What size should a sump pit be?
The hole constrains everything that goes in it. Pit dimensions rule out more pumps than horsepower ever will.
A sump pit does two jobs. It collects water from the drain tile, and it gives the pump somewhere to sit with enough depth to draw down properly between cycles. Both jobs get harder as the pit gets smaller, and neither can be fixed by buying a better pump.
The numbers
| Dimension | Practical minimum | Better | What it decides |
|---|---|---|---|
| Diameter | 18 in | 24 in | Switch type, and whether a backup pump will fit |
| Depth | 22 in | 30 in | Draw-down per cycle, and therefore motor starts |
| Gallons per inch | About 1.1 at 18 in | About 2.0 at 24 in | How quickly the level rises |
Why diameter matters more than people expect
A tethered float needs lateral room to swing. In a pit narrower than about 14 inches it will eventually catch on the wall or wrap around the discharge pipe, and a caught float either holds the pump on — which runs it dry — or holds it off, which floods the basement. Narrow pits therefore force a vertical float or a solid-state switch.
Diameter also decides whether a backup pump can ever be added. Two pumps and up to four floats need room to operate without interfering, and retrofitting a backup into an 18-inch crock is frequently impossible.
Why depth matters
Depth is draw-down. A deeper pit lets the pump move more water per cycle, which means fewer starts for the same total volume — and motor starts, not running hours, are what wear a pump out. A shallow pit produces a pump that runs for twelve seconds every three minutes during rain, which is the worst duty cycle available.
The constraint is that the pit floor has to sit below the drain tile inlet, which the original construction usually fixed. Within that, deeper is better.
Jackel SF20 sump basin (18 x 22 in)
18 by 22 inches, which takes any common residential pump and any switch type.
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| Size | 18 in diameter x 22 in deep |
|---|---|
| Capacity | 20 gallons |
| Material | Injection-molded structural foam polyethylene, one piece |
| Rim | Pre-drilled holes with metal inserts to accept a cover |
| Sealing | Gas and radon tight when fitted with a sealed cover |
| Weight | 11 lb |
If you are installing from scratch, start here and go larger if the floor allows. The marginal cost of a bigger basin at install time is trivial against the cost of widening a pit later.
What it gets right
- Fits tethered and vertical floats alike
- Standard lid sizes available
- Good draw-down depth
Where it falls short
- Tight once you add a backup pump
- Replacing an old basin means breaking concrete
Specifications from 18 x 22 in radon sump basin distributor listing.
If your pit is too small
- Fit a vertical float or solid-state switch instead of a tether. Cheapest and often sufficient.
- Accept shorter cycles and plan to replace the switch more often than average.
- Consider a second pit elsewhere in the basement rather than enlarging this one, if inflow is the problem.
- Enlarge it properly only if you are already breaking concrete for something else — it is a demolition job.
Questions people ask
What is the standard sump pit size?
18 inches in diameter by 22 inches deep is the common residential standard and the practical minimum. 24 by 30 is noticeably better and worth it in a new installation.
Can a sump pit be too deep?
Rarely. Depth gives longer draw-down and fewer motor starts. The practical limit is the depth of the drain tile that feeds it and how far you are prepared to dig.
How many gallons is a sump pit?
An 18-inch basin holds roughly 1.1 gallons per inch of depth; a 24-inch basin about 2.0. Useful numbers when you are converting a measured one-inch rise into an inflow rate.
Do I need a bigger pit for a backup pump?
Usually yes. Two pumps and their floats need room to work independently, and an 18-inch basin is tight for that. Size up if a backup is anywhere in your plans.
Sources
Read next
The best sump pit basins
The hole constrains everything else. Get it wrong and you spend the next decade choosing pumps around a basin that is too small.
What size sump pump do I need?
Two measurements and ten minutes give you a real answer. Almost everyone skips both and buys on horsepower instead.
The best sump pump float switches
Motors rarely die first. Switches do — and unlike a motor, a switch is a part you can replace in twenty minutes for the price of a takeout meal.
How to install a sump pump
A like-for-like replacement in an existing pit is a two-hour job. Cutting a new pit into a slab is not, and this page is honest about which is which.
The best submersible sump pumps
A submersible sits in the water it is moving, which makes it quieter, cooler and almost always the right choice — provided the pit is big enough for it.