Free Plumbing Tool
Sump Pump Sizing Calculator
Horsepower is the wrong spec. Size on gallons per hour actually delivered at your total dynamic head — estimate inflow from soil and footprint, or run the one-inch pit rise test and use a real measured number.
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Book a DemoPit on-level up to the discharge exit
Adds 6 ft equivalent friction
Peak inflow
1,075 GPH
estimated
Total dynamic head
15 ft
9 ft lift + 6 ft friction
Recommended pump
1/2 hp
needs 1,613 GPH at 15 ft
What each pump actually delivers at 15 ft of head
| Pump | GPH at your head | Covers inflow? |
|---|---|---|
| 1/3 hp | 1,500 | No |
| 1/2 hp | 2,400 | Yes |
| 3/4 hp | 3,600 | Yes |
| 1 hp | 4,700 | Yes |
Inflow rates by soil type are planning averages and real sites vary widely with grading, downspout discharge, footing drain condition and seasonal water table. Pump curves shown are representative of common residential models, not any specific manufacturer, and friction allowances approximate a typical discharge run rather than a fitting-by-fitting takeoff. All calculations are estimates based on historical information and should be verified by the user with a licensed plumbing contractor. This tool is provided as a free service for planning purposes only and is not a substitute for a professional assessment, a supplier quote, or applicable local code requirements.
Why Horsepower Is the Wrong Sump Pump Spec
Ask what size sump pump a basement needs and the answer usually comes back in horsepower. But horsepower describes the motor, not the water moved, and two 1/2 hp pumps from different manufacturers can differ substantially in delivered flow. What matters is gallons per hour at the head the pump actually works against.
Head is where most sizing goes wrong. A 1/2 hp pump rated at 3,000 gallons per hour is usually quoting performance at 10 feet. Put it in a pit with 12 feet of lift and 60 feet of horizontal discharge and it delivers closer to 1,700 — a 43 percent drop, with nothing wrong with the pump. Static lift plus friction loss is the number to size against.
The one-inch rise test beats every estimate. During steady rain, unplug the pump, let the pit fill, and time how long the water takes to rise one inch. An 18-inch pit holds about 1.1 gallons per inch, so a one-inch rise in two minutes is roughly 33 gallons per hour of inflow. That single measurement replaces every soil assumption on this page, because it measures the site instead of guessing at it.
Resist the urge to just go bigger. An oversized pump empties a small pit faster than water enters it and short cycles, wearing out the switch and motor far quicker than steady running would. Pair adequate capacity with a pit large enough for a sensible run time, and add a battery backup on its own float — severe storms bring peak inflow and power outages at the same moment. For discharge pipe sizing, the flow rate calculator checks velocity and friction on the run you are actually piping.
Frequently Asked Questions
- What size sump pump do I need?
- Most residential basements are served by a 1/3 or 1/2 horsepower pump, but horsepower alone is the wrong specification. What matters is gallons per hour delivered at your actual total dynamic head — the vertical lift from the pit to the discharge point plus friction loss through the discharge run. A 1/2 hp pump that moves 3,000 gallons per hour at 10 feet of head delivers only about 1,700 at 20 feet, so a deep pit with a long horizontal run can need the next size up.
- What is total dynamic head on a sump pump?
- Total dynamic head is the sum of static lift and friction loss. Static lift is the vertical distance from the pump's on level in the pit to the point where the discharge exits above grade. Friction loss is the additional resistance from pipe length, elbows, check valve and discharge fittings, expressed in equivalent feet of head. Pump curves are published against total dynamic head, so sizing against static lift alone consistently overstates what the pump will actually deliver.
- How do you measure how fast water enters a sump pit?
- During steady rain, unplug the pump, let the pit fill, and time how long the water takes to rise one inch. Multiply the pit's gallons-per-inch by 60 and divide by that time in minutes to get inflow in gallons per hour. A standard 18-inch diameter pit holds roughly 1.1 gallons per inch of depth. This is the single most reliable input you can get and it beats any square-footage estimate, because it measures the actual site rather than a soil assumption.
- Should a sump pump have a battery backup?
- In any basement where flooding causes real damage, yes. Severe storms cause both the highest inflow and the highest chance of a power outage, which is precisely when the primary pump is needed most and least likely to run. A battery backup pump sized to at least the calculated inflow rate, on its own float set slightly above the primary, covers both power loss and primary pump failure. Check the backup's rated runtime at your actual head, not the headline number.
- Is a bigger sump pump always better?
- No. An oversized pump empties the pit faster than water enters it, which causes short cycling — rapid on-off starts that wear the motor and switch far faster than steady running. If the pit is small, an oversized pump can cycle every few seconds during a storm. The right answer is a pump that comfortably exceeds peak inflow at your actual head, paired with a pit large enough to give it a reasonable run time per cycle.
Quote, Finance, and Schedule Every Job in One Platform
SubcontractorHub is the platform plumbing contractors use to turn sizing and cost numbers like these into a branded proposal, financing the homeowner can accept on the spot, and a scheduled job — without re-entering anything. This tool is free to use; the platform is here if you want a closer look.
Book a Free DemoSizing output on this page is a planning estimate. Actual pump selection should confirm the specific manufacturer performance curve at the measured total dynamic head, verify discharge pipe size and check valve selection, and comply with local plumbing code on discharge location and backflow protection. All calculations are estimates based on historical information and should be verified by the user. This tool is provided as a free service for planning purposes only and is not a substitute for a professional assessment, a supplier quote, or applicable local code requirements.