Measure the inflow, do not estimate it
Every other approach to sizing a sump pump is a guess dressed up as a rule. The measurement takes two minutes: unplug the pump, watch the water climb, note how far it rose and how long it took. Multiply the rise in inches by the gallons the pit holds per inch, scale to an hour, and that is your inflow rate.
The pit holds a predictable amount per inch because it is a straight-sided cylinder or box. Area in square inches divided by 231 cubic inches per gallon gives gallons per inch of depth. An 18 inch round liner has 254 square inches of surface, so 1.10 gallons per inch. A 24 inch liner has 452 square inches and holds 1.96 gallons per inch, nearly twice as much for a third more diameter, because area goes with the square of the radius.
The measurement is only worth what the conditions were when you took it. A pit in July tells you nothing about the same pit during spring thaw with the water table up. Take the reading during heavy sustained rain or during the wettest week of your year, and if you cannot, treat whatever you measured as a floor and put a larger margin on it.
Drawdown, cycles, and why the switch travel matters
Drawdown is the volume between the level that starts the pump and the level that stops it. It is the pit area times the vertical travel of the switch, and it decides how many times an hour the pump starts.
Cycles per hour = inflow in GPH ÷ drawdown per cycle in gallons
An 18 inch pit with 8 inches of switch travel clears 8.8 gallons per cycle. At 600 gallons an hour of inflow that is 68 starts an hour, or one every 53 seconds, which is punishing for a float switch. Increase the travel to 14 inches and the same pit clears 15.4 gallons per cycle and starts 39 times an hour. Nothing about the pump changed; the switch settings did.
There are limits at both ends. Too little travel and the motor short-cycles, which is the most common way a sump pump dies before its time. Too much and the water level climbs high enough to come back through the perimeter drain or reach the floor slab, which defeats the purpose. Existing pits usually cannot be made deeper, so the travel available is what it is, and that constraint should shape the pump you buy rather than the reverse.
Head, and the number on the carton
A pump's output falls as the head it works against rises. The relationship is a curve, published by the manufacturer, and reading it at your head rather than at zero is the single most important thing you can do while shopping. A unit advertised at 3,000 gallons per hour will commonly deliver 2,400 at 10 feet and 1,500 at 20, and if your discharge climbs nine feet out of the pit and runs thirty feet to daylight, twenty feet of equivalent head is not an unusual total.
| Component of head | What to count |
|---|---|
| Static lift | Pump-off water level up to the highest point in the discharge |
| Discharge pipe friction | Grows with length and shrinks fast with diameter. 1-1/2" costs far less than 1-1/4" at the same flow. |
| Check valve | A real loss, several feet on some designs. It is also not optional — without one the discharge pipe drains back into the pit and the pump cycles on its own water. |
| Elbows and fittings | Each one adds equivalent length. A discharge with four elbows is a longer pipe than it looks. |
The friction side of that table is the same arithmetic as any other pipe run, and the pressure drop calculator will convert a discharge layout into feet of head if you want a figure rather than an allowance. One foot of water head is 0.433 psi, if you need to move between the two units.
The failure modes worth thinking about
A sump pump is a single point of failure protecting something expensive. The three ways it goes wrong are the motor failing, the switch failing, and the power going out during the storm that made the pump necessary in the first place — which is not a coincidence, because the wind that takes the lines down brings the rain. Sizing has nothing to say about any of those. Backup arrangements, alarms and a check on the switch every autumn are what address them, and they matter more than the last few hundred gallons per hour of capacity.
Where the water is arriving from is also worth asking before buying a bigger pump. A sump that suddenly runs constantly is often reporting a gutter discharging against the foundation, a grade that has settled toward the house, or a failed downspout line rather than a rising water table. Those are cheaper to fix than to pump against forever, and the water leak guide covers the indoor side of tracing where water is coming from. Plumbing work is regulated, and the requirements are not the same from one town to the next. There are several model codes, jurisdictions adopt different ones and then amend them, and the local authority having jurisdiction decides what applies to your building. Most permanent work needs a permit and an inspection. Anything that touches the water service, the gas line, or a backflow-prevention point is licensed work in essentially every jurisdiction. What is on this page is common working practice and arithmetic, not a requirement you can hold up to an inspector.
Questions people ask
What size sump pump do I need for my basement?
It is set by the inflow into your pit and the head your discharge imposes, not by the size of the basement or the horsepower on the carton. Measure the rise with the pump unplugged during wet conditions, convert it to gallons per hour using the pit area, add a margin for a worse storm than the one you measured, and shop for a pump whose published curve delivers that at your total head. Two identical houses on different lots can need pumps a long way apart, because one sits above the water table and the other does not.
Is a 1/2 horsepower pump better than a 1/3?
Not necessarily, and horsepower is the wrong axis to compare on. What you need is gallons per hour at your head, and pumps of the same nominal horsepower differ substantially in what they deliver because impeller design and discharge size differ. A larger motor also draws more current and, if the pit is small, clears the drawdown faster and cycles more often. Compare published curves at your head. If two pumps meet the requirement, other things — switch design, discharge size, whether it is repairable — matter more than the horsepower figure.
How many times an hour should a sump pump cycle?
There is no single right number, but the direction of concern is clear: frequent short cycles wear the motor and the float switch faster than fewer long ones. If your arithmetic shows a run of only a few seconds per cycle, the fix is normally more switch travel rather than a smaller pump, since travel raises the gallons cleared per start without touching anything else. During a genuine storm, continuous or near-continuous running is expected and is not by itself a fault.
Does the discharge pipe size change the sizing?
Yes, through the head. Friction loss falls steeply with diameter at a given flow, so a long run in 1-1/4 inch pipe can cost several times the head of the same run in 1-1/2 inch, and that head comes straight off the pump output. If the discharge is long or climbs a long way, sizing the pipe up is often cheaper than buying a bigger pump to overcome the pipe. Where the discharge is allowed to terminate, and whether it may connect to anything, is set by local rules and is worth confirming before trenching.
Can I use this for a sewage ejector or a condensate pump?
The volume and head arithmetic is identical, and you can use the page for either. What does not carry across is everything else. Sewage ejector basins are sealed and vented, the pump has to pass solids of a specified size, and the whole installation is code-regulated work in a way a groundwater sump usually is not. Condensate pumps are at the other extreme in scale and are usually sized by the equipment manufacturer. Use the numbers, not the assumptions.