Sump Pump Capacity Calculator

Q: How do I size for duty cycle?

Pumps should run < 50% of any 5-minute window or motors overheat. If inflow approaches pump GPH at design head, install a larger pump or add a battery backup pump for redundancy.

Frequently Asked Questions

How is required sump pump GPH calculated?

Measure inflow during a storm: fill rate (in/min) × pit area (sq in) × 0.26 = GPH. A 24 in diameter pit (≈ 452 sq in) filling 1 in/min = 1 × 452 × 0.26 ≈ 118 GPH; design for 2× peak.

How does discharge head affect capacity?

Pump curves drop sharply with head. A ⅓ HP rated 2,500 GPH at 0 ft may deliver only 1,800 GPH at 10 ft of lift + friction. Sum vertical rise + friction loss in the discharge pipe to find true output.

What pipe size and discharge layout matter most?

Use full-size 1½ or 2 in PVC straight up with one sweep elbow; avoid long horizontal runs. Each 90° elbow adds ≈ 3 ft of equivalent head - three elbows can cost 200–400 GPH of capacity.

How do I size for duty cycle?

Pumps should run < 50% of any 5-minute window or motors overheat. If inflow approaches pump GPH at design head, install a larger pump or add a battery backup pump for redundancy.

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This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

A sump pump is sized so that on the heaviest storm your basin can fill faster than the pump can empty it. The two design numbers are the basin's peak inflow rate in gallons per minute (or per hour) and the total dynamic head the pump must lift water against. Peak inflow is measured by timing how fast the basin refills with the pump unplugged during a heavy rain - or estimated from foundation perimeter, drain-tile loop length, and the local rainfall intensity. Total head is the vertical lift from the float-off level in the basin up to the discharge daylight or sanitary connection, plus the friction loss in the discharge pipe and fittings.

Formula / Method

Required pump capacity (gpm) ≈ basin peak inflow rate (gpm), with margin so the pump runs roughly 30–50% of the time rather than continuously. Total dynamic head = static lift + friction head. Friction head for 1-1/4 in or 1-1/2 in PVC discharge over 25–40 ft typical residential runs is small - 1–3 ft - but a long horizontal run or a 1-in restricted discharge can add 5–10 ft. Manufacturers publish a pump curve (gpm at a given lift); read the curve at your computed total head to confirm the model delivers enough gpm at that point. Round up: a pump operating at the absolute edge of its curve has zero margin for filter screen fouling or float-switch lag.

Worked Example

A finished basement with 10 gpm peak basin inflow during a 2-year storm, 10 ft vertical lift to the discharge daylight, and 25 ft of 1-1/2 in PVC discharge. Friction (Hazen-Williams, C = 150) is about 0.5 ft; total dynamic head is 10.5 ft. A common 1/3 HP cast-iron sump rated 35 gpm at 10 ft delivers about 30 gpm at the actual 10.5 ft point - three times the inflow, so the pump runs about 1 minute on, 2 minutes off. For a heavier 30 gpm inflow basement, a 1/2 HP pump rated 55 gpm at 10 ft is the right call; over 30 gpm sustained pretty much demands a battery-backup secondary because the primary will cycle continuously and a power outage drowns the basement in minutes.

Common Mistakes

Reading the box rating "1800 gph" at zero head - real lift cuts that number in half.

Forgetting the check valve adds 2–4 ft of equivalent head loss.

Discharging into a 1-in garden-hose line - chokes the pump and slashes capacity.

No backup pump in a finished basement - the primary fails or the power goes out during the worst storms.

Undersized basin - a small basin cycles the pump dozens of times per hour and wears the motor out in years.

Tips & FAQ

How do I measure peak inflow? Unplug the pump during a heavy storm, time how many seconds it takes the basin to fill 1 inch, then multiply by basin area and convert to gpm. Or measure how often a healthy pump cycles and multiply cycle drawdown by cycles per minute. 1/3 HP or 1/2 HP? 1/3 HP handles most basements with under 20 gpm inflow and 10 ft lift; 1/2 HP for higher water tables, longer runs, or anyone who has finished the basement. Pedestal or submersible? Submersibles run quieter, last longer, and fit smaller basins; pedestals are cheaper and easier to service. Backup options: battery-backup secondary pumps run on a deep-cycle marine battery for 4–8 hours at typical cycling; water-powered backups use city water pressure with no electricity needed but waste 1 gallon city for every 2 gallons pumped. Check valve: always install one on the discharge to keep the column of water from falling back when the pump shuts off (otherwise the pump cycles within seconds). Discharge: 1-1/2 in or 2 in PVC, sloped, with a freeze-resistant termination above grade; never discharge to a sanitary sewer. Maintenance: clean the basin and inlet screen annually, test by lifting the float monthly, replace the pump every 7–10 years before it fails. This is a planning estimate - for flood-prone homes or finished basements get a professional sizing and add backup.

Sump Pump Capacity Calculator

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