Formula & Engineering Reference
| Symbol | Variable | Unit (SI) |
|---|---|---|
| inflow | Flow into the sump pit | L/s |
| SF | Safety factor on inflow | — |
| Qdesign | Required pumping capacity | L/s |
| Qduty | Capacity per duty pump | L/s |
| net | Qduty − inflow (drains the pit) | L/s |
| Vusable | Volume between ON and OFF floats | m³ |
| runtime | Pump-on time per cycle | min |
Capacity is the inflow scaled by a safety factor. Because water keeps flowing in while the pump runs, the pit only empties at the net rate, which sets the run time and the starts per hour. The optional motor power uses ρgQH at the per-duty flow and the lift-plus-losses head.
A pit takes 3 L/s of inflow, with a 1.3 safety factor, 0.25 m³ of usable volume between floats, one duty and one standby pump, lifting 6 m with 2 m of discharge loss.
Design capacity = 3 × 1.3 = 3.9 L/s, all on the single duty pump. Net rate = 3.9 − 3.0 = 0.9 L/s — only the difference actually lowers the level.
Run time = 250 L / 0.9 L/s = 277.8 s = 4.63 min per cycle, so about 13 starts/hour. TDH = 6 + 2 = 8 m, giving a motor of roughly 0.52 kW at 65%/90% efficiency. Thirteen starts an hour is on the high side — a slightly larger pit would calm the cycling.
Sizing the pump barely above the inflow. If the duty capacity only just beats the inflow, the net rate is tiny, run times stretch out, and the pit struggles in a surge. Give the pump real headroom over the peak inflow, not a sliver.
Underestimating the inflow. For storm and groundwater sumps the peak comes during heavy rain, and guessing low here means a flooded basement when it matters most. Base the inflow on the drainage area or infiltration, then keep the safety factor.
Forgetting cycling. A pump that can clear the water but starts thirty times an hour will wear out fast. Run time and starts per hour are as much a part of sizing as capacity — usually fixed by enlarging the pit volume between floats.
Leaving out the standby. Drainage protects the building, so redundancy is the norm. One duty plus one standby keeps the pit covered if a pump fails; skipping it saves a little money and risks a lot.
Ignoring power and alarms. A sump that must keep running through a storm needs reliable power, and often a high-level alarm and emergency backup. The capacity figure is necessary but not sufficient — the reliability around it matters just as much.
Scale the inflow by a safety factor for the design capacity, then use the usable pit volume to find run time and starts per hour. The pump must out-pump the inflow or the pit never empties.
How often the pump starts per hour. Too many starts wears the motor. Run time is usable volume over net rate; starts per hour is 60 over the run time.
Inflow keeps arriving while the pump runs, so the level falls only at the pump capacity minus the inflow. That net rate sets the run time.
Drainage is protective, so redundancy is standard. One duty plus one standby lets a second pump take over if the first fails or the level keeps rising.
The peak the sump will see — for storm or groundwater that is heavy rain. Underestimating is risky, so a safety factor is applied on top.
Sump Pump Drainage Guide
3 topics • Capacity & cycling referenceA sump pump is the unglamorous guardian of basements, lift pits, and plant rooms — it sits in a hole collecting whatever water seeps or pours in and lifts it away before it floods anything. Sizing one is not about raw power; the heads are usually modest. It is about two things working together: a capacity that comfortably beats the inflow, and a pit that lets the pump run in sensible bursts rather than hammering on and off all day.
This calculator handles both. It scales the inflow into a design capacity, then uses the pit volume to predict run time and starts per hour, flagging when the pump would cycle too hard. The guide explains why the net rate, not the raw capacity, controls the timing, and why redundancy is non-negotiable for a pump whose job is to stop a flood.