Sizing calculator

Work out the sump pump you need from the water, not the price tag

A sump pump is bought on one number — gallons per hour, printed large on the box — and installed against a different one. The advertised figure is measured at some particular lift, which may not be yours. The example loaded below puts the basin under 9 feet of vertical rise, 25 feet of discharge line, 3 elbows and a check valve. A pump moves less water against more head, so the flow printed for a lower head is not the flow it delivers here, and the gap is not a defect in the pump. It is the difference between the head it was rated at and the head it actually has to work against.

This tool starts from the opposite end. It asks what you can see in your own basement — how wide the crock is, how far the water climbed while you watched it, how high the pipe goes and how far it runs — and it derives the two numbers you need in order to shop: the flow the pump has to deliver, and the head it has to deliver that flow against. Those two together are a specification. Either one alone is marketing.

Size the pump from your own basin

The basin itself

Inside diameter of the pit

Measure the liner, not the lid. The 18 inches filled in here is an assumed starting value for the worked example, not a sourced typical size, so replace it with your own reading.

Float travel between pump-on and pump-off

Watch one cycle. The vertical distance the water drops before the pump shuts off is the drawdown, and it decides how often the switch is asked to work.

How fast water arrives

Do you have a measurement?

A measurement replaces the estimate's weakest input. The estimate carries an assumed capture fraction (35%) whose own range on this site runs from 10% to 70%, so it can be several times too high or too low.

How far the water rose

How long that took

sec

Roof and hard surface draining toward the house

sq ft

Only used when you have no measurement.

Design rainfall intensity

in/hr

NOAA's precipitation frequency data server gives estimates for a location you choose. The 1.0 filled in here is an assumed starting value, not a sourced one.

Where the water has to go

Vertical rise from the pump to the outlet

Horizontal run of discharge pipe

Discharge pipe size

Number of 90-degree elbows

Number of 45-degree elbows

Check valve on the discharge

Illustrative assumption — not a market quote: every value filled in above is a worked-example starting point chosen by this site, not a measurement of your house. Replace each one with your own.

Take this to any pump curve

Deliver at least 6.6 gallons per minute (397 gallons per hour) at 9.2 feet of total head.

Both halves matter. A pump that hits the flow at a lower head than yours does not hit it here.

Water arriving 4.4 gpm From the pit width and timed rise, or from the area estimate if you switch to it.

Total dynamic head 9.2 ft Lift plus pipe friction at the required flow, using the assumed pipe roughness, fitting equivalents and pipe bore listed below.

Required flow 6.6 gpm Inflow multiplied by a 1.5x margin.

Shelf to shop on 1/4 HP class A shortlist, not a performance claim.

Inflow, low to high 3.7 to 5.1 gpm

How the band is set: on the measured path it is the reading plus or minus 15%, an allowance for reading error chosen by this site (assumed, no direct source); on the estimate path it uses capture fractions of 10% and 70%, this site's own range.

Biggest swing factor How representative your timed test was of a real storm. A measurement taken in light rain understates the peak you are actually sizing for; repeat it during heavy rain and take the worst reading.

Discharge velocity is only 1.0 ft/s over a 25 ft run. This tool flags velocities under 2 ft/s on runs over 20 ft (a threshold chosen by this site, no direct source): slow water can leave silt behind, and water left standing in a horizontal line between cycles can freeze.

What this specification deliberately leaves out

Any performance claim about a specific pump. We compute what you need; the manufacturer curve says what a pump gives.
Groundwater arriving through the slab or the walls rather than the footing drain, which no pump specification can fix.
A failed or collapsed footing drain, which changes the answer completely and is a drainage repair, not a pump purchase.
Sewer backup, which is a different failure with a different device — a sump pump does not protect against it.

The number worth keeping down: starts

Every start works the switch, so the number of starts is worth keeping down. With the figures in the tool the pump runs about 240 seconds, rests about 120 seconds, and therefore starts roughly 10 times an hour while water is arriving at that rate.

In this tool's arithmetic a bigger pump empties the same basin faster, which shortens each run and does not reduce the start count. What reduces starts is a wider basin, a longer float travel, or less water arriving in the first place; redirecting a downspout is one way to get the last.

With the figures in the tool, the pit holds 1.10 gallons per inch of depth, so a 8-inch drawdown moves 8.8 gallons per cycle. Two more inches of float travel would add 2.2 gallons to every cycle and cut the start count from about 10 to about 8 an hour, where the float and the basin depth allow it.

The assumptions on this page, stated plainly

Pipe roughness, fittings and bore — assumed, no direct source

Assumed, no direct source: we use C = 150 for new smooth PVC. The Hazen-Williams equation is a published empirical standard, but we could not retrieve a primary document we can quote for this specific coefficient, so it is labeled an assumption rather than a sourced value.

Assumed, no direct source: fitting losses are modeled as an equivalent straight-pipe length in pipe diameters (90-degree elbow 30, 45-degree elbow 16, swing check valve 100, spring check valve 135). These are conventional practice values; we could not retrieve a primary document we can quote, so they are labeled assumptions.

Assumed, no direct source: inside diameters are the conventional Schedule 40 PVC values. Schedule 80, ABS, and flexible corrugated discharge all differ, and corrugated pipe is rougher than smooth PVC. The 1.5-inch pipe in the example is taken as a 1.61-inch bore. At the example inputs the elbows and check valve add 25.5 feet of equivalent pipe to the friction calculation.

Assumed, no direct source: the inflow low-to-high band on the measured path is the reading plus or minus 15%, an allowance for reading error chosen by this site.

If you switch the inflow method to the estimate, two further assumptions come into play — the capture fraction (35%) and the assumed rainfall intensity (1.0 in/hr). Both are described on the methodology page, and the capture fraction is the weakest number on this site. Do the pit inflow test during real rain instead. A measured rise rate replaces this assumption entirely and is the number the sizing tool prefers whenever you have it.

Where the numbers underneath this come from

On the estimate path, one figure on this page comes from a federal source: the conversion that turns a roof and a driveway into gallons. The United States Geological Survey publishes it under the water-equivalents heading on its rain and precipitation page:

One inch of rain falling on 1 acre of ground is equal to about 27,154 gallons and weighs about 113 tons.

U.S. Geological Survey, Water Science School — source page; original retrieval recorded 2026-08-06; that copy was not retained; checked against our saved copy of 2026-10-05.

What that sentence does for your decision is set a ceiling, not give an answer. USGS says about and means it — the figure is rounded — but the rounding is a fraction of a percent, which is why our own cross-check against the roof example on the same page lands two gallons apart in seventeen hundred. It pins down how much water reaches the ground, which means every remaining doubt about the estimate path is doubt about what share of it reaches your footing drain — and that share is our 35% capture assumption, not the USGS figure. The federal half of the chain is arithmetic; the half we supply is an assumption whose own range on this site runs from 10% to 70%. That asymmetry is the argument for running the timed inflow test instead of trusting the estimate, and it is why the tool above prefers a measured rise whenever you have one.

Constant	Value	Label	Source	Date
Rain per acre-inch	27,154 gal	sourced	U.S. Geological Survey, Water Science School — source page; original retrieval recorded 2026-08-06; that copy was not retained; checked against our saved copy of 2026-10-05.	saved copy 2026-10-05
Pit gallons per inch of depth	0.00340 x diameter squared	derived	Cylinder geometry and the 231-cubic-inch gallon	definition
Sizing margin	1.5x	assumed	Our own adjustment, disclosed above	set 2026-08-06
Capture fraction	35%	assumed	Our own adjustment, disclosed above	set 2026-08-06

Every registered figure carries one of three labels — sourced, derived, or assumed — and the methodology page lists them with their sources and dates, along with the assumed tables and the screening thresholds the tools use.

Where to go from here

Run the pit inflow test properly — the timed procedure, repeated over several storms, that replaces the weakest assumption on this page with a measurement.
Break down the head — see how much of your 9.2 feet is lift you cannot change and how much is pipe you can.
Decide pedestal or submersible — a fit question your basin can answer for you.
Check a quote against the scope — which line items this job genuinely requires.
Two full worked examples — see this same math run for two example households, start to finish.