Methodology, sources, and what we refuse to guess

S1_RAIN_GAL_PER_ACRE_INCH

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.

S1_RAIN_ROOF_EXAMPLE

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

A roof (40 x 70 feet) | 0.0001 | 0.000257 | 1,743 | 6,601

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

S1_WATER_LB_PER_GAL

8.329 lb per gallon at 70F

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

S1_ELECTRICITY_CENTS_PER_KWH

18 cents per kWh, US residential average

Table 5.3. Average Price of Electricity to Ultimate Customers: Total by End-Use Sector, 2016 - May 2026 (Cents per Kilowatthour)

Read from that table: the Residential column for May 2026 shows 18.44 cents per kilowatthour. EIA marks 2026 and 2025 values as preliminary estimates from a cutoff model sample. 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 claim: discharge into sanitary sewers

Inappropriate connections – Connections of sources of water such as sump pumps, roof leaders, foundation drains and area drains

The same EPA page lists this among the causes that "can overload a sanitary sewer".

D1_RAIN_GAL_PER_SQFT_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: S1_RAIN_GAL_PER_ACRE_INCH, DEFINITIONS.squareFeetPerAcre

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").

D2_GAL_PER_CUFT

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: DEFINITIONS.cubicInchesPerCubicFoot, DEFINITIONS.cubicInchesPerUsGallon

D3_GAL_PER_INCH_DEPTH_FACTOR

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: DEFINITIONS.cubicInchesPerUsGallon

D4_ELECTRICITY_USD_PER_KWH

0.184400 USD per kWh

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

Derived from: S1_ELECTRICITY_CENTS_PER_KWH

A1_HAZEN_WILLIAMS_C_PVC

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. If your discharge line is corrugated or has mineral scale, drop C toward 120 and re-run; the tool will show you how much head that costs you.

A3_FOUNDATION_CAPTURE_FRACTION

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, and it can plausibly run anywhere from under 10% to over 70%. 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.

A4_DESIGN_RAIN_INTENSITY

1.0 inches per hour

Assumed, no direct source: 1.0 inch per hour is a placeholder design storm, not your storm. Short-duration intensities vary by more than a factor of four across the United States.

How to replace it: NOAA Atlas 14 publishes precipitation-frequency estimates for your exact coordinates. Look up the 1-hour depth for the return period you care about and type it in.

A5_CAPACITY_SAFETY_FACTOR

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: Set it to 1.0 to see the bare break-even flow, or higher if your basement is finished and you want more margin.

A6_BATTERY_USABLE_FRACTION

0.50 usable fraction of nameplate amp-hours

Assumed, no direct source: we treat half the nameplate amp-hours as usable. Deep-cycle lead-acid batteries lose life fast below roughly 50% discharge, and a battery does not deliver its rated capacity 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.

A7_BATTERY_AGE_DERATE_PER_YEAR

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.

A2_FITTING_EQUIV_DIAMETERS

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.

A8_HP_BANDING

Assumed, no direct source: our horsepower bands are a shortlisting aid only. Two pumps of the same nominal horsepower can differ by 30 gallons per minute 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.

A9_PUMP_WATTS_BY_HP

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 $25 plug-in energy meter reads your pump's real watts in about a minute, and that measurement replaces this assumption. The nameplate amps x volts is a second-best substitute.

PIPE_ID_INCHES

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 dramatically rougher.

How to replace it: Measure the actual inside diameter of your discharge pipe if you can reach a cut end. Diameter matters more than anything else in the friction calculation — head loss scales roughly with the inverse 4.87 power of diameter.