Sump Pit Sizing and Pump Capacity

Is Your Sump Pit Too Small?

The typical pit is 30 inches in depth and 18 to 24 inches across. The standard sump pit insert available in home improvement centers is 26 gallons and 18 inches diameter. In many cases the pit needs to be a minimum of 24 inches and up to a depth of 36 inches. But some builders just use a common 5-gallon bucket.

A small pit fills up with water very quickly and the sump pump has to turn on and off frequently, which shortens the life of the pump and of its check valve. Installing a store-sold backup pump in a small basin is impractical because of the risk of its float getting stuck and basement flooding.

In case the sump pit is still too small or crowded, you may need to dig through the bottom of the pail to place the main pump deeper or to cut the concrete and install a full-size sump pit. But many pumps have set turn-on level of 6 to 8" and turn-off at 3" level. Then, making the pit deeper would not affect the short-cycling of the pump. A larger diameter pit will take longer to fill, or install a pump with a switch that turns the pump on at a much higher level.

When the sump pump short-cycles, its thermal overload protection kicks in and shuts the pump down – the basement floods even though the sump pump is not broken and will return back to normal after it cools down.

The Hi & Dry backup pumps will not trip! They are placed well above the pit or the normal water level. Inside the pit, they only need enough space for the suction pipe and a float. And Hi & Dry battery pumps feature a slim-line vertical switch to avoid the risk of entanglement.

What Is Water inflow into My Sump Pit?

If you are an engineering type, you can estimate the needed approximate capacity of the back-up sump pump. As an indication of the minimal pump capacity needed, calculate the volume of water based on the volume of the sump basin. On a rainy day, insert a yard stick into the sump to the low water level. Then, read how many inches the water rises in one minute.  

Sump Water Inflow (gph)
water rise in one minute
    2" 4" 6" 8" 10" 12"
sump diameter 16" 120 240 300 420 540 660
18" 120 300 420 540 660 780
20" 180 360 480 660 840 960
22" 180 420 600 780 1,020 1,200
24" 180 480 720 960 1,200 1,380

Example: An 18" sump with a water rise of 6" per minute represents a flow of 420 US gallons per hour.

Beside emptying the sump pit, the pump also has to handle the water gushing into the pit while it is pumping. Add an extra margin (20% to 50%) to the needed pump capacity.

Then, you have to measure the needed lift – how high has the pump deliver the water from the bottom of the sump pit. A pump can remove much more water at a 5-ft "head pressure" (lift) than if has to pump 10 feet high. Add an extra foot for each elbow.

But how can this tiny pump back up my huge primary sump pump? The primary pump does not pump continuously – it empties the pit and waits until the sump fills up again. Say, your pump capacity is 2,400 gph and it empties the pit in 10 seconds once a minute. Then, it actually pumps for just 600 seconds (60 cycles x 10 seconds) or 10 minutes each hour and moves approximately 400 gph (10/60 x 2,400) of water. The backup pump will take longer to empty the sump (say 20 seconds) but then, it still has to wait for the water level to rise again.