💧 PLUMBING

Free Online Sump Pit & Ejector Pump Calculator

Size a sump pit volume and submersible drainage pump from inflow and pump cycles. NBC Part 9 basis. Free online plumbing tool, no sign-up.

📐 Standard: NBC Part 9
✅ Free to use
📄 PDF export
📱 Mobile friendly
💧
Sump Pit & Ejector Pump Calculator
Reference: NBC Part 9
💧 PLUMBING
Size a sump pit volume and submersible drainage pump from inflow and pump cycles. NBC Part 9 basis. Free online plumbing tool, no sign-up.
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ℹ️ About This Calculator

A sump pit collects drainage or groundwater from basements and lift stations, and a submersible pump lifts it to the gravity drain. This calculator sizes the pit working volume and the pump duty from the inflow rate and the allowable pump cycles per hour, following NBC Part 9 practice.

The pit must hold enough between the pump's start and stop levels that the pump does not short-cycle - starting too often overheats the motor. The pump flow should comfortably exceed the peak inflow, and its head must cover the lift from the pit to the discharge plus pipe friction. Duty-and-standby pumps are standard so drainage continues if one fails.

📐 Sump Pit & Pump Formula

NBC Part 9

V_working = Q_pump / (4 × N)   (per Hydraulic Institute)

V_working = active pit volume between start/stop, L
Q_pump = pump flow, L/min
N = max starts per hour (typically 6-10)
Pump head = static lift + pipe + fitting friction

🧮 Worked Example

Example: A basement sump with a 150 L/min peak inflow, using a pump of 300 L/min against a 6 m lift, limited to 10 starts per hour, needs a working volume of about 300 / (4 × 10) ≈ 7.5 L per cycle minimum - in practice a pit of a few hundred litres is used for reserve. A duty + standby pump pair is provided for reliability.

📊 Sump Pump Design Reference (NBC Part 9)

Key selections for a reliable sump system:

ParameterTypical valueNote
Pump flow vs peak inflow1.5 – 2 ×So the pit draws down quickly
Max starts per hour6 – 10More causes motor overheating
Pump arrangementDuty + standbyAlternating, auto-changeover
Level controlFloat / probe switchesStart, stop, high-level alarm

Design Notes

Size the pump head for the static lift plus pipe and fitting friction to the gravity drain, and fit a non-return valve and isolating valve on each discharge. Provide a high-level alarm above the duty start, and alternate the duty and standby pumps so both share the wear. For sewage (ejector) duty, use a solids-handling pump and vent the pit.

Indicative guidance - confirm against NBC Part 9 and the pump manufacturer's curve.

❓ Frequently Asked Questions

How do I size a sump pump? +
Select a pump flow that comfortably exceeds the peak inflow (1.5-2 times), with a head covering the lift from the pit to the gravity drain plus pipe friction. Then size the pit working volume so the pump does not exceed its allowable starts per hour.
How big should a sump pit be? +
Big enough that the volume between the pump start and stop levels prevents short-cycling - from the pump flow divided by four times the maximum starts per hour. In practice a reserve is added, giving pits of a few hundred litres for building drainage.
What causes a sump pump to short-cycle? +
Too small a working volume between the start and stop floats relative to the pump flow, so the pump empties the pit and restarts too often. This overheats the motor. Increasing the pit volume or the float differential fixes it.
How many starts per hour can a sump pump take? +
Small submersible pumps are usually limited to about 6-10 starts per hour; larger motors fewer. Exceeding this overheats the motor and shortens its life, which is why the pit working volume is sized around the start limit.
Do I need a standby sump pump? +
For any critical drainage - basements, lift stations, plant rooms - yes. A duty-and-standby pair with automatic changeover keeps drainage running if one pump fails, and alternating them shares the wear. A high-level alarm warns of pump failure or excess inflow.
What is the difference between a sump pump and an ejector pump? +
A sump pump handles clear or lightly contaminated drainage and groundwater; a sewage ejector pump handles wastewater with solids and needs a solids-handling impeller and a vented pit. Both lift liquid from a low pit to the gravity drain.
What head does a sump pump need? +
The static lift from the pump's stop level to the discharge point, plus the friction loss in the rising main and fittings. Add a non-return valve to hold the column between cycles and an isolating valve for maintenance.
Where should the high-level alarm be set? +
Above the duty pump's start level but below the point where flooding or backflow occurs, so it warns of pump failure or inflow exceeding pump capacity in time to act. It is essential on unattended basement and plant-room sumps.

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⚠️ Disclaimer: For preliminary engineering design only. Verify all results with a licensed engineer before use. Full disclaimer →