Sump Pump Sizing Calculator

A sump pump sizing calculator returns how many times per hour a pump cycles. It converts the basin's drawdown into gallons, then divides by the net pump rate and the inflow rate. An 18 in. basin with 10 in. drawdown, a 40 GPM pump, and 10 GPM inflow cycles about 41 times per hour.

Last reviewed September 2026Method & sources Report an error

Pump Cycles per Hour

40.8

Breakdown
Pump Cycles per Hour	40.8
Drawdown Volume	11.02 gal
Pump Run Time per Cycle	0.37 min

How to Calculate Sump Pump Sizing

The basin's drawdown volume is how much water the pit holds between the pump's ON and OFF float levels. It comes from the basin's cross-sectional area times the drawdown height, converted to gallons. Each cycle has two phases. In the first, the pump runs at its net rate, meaning pump capacity minus incoming water, since inflow continues while pumping. It runs until it empties that drawdown volume. In the second, the basin refills at the plain inflow rate until the pump switches back on. Adding those two times gives the full cycle time. Sixty divided by that gives cycles per hour.

Example: 18 in. diameter basin, 10 in. drawdown, 40 GPM pump, 10 GPM inflow

An 18 in. diameter basin, 10 in. drawdown, 40 GPM pump, 10 GPM inflow: 1. Basin area: π × (18 ÷ 12 ÷ 2)² = π × 0.5625 = 1.767 sq ft 2. Drawdown volume: 1.767 × (10 ÷ 12) × 7.48052 = 11.02 gal 3. Net pump rate: 40 − 10 = 30 GPM 4. Run time: 11.02 ÷ 30 = 0.37 min 5. Refill time: 11.02 ÷ 10 = 1.10 min 6. Cycle time: 0.37 + 1.10 = 1.47 min 7. Cycles per hour: 60 ÷ 1.47 ≈ 40.8 This setup cycles about 41 times per hour, more frequent than the commonly recommended limit. A larger basin or drawdown, or a pump closer to the actual inflow rate, would reduce wear.

Sump Pump Sizing: Compare Scenarios

Scenario	Pump Cycles per Hour	Drawdown Volume	Pump Run Time per Cycle
Small basin, light inflow (12 in., 5 GPM)	81.7	2.94 gal	0.15 min
Standard setup (18 in., 10 GPM)	40.8	11.02 gal	0.37 min
Large basin, heavy inflow (24 in., 30 GPM)	31.9	35.25 gal	0.71 min

What Is Sump Pump Sizing?

A sump pump sizing calculator checks how frequently a pump will turn on and off. It uses a basin's size, its drawdown range, the pump's capacity, and how fast water enters the pit. Too-frequent cycling wears out a pump's motor and switch prematurely. The calculator helps catch an undersized basin, or an oversized pump relative to the actual water inflow, before it becomes a maintenance problem.

Sump Pump Sizing: Pros and Cons

Pros

Accounts for the fact that inflow continues while the pump runs, using the net rate for the pump-down phase rather than the pump's full rated capacity
Gives a direct cycles-per-hour figure, the metric most sump pump buying guides and manufacturers reference for avoiding excessive wear
Works for any basin size, drawdown setting, pump capacity, and inflow rate combination

Cons

Doesn't measure your actual water inflow rate for you. You typically need to estimate it from experience, such as how fast your existing pit fills, or from a percolation or drainage assessment. It varies by soil, water table, and weather
A pump oversized relative to inflow cycles more often. It can also be counterproductive if it's rated well beyond what's needed. More starts per hour is itself a source of wear, regardless of pump size
Doesn't account for a battery backup or secondary pump system, which some sump setups use for redundancy during power outages or heavy inflow

Sump Pump Sizing: What a Pro Checks

Keep cycling under the roughly 20-30 times per hour most manufacturers recommend. Above that, motor and switch wear accelerates.
Don't just grab the biggest pump available. A pump rated far beyond your actual inflow rate can be counterproductive, since more starts per hour is itself a source of wear regardless of pump size.
If cycling comes out too frequent, try increasing the float switch's drawdown range or the basin diameter before assuming you need a different pump. More volume per cycle means fewer, longer cycles.

Frequently Asked Questions

How many times per hour should a sump pump cycle?

Most manufacturers recommend keeping cycling under roughly 20-30 times per hour to avoid excessive motor and switch wear. Check your specific pump's manual for its stated maximum recommended cycle rate.

What is drawdown in a sump pump system?

Drawdown is the vertical distance between the float switch's ON level, when the pump starts, and its OFF level, when it stops. A larger drawdown means more water is pumped per cycle. That generally means fewer, longer cycles rather than many short ones.

How do I reduce how often my sump pump cycles?

Increase the drawdown range by adjusting the float switch settings further apart, if adjustable. You can also use a larger diameter basin, which holds more volume per inch of drawdown. Or choose a pump capacity closer to your actual inflow rate rather than a much larger one.

What size sump pump do I need?

Pump capacity should comfortably exceed your expected peak water inflow rate. But a pump vastly oversized relative to inflow can cause more frequent short-cycling rather than solving the problem. Matching capacity reasonably close to your actual inflow, with some margin, generally works better than choosing the largest available pump.

How do I know my sump pit's water inflow rate?

A rough estimate comes from timing how long it takes your existing pit to refill from the OFF level to the ON level during wet conditions. Then divide the drawdown volume by that time. Inflow varies with rainfall, season, and water table, so consider your heaviest expected conditions rather than a typical day.

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