Sizing a sump pump for the storm that will actually arrive
Converting drainage area and rainfall intensity into inflow, then applying the margin a real storm will need.
The engineering question
The sump pump runs constantly in heavy rain. Is it undersized, badly levelled, or overwhelmed by inflow?
Worked example
The figures below were produced by running the Sump Pump Sizing Calculator with the same inputs — during the build, from its authoritative engine.
| Required pump capacity | 4.5 L/s |
|---|---|
| Required pump capacity | 71.325 GPM |
| Calculated inflow | 3 L/s |
| Hydraulic power | 220.725 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 is area × intensity × runoff coefficient, converted to litres per second.
- Apply a safety factor because real storms exceed the design average.
- Hydraulic power follows from the head; the motor is larger.
Sanity check
A pump cycling constantly in light rain points to a float switch or a stuck impeller, not to capacity.
Tools for this problem
These are the EngiMetric calculators that compute the numbers in this topic. Each one shows its formula, its units and its limits rather than returning a bare result.
References & standards
- Local stormwater drainage design manuals
- US EPA — basement sump guidance
Related problems
Sizing drainage pipe by fixture units rather than by flow
Why drainage is sized by a loading unit, and reading the code table to get the building drain.
When the ground can absorb the water, and when it cannot
Sizing infiltration storage and soakaways, and recognising the soils that will not drain.
Also: Geotechnical