Sump Pump Size Calculator
Estimate how quickly water is entering a round sump basin, apply a planning margin, and use the result as a target flow to compare against manufacturer pump curves.
The calculator does not select a horsepower for you. A pump should be chosen by the flow it can deliver at the system’s actual total dynamic head (TDH), not by 1/3 HP, 1/2 HP, or another motor rating alone.
Safety: do not switch off a working sump pump during active rain just to create a measurement. Use an observation you already have, a controlled test that can be stopped safely, or professional measurements. Stop immediately if the basin approaches overflow.
Estimate Incoming Flow
Measure how quickly water rises in a round sump basin. The result is a flow target—not a final pump specification.
How the Sump Pump Sizing Math Works
For a round basin, the calculator first determines the cross-sectional area. Dividing that area by 231 converts one vertical inch of basin volume from cubic inches to gallons.
Gallons per inch ≈ π × (diameter ÷ 2)² ÷ 231
It then converts the observed water rise into a flow rate:
Incoming GPM ≈ gallons per inch × observed rise in inches × 60 ÷ elapsed seconds
The selected planning margin is applied to that observed inflow. It is not a code requirement or a manufacturer-prescribed universal safety factor.
Why Total Dynamic Head Matters
Liberty Pumps currently defines total dynamic head as the combination of static head and friction head. Its guidance explains that a pump performance curve shows the GPM a specific model can deliver across a range of TDH values and is used to select the correct pump for an application.
Liberty Pumps: performance curves and TDH →
Zoeller likewise provides a current System and Pump Curve Generator for matching system conditions to pump performance.
Zoeller System and Pump Curve Generator →
This calculator asks for static lift because it is easy to estimate, but it deliberately does not pretend to calculate friction head without the actual discharge-pipe diameter, length, fittings and valve configuration.
Why the Calculator Does Not Pick 1/3 HP or 1/2 HP
Horsepower is not a direct flow rating. Different pump models with the same horsepower can have different performance curves.
A useful comparison asks:
- How much inflow needs to be handled?
- What planning margin is appropriate for the property?
- What is the actual TDH?
- What flow does the exact pump model deliver at that TDH?
- Does the pump and float physically fit the basin?
- Can the discharge system carry the resulting flow?
- Is independent backup protection needed?
A 1/2 HP pump can outperform a 1/3 HP pump in one comparison, but horsepower alone is not enough to specify a system.
Chicago Sump Pit Size
Chicago’s current plumbing code generally requires a sump pit to be at least 18 inches in diameter and 30 inches deep unless otherwise approved by the Department of Water Management. The pit also needs a removable cover adequate for anticipated loads, and the pit floor or cover must provide permanent support for the pump.
Chicago Plumbing Code 18-29-1111.3.2 →
The code minimum does not guarantee that every pump, float and backup combination will have enough operating clearance. Basin layout still matters.
Why Basin Size Affects Cycling
A basin with less usable volume between the pump’s ON and OFF levels fills more quickly, so the pump may cycle more often even when its flow capacity is adequate.
The basin also needs unobstructed room for the primary float and any backup control. A high-capacity pump in a crowded or low-volume basin can create a cycling problem rather than solve one.
When the Flow Measurement Can Mislead You
- The observation was made during a mild event rather than the property’s heaviest known inflow
- The basin is tapered, irregular, or not round
- Water is entering from more than one level in the basin
- The measured rise is too small or the timing is inaccurate
- The discharge is already returning water to the basin
- The property has highly variable groundwater conditions
- A drain, plumbing leak, or other source is adding water that is not normal groundwater inflow
Repeat safe observations when possible and compare the result with the property’s known storm history.
What the Calculator Does Not Measure
- Exact friction head
- Exact total dynamic head
- Maximum future inflow that has never been observed
- Whether the existing pump is mechanically healthy
- Whether the check valve is allowing backflow
- Whether the discharge line can carry the target flow
- Municipal discharge requirements
- Whether the basin and float arrangement are correctly designed
- Whether a backup or second pump is needed
For new-system planning, use Sump Pump Installation. If the existing system cannot keep up or is behaving abnormally, use Sump Pump Repair or Sump Pump Replacement.
Sump Pump Size Calculator FAQ
How do I calculate sump pump GPM from a sump basin?
For a round basin, calculate gallons per vertical inch from the basin diameter, multiply by the observed water rise, then convert the elapsed time to a per-minute rate. The calculator performs those steps automatically.
Should I turn off my sump pump to measure water rise?
Do not switch off a working pump during active rain or any condition where the basin could overflow. Use a safe observation you already have, a controlled test that can be stopped safely, or professional measurements.
Does the calculator include friction loss?
No. It shows the static-lift reference and warns about discharge complexity, but exact friction head requires the actual pipe diameter, length, fittings, valves, and flow. Add static and friction head to get TDH before using the pump curve.
Does a 1/2 HP sump pump always move more water than a 1/3 HP pump?
No. Compare the exact manufacturer performance curves at the same total dynamic head. Pump design matters in addition to horsepower.
What planning margin should I use?
There is no universal margin required for every property. The tool lets you choose 25%, 50%, or 100% because a short observation may not capture the property’s highest inflow. The right design margin depends on measurement quality, storm history, backup protection, and the consequences of failure.
Can this calculator tell me which pump model to buy?
No. Use the calculated target flow as one input, calculate the system’s actual TDH, then compare manufacturer curves and confirm basin, float, discharge, electrical, code, and backup requirements.
Next Step After the Calculation
Save the target GPM and static-lift result, then compare exact pump models at the calculated TDH. If you cannot determine the friction component or the basin/discharge system needs correction, treat the calculator result as a screening estimate rather than a final specification.