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
{{ row.signal }}	{{ row.value }}	{{ row.basis }}

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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Basement water removal depends on two numbers meeting at the same point: how fast water reaches the sump basin and how much water a pump can move after it has lifted that water out of the pit. A pump that looks strong on the box can still fall short when the discharge route rises two stories, turns through several fittings, or uses a narrow pipe that adds friction.

Good sump pump sizing starts before horsepower. The drainage system around a foundation may collect groundwater, roof runoff, seepage through footing drains, or water that appears only during heavy rain. Those sources do not always arrive at a steady rate, so a single dry-weather test can understate the flow that matters during a storm.

Inflow: Water entering the basin during the design condition, usually expressed in gallons per minute.
Static head: The vertical lift from the basin to the highest point in the discharge route.
Friction head: Extra head caused by pipe length, elbows, valves, pipe size, roughness, and flow rate.
Cycle volume: The usable basin volume between pump-off and pump-on levels, controlled by basin diameter and float-switch travel.

Sump pump setup showing basin water, vertical lift, horizontal discharge run, and outlet flow

A timed basin rise is often the most useful field measurement because it observes water reaching the actual pit. The basin diameter and water rise give a volume, and the time for that rise turns the observation into flow. Drainage-area estimates can help when a wet-weather test is unavailable, but they depend heavily on the catchment area, rainfall intensity, and runoff factor chosen.

Pump curves explain why horsepower labels mislead. A pump has one flow at 5 ft of head, a lower flow at 15 ft, and eventually no flow at its shutoff head. The useful shopping question is not whether the motor is 1/3 HP or 1/2 HP by itself; it is whether the published curve still reaches the needed gallons per minute at the total dynamic head for the installed route.

Sizing also has a wear-and-reliability side. A pump with enough flow can still cycle too often when the pit is small or the float switch has little travel. The final choice should be checked against the exact model curve, the discharge instructions, local plumbing rules, power reliability, alarms, and backup capacity for the risk level of the space being protected.

How to Use This Tool:

Choose the inflow evidence first, then describe the discharge route so the target flow is compared at the right head.

Select a Project preset and Unit system. Metric mode changes input labels, while pump-curve comparison remains visible in GPM at feet of head.
Set Water inflow method. Use Timed basin rise test for basin diameter, observed water rise, and rise time. Use Known inflow rate when a design flow is already known. Use Drainage area estimate when you only have catchment area, rainfall intensity, and runoff factor.
Enter Sump basin diameter and Float switch travel. These fields affect the timed-rise calculation and the cycle-volume warning.
Describe the route with Vertical lift, Horizontal discharge run, Discharge pipe size, 90 degree elbows, Check valves, and Sizing margin.
Open Advanced when the route includes 45 degree elbows, older pipe, a different preferred run time, or a candidate model with a published capacity at the calculated head.
Review Sizing Worksheet first, then use Pump Curve Ladder, Install Checks, and Capacity at TDH to compare the result with an actual pump curve.

If the summary shows Sizing paused, fix the named measurement before interpreting any recommendation. Positive basin dimensions, vertical lift, and the active inflow-method fields are required for a usable target.

Interpreting Results:

Target pump capacity and Total dynamic head are the two values to carry into pump shopping. The target includes the selected margin, and the head includes vertical lift plus estimated friction through the pipe and fittings.

Measured inflow is the water entering the pit before margin. Repeat the measurement if the storm condition was mild or the pit was not actively filling.
Reference shopping class is only a starting class from built-in curve bands. A real model must meet the target on its manufacturer curve at the same head.
Install Checks warns about high head, friction-heavy routes, short cycling, backup pump demand, and candidate models that are short at the calculated head.
Capacity at TDH makes the capacity loss at higher head easier to see across the reference classes.

A Comfortable fit does not prove the installation is safe, and a larger motor is not automatically better. Verify the exact curve value, discharge-pipe requirements, check-valve placement, power source, alarm, backup pump, and code requirements before buying or installing a pump.

Technical Details:

Sump pump sizing is a flow-at-head estimate. Inflow defines the flow the pump must outrun, margin keeps the selection away from the break-even point, and total dynamic head reduces the flow available from any centrifugal pump curve. Static lift usually dominates short residential routes, while friction grows quickly as flow increases or pipe diameter gets smaller.

Basin testing treats the pit as a cylinder. Drainage-area estimating treats rainfall as depth over area and then reduces or scales that water with a runoff factor. Both paths produce a gallons-per-minute inflow value before margin, but the timed test is tied to the actual pit and the drainage estimate is a planning approximation.

Formula Core:

The calculation uses U.S. pump-curve units internally so capacity can be compared as GPM at feet of head.

\begin{array}{lcl} V_{rise} & = & \frac{π {(\frac{d}{2})}^{2} h}{231} \\ Q_{basin} & = & \frac{V_{rise}}{t / 60} \\ Q_{drainage} & = & \frac{A \frac{r}{12} c 7.4805}{60} \\ Q_{target} & = & Q_{inflow} (1 + \frac{m}{100}) \\ L_{eq} & = & L_{run} + n_{90} e_{90} + n_{45} e_{45} + n_{valve} e_{valve} \\ H_{friction} & = & \frac{4.52 Q_{target}^{1.85}}{C^{1.85} D^{4.87}} \frac{L_{eq}}{100} \\ H_{TDH} & = & H_{static} + H_{friction} \end{array}

Symbols used in sump pump sizing formulas
Symbol	Meaning	Unit basis
`d`	Basin diameter	inches
`h`	Observed water rise for a timed basin test	inches
`t`	Rise time	seconds
`A`, `r`, `c`	Drainage area, rainfall intensity, and runoff factor	square feet, inches per hour, ratio
`D`, `C`	Pipe inside diameter and Hazen-Williams roughness coefficient	inches, coefficient
`L`_eq	Horizontal run plus equivalent fitting length	feet

For example, an 18 in basin with a 6 in rise holds about 6.6 gal for that rise. If the rise takes 32 seconds, the measured inflow is about 12.4 GPM. A 50% margin raises the target to about 18.6 GPM, and an 11 ft vertical lift route must be checked near 11.0 ft total dynamic head after the selected pipe and fitting losses are added.

Pump curve fit rules used by the sizing result
Fit label	Rule at calculated TDH	Meaning
Comfortable	Capacity ≥ 115% of Target pump capacity	Reference class has reserve before the exact model curve is checked.
Meets target	Capacity ≥ target and < 115% of target	Reference class clears the selected margin with little extra reserve.
No reserve	Capacity ≥ Measured inflow and < target	Reference class may keep up with raw inflow but does not satisfy the margin.
Short	Capacity < Measured inflow	Reference class is below the observed or estimated inflow at the calculated head.

Sump pump calculation methods and boundaries
Quantity	Modeled rule	Boundary
Basin rise inflow	Cylindrical rise volume divided by timed rise	Most useful when repeated during active wet-weather inflow.
Drainage estimate	Area times rainfall depth times runoff factor, converted to GPM	Useful for planning when pit inflow is unknown, but sensitive to assumptions.
Pipe friction	Hazen-Williams estimate using target flow, equivalent length, pipe diameter, and roughness	Approximates clear-water pipe loss and does not replace engineered hydraulic design.
Pump volume per cycle	Basin area times float-switch travel, converted to gallons	Small usable volume can cause frequent starts even when flow capacity is adequate.
Candidate model	Entered capacity at this head compared with target capacity	Use the model's published curve value at the calculated head, not maximum or zero-head flow.

Displayed values are rounded for reading, while the calculations keep more precision between steps. Manufacturer curves, actual pipe inside diameter, fittings, check-valve design, debris, discharge freezing, and local code can change the real installation limit.

Accuracy Notes:

The result is a sizing estimate for clear-water residential sump pump selection. It does not design a full drainage system, certify flood protection, or replace a pump manual, plumbing code, or site-specific professional review.

Repeat timed basin tests when inflow is active and safe to observe.
Use more margin when measurements are uncertain, the basement is finished, or the site has a high water table.
Verify the chosen pump on the exact manufacturer curve at the calculated total dynamic head.
Check backup pump capacity, battery runtime, alarm, discharge freezing, check-valve installation, and legal discharge location separately.

Worked Examples:

Timed basin rise in an 18 in pit

An 18 in Sump basin diameter, 6 in Observed water rise, and 32 sec Rise time produce Measured inflow near 12.4 GPM. With a 50% Sizing margin, Target pump capacity is about 18.6 GPM at roughly 11.0 ft Total dynamic head. The reference result reaches a 1/3 HP residential class, but the cycle check still deserves attention if the estimated run time is short.

Drainage estimate before a storm test is possible

A 1,400 sq ft Drainage area feeding pit, 3.0 in/hr Design rainfall intensity, and 0.50 Runoff factor estimate Measured inflow near 21.8 GPM. A 45% margin raises Target pump capacity to about 31.6 GPM. Treat the result as a planning value until a timed pit measurement can confirm the inflow.

Long route with short cycling risk

A known 24 GPM inflow with 22 ft Vertical lift, 90 ft Horizontal discharge run, four 90 degree elbows, one 45 degree elbow, and one check valve lands near 32.4 GPM target at about 22.0 ft TDH. The Pump Curve Ladder can move toward a 3/4 HP reference class, while Install Checks may flag Short cycling risk because the selected capacity empties the usable basin volume too quickly.

Candidate model entered from a published curve

For a 14 GPM known inflow with a 35% margin, the target is about 18.9 GPM. If the model curve says the pump delivers 22 GPM at the calculated head, entering 22 in Candidate pump rating at this head lets Install Checks mark Candidate model as Meets target with about 3.1 GPM of headroom.

Advanced Tips:

Use Timed basin rise test during active wet-weather inflow when it is safe to observe. A dry-weather rise can undersize the pump for the storm condition that matters.
Keep Unit system consistent while measuring. Metric inputs are converted for the calculation, but published pump curves are commonly read as GPM at feet of head.
Raise Sizing margin when the inflow measurement is uncertain, the basement is finished, or the discharge route may partly freeze, clog, or share a long run.
Use Discharge pipe size and fitting counts as installation checks, not decoration. Narrow pipe, several elbows, and check valves can make friction head large enough to change the reference class.
Enter Candidate pump rating at this head from the manufacturer curve at the calculated total dynamic head. Do not enter zero-head flow or the largest number printed on the box.
Read Install Checks before choosing a larger pump. A high-capacity model can pass the flow target while increasing short cycling when the basin volume and switch travel are small.

FAQ:

Why does the result use GPM at TDH?

Pump curves are read by flow at head. Total dynamic head combines vertical lift and estimated friction, so a pump's useful capacity is the GPM it can still deliver at that head.

Is a higher horsepower pump always better?

No. A larger motor may still be weak at the calculated head, and an oversized pump can short cycle when Pump volume per cycle is small.

When should I use Drainage area estimate?

Use it when no timed basin test or known inflow is available. The result depends on Drainage area feeding pit, Design rainfall intensity, and Runoff factor, so verify it later with a wet-weather basin rise if possible.

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 the maximum flow printed for zero head.

What should I fix when sizing is paused?

Read the message under Check sump pump inputs. The common fixes are a positive Vertical lift, a valid Rise time and Observed water rise for timed tests, or a positive Known inflow rate for direct-flow sizing.

Glossary:

GPM: Gallons per minute, the flow unit used for pump-curve comparison.
Total dynamic head: Static head plus estimated friction head through the discharge route.
Static head: The vertical lift from the sump basin to the discharge high point.
Friction head: Head loss from pipe length, fittings, valves, pipe roughness, and flow rate.
Pump curve: A manufacturer chart showing how much flow a pump delivers at different head values.
Short cycling: Frequent brief pump runs caused by small usable basin volume, limited switch travel, or excessive pump capacity.
Runoff factor: The fraction of rainfall over the chosen drainage area assumed to reach the sump basin.

References:

Sump Pump Questions, NDSU Extension, reviewed August 2021.
Model 88 Submersible Sump Pump Data Sheet, Zoeller.
Hazen-Williams Pressure Loss Equation, Engineering ToolBox.
Sump Pit Sizing and Pump Capacity, RadonSeal.