Sump Pump Size Calculator

Check sump pump inputs

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Project preset:

Start from the closest basement or backup-pump scenario, then edit the measurements.

Unit system:

Switch input labels and exported display units; pump-curve outputs still show GPM at feet of head.

Water inflow method:

Choose how you know the water entering the pit during a storm or wet-weather test.

Sump basin diameter: {{ formatSmallLength(base.basinDiameterIn) }}

Used for timed rise tests and pump-cycle volume checks.

Observed water rise: {{ formatSmallLength(base.riseHeightIn) }}

Measure the water-level change used for the timed fill test.

Rise time:

Shorter fill times increase the required pump capacity.

sec

Known inflow rate: {{ formatFlow(base.directInflowGpm) }}

This rate is multiplied by the safety margin before pump selection.

Drainage area feeding pit:

The estimate converts rainfall over catchment area into gallons per minute.

Design rainfall intensity: {{ formatRain(base.rainfallInHr) }}

Higher rainfall intensity increases the estimated inflow.

Runoff factor: {{ formatPercent(base.runoffCoeff * 100, 0) }}

This accounts for how much rainfall can actually reach the sump.

ratio

Float switch travel: {{ formatSmallLength(base.switchTravelIn) }}

More usable travel gives the pump a longer run and reduces short cycling.

Vertical lift: {{ formatHead(base.verticalLiftFt) }}

This is the static head the pump must lift before friction losses.

Horizontal discharge run: {{ formatRun(base.horizontalRunFt) }}

This length is combined with fitting equivalent lengths for friction loss.

Discharge pipe size:

Pipe size changes equivalent fitting lengths and friction head.

90 degree elbows:

Include the elbow where the vertical riser turns into the horizontal discharge.

elbows

Check valves:

Most installations have one valve above the pump; count additional valves if present.

valves

Sizing margin: {{ formatPercent(base.safetyMarginPct, 0) }}

Use more margin for high water tables, uncertain measurements, or critical finished basements.

45 degree elbows:

elbows

Pipe roughness:

Minimum preferred run time:

sec

Candidate pump rating at this head:

Leave at 0 if you are only choosing a shopping class.

Signal	Value	Basis	Copy
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Class	Capacity at TDH	Headroom	Fit	Copy
{{ row.classLabel }}	{{ row.capacity }}	{{ row.headroom }}	{{ row.fit }}

Check	Status	Action	Copy
{{ row.check }}	{{ row.status }}	{{ row.action }}

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Sump pump sizing is a flow-at-head problem. The pump must move water out of the pit as fast as water enters during the design condition, and it must do that after lifting water vertically and pushing it through the discharge pipe, elbows, and check valve.

Horsepower alone is a weak sizing shortcut. A pump that moves a large flow at zero head may move much less after 10, 20, or 30 ft of total dynamic head. The useful comparison is gallons per minute at the actual head for the installed route.

Sump pump diagram showing basin water, vertical lift, horizontal discharge run, and pump flow path

The best field input is often a timed basin rise during wet conditions. Basin diameter and water-rise height give a volume. The time for that rise gives an inflow rate. A direct measured inflow or a drainage-area estimate can also be useful, but each method represents a different level of certainty.

A pump also needs reasonable cycling behavior. A small basin or narrow float-switch travel can make a strong pump run for only a few seconds at a time. Short cycling wears switches and motors, so capacity, basin volume, and switch travel should be reviewed together.

How to Use This Tool:

Pick the inflow method that matches your evidence, then compare the target flow with pump capacity at the calculated head.

Choose a Project preset and Unit system. Pump-curve comparisons remain visible as GPM at feet of head because that is how many pump charts are published.
Select Water inflow method. For Timed basin rise test, enter basin diameter, observed water rise, and rise time. For Known inflow rate, enter the design inflow. For Drainage area estimate, enter drainage area, rainfall intensity, and runoff factor.
Set Float switch travel, Vertical lift, Horizontal discharge run, Discharge pipe size, 90 degree elbows, Check valves, and Sizing margin.
Use Advanced for 45 degree elbows, pipe roughness, minimum preferred run time, and a candidate pump rating at the calculated head.
Review Sizing Worksheet, Pump Curve Ladder, Install Checks, Capacity at TDH, and JSON before comparing a real pump model.

If the result pauses, fix the named measurement first. Basin diameter, float switch travel, vertical lift, and the selected inflow method inputs must be positive enough to produce a target flow.

Interpreting Results:

Target pump capacity is the inflow rate plus the selected sizing margin. It must be met at Total dynamic head, not at zero lift.

Measured inflow is water entering the pit before margin.
Static head is the vertical lift from basin to discharge high point.
Friction head is the estimated pipe and fitting loss through the selected discharge route.
Reference shopping class is a rough horsepower class from built-in curve bands; verify the exact model curve.
Cycle behavior warns when the selected capacity and basin volume imply short run times or frequent cycling.

A comfortable ladder fit is not a purchase approval. Use the exact manufacturer curve at the calculated total dynamic head, then check discharge size, check valve, power, alarms, backup capacity, and local plumbing requirements.

Technical Details:

Inflow can be computed from basin volume, entered directly, or estimated from rainfall over a drainage area. The timed-rise method treats the basin as a cylinder, converting the measured water rise into gallons and then dividing by elapsed time.

Total dynamic head combines static lift with friction head. Friction is estimated with a simplified Hazen-Williams form using target flow, pipe inside diameter, equivalent route length, and a roughness coefficient. The result is an estimate for clear water, not a substitute for a manufacturer pump-curve table or engineered hydraulic design.

Formula Core:

The core sizing target is flow at head. U.S. units are used internally for pump-curve comparison.

\begin{array}{lcl} V_{rise} & = & \frac{π \times {(\frac{d}{2})}^{2} \times h}{231} \\ Q_{inflow} & = & \frac{V_{rise}}{t / 60} \\ Q_{target} & = & Q_{inflow} \times (1 + \frac{m}{100}) \\ H_{TDH} & = & H_{static} + H_{friction} \end{array}

Sump pump calculation methods and boundaries
Quantity	Modeled rule	Boundary
Basin rise inflow	Cylindrical rise volume divided by timed rise	Best repeated during wet conditions when the pit is actively filling.
Drainage estimate	Area x rainfall depth x runoff factor, converted to GPM	A rough planning path for uncertain catchments, not a measured pit inflow.
Equivalent length	Horizontal run plus fitting allowances for elbows and check valves	Real fittings and pipe condition can differ from the reference values.
Pump class ladder	Interpolated capacity at TDH from reference curve bands	Use manufacturer curves for the actual pump model.
Cycle volume	Basin area x float-switch travel, converted to gallons	Too little usable volume can cause short cycling.

An 18 in basin rising 6 in in 32 seconds holds about 6.6 gallons for that rise, or about 12.4 GPM before margin. With a 50% margin, the target is about 18.6 GPM. If vertical lift and pipe losses total 13 ft of head, the pump curve must still meet that target at 13 ft TDH.

Accuracy Notes:

The calculation is an estimating worksheet for clear-water sump pump selection. It does not design a drainage system, guarantee flood protection, or replace the pump manual, plumbing code, or site-specific professional judgment.

Repeat timed rise tests when inflow is active and safe to observe.
Verify the selected model on the manufacturer's pump curve at the calculated total dynamic head.
Check backup pump capacity, battery runtime, alarm, discharge freeze protection, and check-valve installation separately.
Increase margin or seek professional design for finished basements, high water tables, long discharge routes, or critical flood-risk locations.

Worked Examples:

Timed basin rise in a typical pit

With Timed basin rise test, an 18 in Sump basin diameter, 6 in Observed water rise, and 32 sec Rise time produce a Measured inflow near 12.4 GPM. A 50% Sizing margin raises Target pump capacity to about 18.6 GPM before head is considered.

Long discharge route

A direct 24 GPM inflow with 22 ft Vertical lift, 90 ft Horizontal discharge run, several elbows, and a check valve can push Total dynamic head high enough that the Pump Curve Ladder moves from a common residential class toward a high-head selection.

Candidate model check

After choosing a pump, enter its published capacity at the calculated head in Candidate pump rating at this head. Install Checks reports whether that candidate meets the target or is short.

FAQ:

Why does the result say GPM at TDH?

Pump curves are compared by flow at head. Total dynamic head includes vertical lift plus estimated friction, so the useful rating is capacity at that head.

Is a higher horsepower pump always better?

No. A larger pump may still be wrong if its curve is weak at the calculated head, and it can short cycle if basin volume and switch travel are small.

When should I use the drainage area estimate?

Use Drainage area estimate when no timed pit measurement or known inflow is available. Treat it as a rough planning value because runoff factor and storm intensity can be uncertain.

Why is my candidate pump marked short?

Candidate model compares the entered pump capacity at this head with Target pump capacity. Enter the model's curve value at the calculated TDH, not its maximum or zero-head rating.

Glossary:

GPM: Gallons per minute, the flow rate used for pump-curve comparison.
Total dynamic head: Static lift plus estimated friction losses through the discharge route.
Static head: The vertical lift from basin to the discharge high point.
Friction head: Head loss caused by pipe length, fittings, valves, roughness, and flow rate.
Short cycling: Frequent brief pump runs caused by small usable basin volume, high inflow, or oversized capacity.

References:

Sizing Up a Sump Pump, Waterproof! Magazine, April 9, 2013.
Sump Pit Sizing and Pump Capacity, RadonSeal.
Hazen-Williams Water Flow Formula: Head Loss, Data, Charts & Calculator, Engineering ToolBox, 2004.