Sump Pump Runtime Calculator
Calculate cycle time, cycles per hour, daily runtime, and duty cycle from your pump capacity, pit size, switch differential, and water inflow rate.
Planning for a power outage during a storm? Pair with the generator size calculator to size a backup that can run your pump.
Calculate your sump pump runtime
Enter pump capacity at your discharge head, pit diameter, switch differential, and inflow rate to see cycle time, cycles per hour, and daily runtime.
11 min/day
2,400 GPH pump handling 120 GPH inflow in an 18 in pit with a 6 in switch differential.
Quick tips
- Sump pumps fail from short-cycling, not long runs. Most motors are rated for about 1 million cycles. At 20 cycles per hour, that lifespan is gone in under 6 years. Use a wider pit or a longer switch differential to cut cycle count.
- Pump capacity drops sharply with discharge height. A pump rated 3,000 GPH at 0 ft of head may deliver only 2,100 GPH at 10 ft and 1,200 GPH at 20 ft. Look up the pump curve and use the number at your actual lift.
- Measure actual inflow during a rain event. Unplug the pump, let the pit fill, and time how many inches the water rises per minute. Multiply by pit area to get gallons per hour.
- Switch differential matters more than pit diameter for cycle rate. A 6 in differential gives twice the per-cycle runtime of a 3 in differential in the same pit, which roughly doubles motor life.
- Install a battery backup sump pump. Most primary pump failures happen during power outages in the same storm causing the water inflow. A dedicated marine battery with an automatic charger gives 4–8 hours of backup runtime.
Practical note
If inflow exceeds pump capacity at your discharge head, the pit overflows and you get a flooded basement. Always size your pump for the highest expected inflow (a 25-year storm event), not the average. A typical residential basement sees 120–240 GPH during moderate rain and 400–800 GPH during a heavy storm. If this calculator shows runtime over 15 minutes per cycle or more than 60% duty cycle, upgrade to a larger pump or add a secondary pump in parallel.
How to use this calculator
Enter your pump capacity in gallons per hour at the actual discharge head (not the zero-head maximum on the box). Select your pit diameter and enter the switch differential in inches. Enter your measured or estimated water inflow rate in GPH. The calculator returns gallons moved per cycle, runtime per cycle in seconds, cycles per hour, and total daily runtime in minutes.
How the math works
- Gallons per cycle. Pit area times switch differential gives the volume moved per cycle. A circular pit uses pi × radius² × differential, converted from cubic inches to gallons at 231 cubic inches per gallon.
- Net drain rate. Pump capacity minus inflow equals the effective rate at which water leaves the pit. If net drain is zero or negative, the pump cannot keep up and the pit will overflow.
- Runtime per cycle. Gallons per cycle divided by net drain rate. This is how long the pump runs each time it activates.
- Off time per cycle. Gallons per cycle divided by inflow rate. This is how long the pit takes to refill between cycles.
- Cycle period. Runtime plus off time equals full cycle length. Cycles per hour equals 60 divided by cycle period in minutes.
- Daily runtime. Cycles per day times runtime per cycle. This is total pump motor hours per 24-hour period, which drives electrical cost and pump wear.
Common mistakes
- Using the zero-head rating. The box says 3,300 GPH, but that's with no lift. At 10 ft of discharge head, the same pump may deliver only 2,000–2,200 GPH. Always read the pump curve at your actual discharge height.
- Ignoring the check valve. A failing check valve lets pumped water flow back into the pit, creating continuous cycling even when the pump is perfectly sized. If your pump runs far more than this calculator predicts, replace the check valve first before buying a new pump.
- Setting the differential too short. A 3-inch differential in an 18-inch pit holds only about 3 gallons between cycles. Short differentials cause short-cycling that wears out a motor in 2–4 years instead of 8–10.
- Sizing for average rain, not the worst storm. Your pump will sit idle during dry weeks and get pounded during a 25-year storm. If moderate rain brings 120 GPH, a severe storm can bring 400–800 GPH. Size for the peak, not the typical.
- No battery backup. Most sump pump failures happen during power outages caused by the same storm creating the inflow. A dedicated marine battery with an automatic charger gives 4–8 hours of backup runtime for under $200.
FAQ
How often should my sump pump run?
During dry weather, your pump should not run at all or cycle only once every few hours from minor moisture. In moderate rain, cycling every 10–20 minutes is normal. During heavy storms, every 2–5 minutes is typical. If your pump runs continuously or cycles more than 20 times per hour, either inflow is too high or the switch differential is too short. Check for a stuck check valve or a failing float switch first.
How long should a sump pump run per cycle?
A healthy sump pump should run 10–30 seconds per cycle to move 5–10 gallons, then shut off. Runtimes under 5 seconds indicate short-cycling and will burn out the motor fast. Runtimes over 60 seconds per cycle in non-storm conditions suggest the pump is undersized, the check valve is bad, or the discharge line is partially blocked.
Why does my sump pump run constantly?
The three most common causes are: a stuck check valve letting pumped water fall back into the pit, a float switch stuck in the up position, or a discharge line that is frozen, clogged, or disconnected. If none of those apply, the pump is simply too small for the inflow rate. Upgrade to a 1/2 HP or 3/4 HP pump, or install a second pump in parallel.
How do I measure my actual inflow rate?
During a rain event, unplug the pump after it empties the pit. Let water rise while timing how long it takes to fill a known volume. Mark the pit at 1 in increments with tape, then time how many inches rise per minute. Multiply inches per minute by pit area in square feet by 7.48 (gallons per cubic foot) by 60 (minutes per hour) divided by 12 (inches per foot) to get GPH.
What size sump pump do I need for my house?
For a typical basement with moderate water table issues, a 1/3 HP pump (2,500–3,300 GPH at zero head, roughly 1,800–2,400 at 10 ft lift) handles most rain events. Homes with high water tables, large roof area draining to foundation drains, or heavy clay soils should start with a 1/2 HP pump (3,600–4,800 GPH). Below-grade basements with drain tile in flood-prone areas need 3/4 HP or a primary plus backup system.
How much electricity does a sump pump use?
A 1/3 HP pump draws roughly 450 watts while running. At 11 minutes of daily runtime, that is 0.08 kWh per day or under $0.30 per month at $0.15 per kWh. During heavy rain weeks with 2–4 hours of daily runtime, cost can climb to $10–$20 for the month. Pumps running continuously (50% duty cycle or more) use 150–250 kWh per month.