Methodology, sources, and what we refuse to guess

Rain per acre-inch (USGS)

27,154 gal per acre-inch

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

USGS roof example

1,743 gal from a 1-inch rainstorm on a 40x70 ft roof

Transcribed table cells (not a quotation):

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.

Used as an independent cross-check on D1, not as a calculator input.

Weight of a gallon of water at 70°F (USGS)

8.329 lb per gallon at 70F

That gallon of tap water at 70°F weighed 8.329 pounds

Not an input to any calculator on this site. Listed because it was registered on 2026-08-06; the sentence has since been removed from the live page.

US residential electricity price (EIA)

18.44 cents per kWh, US residential average

Transcribed table cell (not a quotation): row U.S. Total, column Residential, May 2026, page 137: 18.44 (cents per kilowatthour).

Historical figure from the publisher's archived issue — U.S. Energy Information Administration, Electric Power Monthly, July 2026 issue, Table 5.6.A, May 2026 data, marked preliminary by the publisher: Electric Power Monthly, July 2026 issue (PDF); original retrieval recorded 2026-08-24; that copy was not retained; checked against our saved copy of 2026-10-05.

Transcribed cell: the U.S. Total row of Table 5.6.A in EIA's July 2026 issue, Residential column, May 2026: 18.44 cents per kilowatthour. EIA marks these values as preliminary estimates. This is a national average and is the wrong number for any specific house; the tool takes your own rate off your own bill and only falls back to this.

Regulatory context: discharge into sanitary sewers

Inappropriate connections – Connections of sources of water such as sump pumps, roof leaders, foundation drains and area drains can overload a sanitary sewer

It is one item in the page's list of reasons sewers overflow. The page does not say whether such a connection is prohibited or inspected where you live; that is a local question.

Rain per square foot per inch

0.623370 gal per sq ft per inch of rain

27,154 gal per acre-inch (USGS) / 43,560 sq ft per acre (definition) = 0.623370 gal per sq ft per inch

Derived from: Rain per acre-inch (USGS); Square feet per acre

Cross-check against the USGS roof example on the same page: 2,800 sq ft x 0.623370 = 1,745 gal vs the published 1,743 gal (0.1% apart, which is the rounding in "about 27,154").

Gallons per cubic foot

7.4805 gal per cubic foot

1,728 cubic inches per cubic foot / 231 cubic inches per US gallon = 7.480519 gal per cubic foot

Derived from: Cubic inches per cubic foot; Cubic inches per US gallon

Round-pit gallons per inch of depth

0.003400 gal per inch of depth per square inch of diameter

A 1-inch slice of a round pit holds (pi/4) x d^2 cubic inches. Divide by 231 cubic inches per gallon: (pi/4)/231 = 0.00340 gal per inch of depth for each (inch of diameter) squared. An 18-inch pit therefore holds 0.00340 x 18^2 = 1.10 gal per inch of depth.

Derived from: Cubic inches per US gallon

Electricity price in dollars per kWh

0.184400 USD per kWh

18.44 cents per kWh (EIA, May 2026) / 100 = $0.1844 per kWh

Derived from: US residential electricity price (EIA)

Pipe roughness coefficient C

150.0 Hazen-Williams C, smooth plastic pipe

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.

How to replace it: Older, scaled, or corrugated discharge pipe is rougher and loses more head. The tool holds C at 150 and it cannot be changed on these pages; under the same Hazen-Williams formula, C = 120 raises friction by about 51% at any flow, so treat the friction figure as a floor for rough pipe.

Share of rain reaching the footing drain

0.35 fraction of contributing-area rainfall reaching the pit

Assumed, no direct source: we assume 35% of the rain landing on the contributing area reaches your footing drain. The true figure depends on soil, grading, downspout routing, and how much of the drain is still open. The low-to-high band the estimate prints uses 10% and 70%, a range chosen by this site, so the estimate can be several times too high or too low. This is the biggest single swing factor in the storm estimate.

How to replace it: 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.

Starting design rainfall intensity

1.0 inches per hour

Assumed, no direct source: 1.0 inch per hour is an assumed design storm, not your storm. It is a starting value for the form, not a national average.

How to replace it: NOAA's Precipitation Frequency Data Server lets you choose a location and read precipitation frequency estimates for it. Use the depth it gives for the duration and return period you want to plan for, and type it in.

Sizing margin on inflow

1.5 multiplier on peak inflow

Assumed, no direct source: we size the pump to 1.5x the peak inflow so it is not running continuously at its limit during the storm that matters. A pump that exactly matches inflow never shuts off and has no margin for a partly blocked line.

How to replace it: This margin is fixed at 1.5x in the tool. To see the bare break-even flow, divide the required flow by 1.5; that is the inflow figure shown as water arriving.

Usable share of battery amp-hours

0.50 usable fraction of nameplate amp-hours

Assumed, no direct source: we treat half the nameplate amp-hours as usable. The half is an allowance chosen by this site for two effects it does not model separately: battery life lost to deep discharge, and the lower capacity a battery delivers at the high current a pump draws.

How to replace it: If your battery manufacturer publishes a capacity at your pump's actual amp draw (rather than the 20-hour rate), use that number instead — it is the honest one.

Battery capacity lost per year of age

0.08 fraction of capacity lost per year

Assumed, no direct source: we derate battery capacity 8% per year of age. Real degradation depends on temperature, charger quality, and how often the battery has been deeply discharged.

How to replace it: Load-test the battery. A measured capacity beats any age-based rule of thumb, including this one.

Fitting equivalent lengths

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.

How to replace it: If your pump or valve manufacturer publishes an equivalent-length or head-loss figure for the exact valve you own, use theirs — it beats any generic table, including ours.

Horsepower shortlist bands

Assumed, no direct source: our horsepower bands are a shortlisting aid only. Two pumps of the same nominal horsepower can deliver different flows at the same head. The manufacturer's published performance curve is the answer; our band only tells you which shelf to look on.

How to replace it: Take the required flow AND the total dynamic head this tool gives you to any pump's published curve and read the flow it actually delivers at that head. If the curve does not reach your required flow at your head, the pump is wrong no matter what its horsepower says.

Running watts by horsepower band

Assumed, no direct source: running wattage by horsepower band is our estimate for planning only. Actual draw depends on motor design, head, and voltage.

How to replace it: A plug-in energy meter reads your pump's real running watts directly, and that measurement replaces this assumption. The nameplate amps x volts is a second-best substitute.

Pipe inside diameters

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.

How to replace it: Measure the actual inside diameter of your discharge pipe if you can reach a cut end. In the Hazen-Williams formula this site uses, head loss scales with the inverse 4.87 power of the inside diameter.