynamic head = vertical lift + 1 ft per 10 ft of horizontal pipe + fittings (add ~10%).
- Match the curve: pick a pump whose GPM rating at your head beats the requirement — a 1/3 HP unit does ~25–35 GPM at 5–10 ft.
Most 18-in basins in normal soil see 0.5–1 in/min during a storm — 1,000–2,000 GPH. High water tables or weeping-tile homes can hit 2× that; that's where the 1/2 HP and backup conversation starts.
Pump size → typical output
Rough capacity at 5–10 ft total head (varies by brand; always read the curve).
| Pump | GPM @ 5 ft | GPM @ 10 ft | Best for |
|---|---|---|---|
| 1/4 HP | ~25 | ~18 | Small basins, low inflow |
| 1/3 HP | ~35 | ~25 | Most homes — the standard pick |
| 1/2 HP | ~50 | ~40 | High water table, long runs |
| 1 HP | ~75 | ~60 | Commercial-grade inflow |
Last updated: October 2026 · Match the pump's capacity curve at your total head, not the HP sticker. Always add a battery backup if losing the basement to a storm-power outage is unacceptable.
Sump pump FAQ
During a storm, mark the water level, time 60 seconds, and note the rise. Basin area × rise × 0.00433 = GPM inflow. That's the number your pump must beat with headroom.
For the vast majority of homes, yes — it moves 1,500–2,000 GPH at typical heads. Upgrade to 1/2 HP only when water rises 2+ in/min, the discharge runs 30+ ft, or your basement floods regularly.
Constant cycling means oversized for the basin, a stuck float, or a check valve missing (water backflows every cycle). Check the check valve before buying a bigger pump.
Yes if a flooded basement is unacceptable — storms that flood are exactly when power fails. A battery backup covers 5–7 hours of normal inflow; a water-powered backup runs indefinitely.
Plan 7–10 years for a well-maintained pump. Test monthly (pour a bucket in), clean the inlet screen yearly, and replace before it fails mid-storm — not after.