Sump Pump Size Calculator

Inflow {{ resultsReady ? formatFlow(computation.values.inflow_gpm) : '—' }} Margin {{ resultsReady ? `${formatNumber(computation.normalizedInputs.safety_margin_percent, 0)}%` : '—' }} Route {{ resultsReady ? formatDistanceCanonical(computation.values.equivalent_length_ft) : '—' }}

Inflow method:

Choose the evidence you have for peak water entering this pit.

Basin inside diameter:

Changing the unit preserves the same physical diameter.

Observed water rise:

Use the level change measured during the same timed interval.

Rise time:

Use seconds for the rise entered above.

Known peak inflow:

Use a measured or engineering design inflow rate.

Drainage area feeding the pit:

This screening method estimates a peak rate, not a routed hydrograph.

Design rainfall intensity:

Local design criteria should supply this value.

Runoff coefficient:

The default 0.50 is only a starting assumption.

Float switch travel:

Used only for the optional exact-model cycle check.

Vertical lift:

This is static head, not total pipe length.

Horizontal discharge run:

The model adds fitting equivalent lengths separately.

Nominal discharge pipe:

Pipe inside diameter drives friction and velocity.

Pipe condition:

Confirm material and condition for a final design.

90° elbows:

Fitting allowance varies with nominal pipe size.

elbows

Swing check valves:

Use additional equivalent length in Advanced for unlisted fittings.

valves

Flow safety margin:

Use project-specific reserve; the default is 50%.

45° elbows:

Leave at zero when none are present.

elbows

Additional equivalent length:

Do not duplicate elbows or check valves already counted above.

Candidate capacity at calculated TDH:

This is not the zero-head or maximum-flow label.

Short-cycle warning threshold:

The neutral default is 0 seconds.

Duty point

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Calculation method:

The result is a flow/head coordinate, not a horsepower class.

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Installation checks

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The maximum-flow number on a pump label does not describe a basement installation. Water must rise from the basin and travel through pipe, elbows, and a check valve. Each part adds resistance, so the capacity that matters is the flow a specific model can deliver at the installation's total dynamic head.

Peak inflow sets the minimum flow requirement. It can come from a timed rise in the actual basin, an established design flow, or a drainage-area estimate. Those methods do not carry equal confidence.

Sump pump inflow evidence methods
Evidence	Best use	Important limit
Timed basin rise	Observed water entering the installed pit while the pump is off.	A dry-weather or short observation may miss the design storm.
Known peak inflow	A measured or engineering design rate already expressed as flow.	The value is only as reliable as its source and design condition.
Drainage estimate	Early screening from area, rainfall intensity, and runoff coefficient.	The Rational Method gives a peak rate rather than a routed hydrograph.

A safety margin keeps the selected flow above the estimated inflow, but extra flow does not solve every problem. Narrow pipe can create high velocity and friction. A small usable basin volume can make a strong pump start and stop too often. A pump whose capacity does not exceed incoming flow cannot lower the water level.

Final selection requires the manufacturer curve for the exact model, the permitted discharge route, local plumbing and electrical rules, reliable power, and suitable alarm or backup protection. The calculation is a sizing aid rather than an installation approval.

How to Use This Tool:

Choose the strongest inflow evidence available, then describe the route all the way to the discharge point.

Select Inflow method. For a timed test, enter basin inside diameter, observed rise, and rise time from the same interval. For a drainage estimate, use a locally appropriate design intensity and runoff coefficient.
Enter Vertical lift separately from the horizontal run. Choose nominal pipe size, pipe condition, 90° elbows, check valves, and the flow safety margin.
Add 45° elbows or unlisted equivalent length when the basic fitting counts do not describe the route.
Read Design flow and Total dynamic head together, then find the exact model's rated capacity at that head on its manufacturer curve.
Enter that capacity for the candidate check. Add float-switch travel and an optional short-cycle threshold only when the basin and exact model are known well enough for a cycle estimate.

Interpreting Results:

Carry the duty point into product selection as a pair: required GPM at total dynamic head. A candidate passes the flow check when its entered capacity at that head is greater than or equal to design flow. Do not compare design flow with a zero-head maximum or use horsepower as a substitute for the curve.

A friction-heavy warning means friction exceeds 30% of total dynamic head. A velocity review appears only above 5 ft/s. Both are model review thresholds, not universal code limits. A larger pipe, fewer fittings, or a shorter route can move the duty point.

Cycle results require an exact-model capacity greater than inflow. If capacity is less than or equal to inflow, the water level continues to rise. When a positive short-cycle threshold is entered, pump-on time below that threshold triggers the warning.

Technical Details:

The hydraulic model normalizes small dimensions to inches, route distances to feet, flow to U.S. gallons per minute, drainage area to square feet, and rainfall to inches per hour. One cubic foot is 7.48051948 U.S. gallons and one gallon is 3.785411784 litres.

Formula Core:

For a cylindrical basin with inside diameter D, observed rise y, and rise time t in seconds, timed inflow is:

Q_{rise} = \frac{π \times {(\frac{D}{2})}^{2} \times y \div 231}{t \div 60}

The drainage-area path converts the Rational Method to gallons per minute. With area A in ft², rainfall I in in/hr, and runoff coefficient C:

Q_{drainage} = \frac{A \times \frac{I}{12} \times C \times 7.48051948}{60}

Whichever inflow method is selected, margin m produces design flow. Total dynamic head adds vertical lift to Hazen–Williams friction:

\begin{aligned} Q_{design} & = Q_{inflow} \times (1 + \frac{m}{100}) \\ H_{f} & = \frac{4.52 \times L \times Q^{1.852}}{C^{1.852} \times d^{4.87}} \\ TDH & = H_{vertical} + H_{f} \end{aligned}

In the friction equation, L is total equivalent length in feet, Q is design flow in GPM, C is the Hazen–Williams coefficient, and d is pipe inside diameter in inches. New smooth PVC uses C = 150; the aged or mixed-plastic allowance uses 130.

Equivalent-Length Lookup Core:

Horizontal run, listed fitting allowances, and any additional equivalent length are added before friction is calculated.

Discharge pipe dimensions and equivalent fitting lengths
Nominal pipe	Inside diameter	90° elbow	45° elbow	Check valve
1¼ in / 32 mm	1.38 in	4 ft	2.4 ft	14 ft
1½ in / 40 mm	1.61 in	5 ft	3 ft	16.8 ft
2 in / 50 mm	2.067 in	7 ft	4 ft	22.4 ft
2½ in / 65 mm	2.469 in	8 ft	5 ft	28 ft

Cycle and Rule Core:

Float travel replaces observed rise in the cylindrical-volume equation to obtain usable cycle volume V. Candidate capacity Q_p is the manufacturer-listed flow at the calculated TDH.

Sump pump cycle calculations and thresholds
Result	Exact rule
Fill time	`V` ÷ inflow × 60 seconds.
Pump-on time	`V` ÷ (candidate capacity − inflow) × 60, only when candidate capacity exceeds inflow.
Cycles per hour	3,600 ÷ (fill time + pump-on time).
Candidate passes	Candidate capacity ≥ design flow.
Continuous rise	Candidate capacity ≤ inflow.
Short cycle	Entered threshold > 0 and pump-on time < that threshold.
Friction heavy	Friction head ÷ TDH > 0.30.
Velocity review	Calculated pipe velocity > 5 ft/s.

With an 18 in basin rising 6 in over 32 seconds, inflow is about 12.393 GPM. A 50% margin raises the duty flow to about 18.589 GPM. A 10 ft lift through 30 ft of 1½ in pipe, two 90° elbows, and one check valve has 56.8 ft of equivalent length; calculated friction is about 0.528 ft and TDH is about 10.528 ft.

Sizing Limits and Safety:

Hazen–Williams and equivalent-length values provide a clear-water planning estimate. Actual pipe schedule, fittings, valve design, fouling, freezing exposure, air locking, discharge restrictions, changing water level, and installation quality can alter the operating point.

Use local design rainfall and runoff criteria; the drainage-area path is not a flood-routing model.
Check the full manufacturer curve and installation manual, not one entered capacity or the shutoff head.
Plan for check-valve access, safe electrical supply, alarms, power failure, pump failure, and backup capacity according to the consequences of flooding.

References:

Stormwater Procedures Manual, City of Frankfort and Franklin County, February 2018.
Residential Code Appendix P: Sizing of Water Piping System, City of Seattle, 2012.
Submersible Sump Pump Performance Curves, Liberty Pumps.
How to Read a Pump Curve, Zoeller Pump Company, October 29, 2024.