Measure how fast your pit fills, instead of guessing

The procedure, in order

1. Wait for real rain. A test run in dry weather measures the water table, which is useful but is not the storm you are sizing for. The number that matters is the peak, so test during sustained heavy rain and take the worst reading you get.
2. Measure the basin, not the lid. Inside diameter of the liner, at the water line. A lid overhangs, and using the lid dimension inflates every gallon figure that follows.
3. Mark the starting water level. A strip of tape on the liner wall beats eyeballing a rough surface. You are going to read a difference of a few inches, and a half-inch reading error on the example's 3-inch rise is about 17 percent.
4. Unplug the pump. Stay there. This is the part of the procedure where an unattended basement floods, and no measurement is worth that.
5. Time the rise. Sixty seconds or more if the pit fills slowly enough to allow it; the shorter the window, the more your reaction time at each end distorts the result.
6. Plug the pump back in immediately and confirm it starts and clears the basin.
7. Repeat during the next storm. One reading is a data point; three readings across different storms tell you whether the first one was the peak or a lull.

Convert your rise into gallons per minute

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.

The arithmetic, shown in full

There is no black box here, so it is worth writing out. A one-inch-deep slice of a round basin is a cylinder: its volume is pi over four, times the diameter squared, times one inch. The United States liquid gallon is defined as exactly 231 cubic inches, so dividing by 231 converts that slice into gallons and gives a constant of 0.00340 gallons per inch of depth for every squared inch of diameter.

For the example 15-inch basin loaded above: 0.00340 multiplied by 15 squared gives 0.765 gallons per inch. A 3-inch rise is therefore 2.29 gallons, and 2.29 gallons in 120 seconds is 1.15 gallons per minute.

Nothing in that chain is estimated. It is geometry and a unit definition, which is precisely why a measurement is worth so much more than the alternative.

Why a long test reads low

There is a bias built into this procedure and it runs in the direction that matters. Water enters the basin because the drain tile around your footing sits at a higher pressure than the basin does, and as the level in the pit climbs, that difference shrinks and the flow into the pit falls with it. Unplug the pump, let the water rise a foot, and you have measured an average taken over a period in which you were steadily throttling the thing you were trying to measure.

So keep the rise small relative to the depth below the tile inlet. A small rise, well below the inlet, keeps this effect small; a rise that reaches the inlet makes it large, and a basin backed up far enough that water is standing in the tile has stopped measuring inflow altogether. If a slow pit forces a long window, read the first minute of the rise rather than the average across five — the first minute is the one taken at close to the level the pump normally holds, which is the condition you are actually sizing for.

Rising water in the pit slows the inflow, so a long test tends to read low. Other mistakes push the other way: measuring the lid instead of the liner, or a downspout discharging beside the foundation during the test, both make the reading high. Check both before trusting a high number.

Reading the result honestly

A rate measured in light rain understates your peak, and sizing a pump to it will leave you short on the night that matters. A rate measured while a downspout was discharging next to the foundation overstates it, and will sell you a pump larger than the house needs. Both errors are avoided the same way: test more than once, note the weather each time, and use the highest credible reading.

If your readings agree closely across several storms, the drainage around the house is behaving consistently. If one storm produces a reading several times the others, something intermittent may be contributing — a gutter that overflows only in heavy rain, a neighbour's downspout, a window well without a drain. That is worth finding before you size a pump around it.

One more check, assumed, no direct source: this tool flags more than 60 gallons a minute for a second look. At a reading like that, look for a broken supply line or a storm connection before you shop for a larger pump.

Was the storm you measured actually a big one?

A measured rate answers how fast, not how rare, and sizing is a question about rarity. A reading taken in what felt like heavy rain might represent a storm your address sees often or one it rarely sees, and the two call for different pumps. Nothing in your pit distinguishes them. A rain gauge standing next to the test does, as long as you look the depth up afterwards.

NOAA's National Weather Service runs a data server for that lookup, which it describes this way:

The Precipitation Frequency Data Server (PFDS) is a point-and-click interface developed to deliver NOAA Atlas 14 precipitation frequency estimates and associated information.

The same page says a user picks a state, identifies a location, and reads the estimates as tables or graphs. For your decision, the useful move is to find where the depth your gauge caught falls among the estimates for your location. If it sits among the frequent storms and the basin was already struggling, the problem is more likely drainage than pump capacity, and a larger pump may only move the failure. If it sits among the rare ones, you caught a storm worth sizing for.

If you caught something ordinary and want to plan for something rare, one rough approach is to scale up: take the ratio of the rarer depth to the depth you measured and multiply your inflow by it. That assumes inflow rises in proportion to rainfall, which this site has not verified, so treat the result as a rough planning figure. Both of its inputs — your measurement and a published estimate for your location — are real, which is not true of the capture-fraction estimate.

Take the measurement back to the sizing tool

• Sizing calculator — enter the same pit diameter, rise and seconds there to get the flow-and-head specification.
• Backup runtime — inflow is what sets the duty cycle, and therefore how long a battery lasts.
• Operating cost — annual run hours come straight from how many gallons have to move.