What this calculator does
A sump pump is sized on the water that will actually arrive in the worst storm. This calculator converts the area above the sump, the design rainfall intensity and the surface runoff coefficient into an inflow, applies a safety factor, and gives the hydraulic power at your stated head.
How to use it
- Enter the drainage area that discharges into the sump and the design rainfall intensity for your return period.
- Set the runoff coefficient — 1.0 for a fully paved area around a basement, lower if ground absorbs some.
- Set the safety factor, 1.5 being common, to cover the peak that a storm audit will always miss.
- The power figure assumes a typical pump efficiency; confirm against the actual pump curve.
Governing formula
Q = A · (i/1000) · C / 3600 ; P = ρgQH/η Where:
-
i= Rainfall intensity [mm/h] -
C= Runoff coefficient [–] -
H= Total dynamic head [m]
Worked example — 120 m² of paving draining to a sump
The figures below were produced by running this page's own engine with the example inputs (Area draining to the sump (m²), Design rainfall intensity (mm/h), Runoff coefficient (–), Safety factor (–), Total dynamic head (m), Pump efficiency (%)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Required pump capacity | 4.5 L/s |
|---|---|
| Required pump capacity | 71.33 GPM |
| Calculated inflow | 3 L/s |
| Hydraulic power | 220.73 W |
| Assessment | Design inflow 3.0 L/s (48 GPM) → pump ≥ 4.5 L/s (71 GPM) at 3 m head. Typical basement sumps run 1–25 L/s; size the pit (often 0.6 m³) accordingly. |
- Inflow = 120 × 0.1 × 0.9 / 3600 = 0.003 m³/s, which is 3 L/s.
- With a 1.5 safety factor the pump should deliver at least 4.5 L/s.
- At 3 m of head that is roughly 220 W of hydraulic power — specify the next standard pump above it.
Assumptions & limitations
Engineering calculation. Estimates the inflow reaching a sump from the drainage area and rainfall intensity, then applies a safety factor. Pump selection by curve and switch settings is outside the calculation.
- Assumes all the drainage area reaches the sump with no attenuation from above-ground storage or infiltration.
- The power figure is hydraulic; motor power is typically 25–50 % higher once motor efficiency is included.
- Sump pit volume, float switch settings and backwater conditions must be checked separately.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- US EPA — basement sump pump and drainage guidance.
- Local stormwater drainage design manuals.