Sizing a Pump: GPH, Head Pressure & HP
Updated February 24, 2026
Pump performance is measured in GPH (gallons per hour) or GPM (gallons per minute) at a specific head pressure (vertical lift). A pump rated at 3,000 GPH at 0 feet of head may only deliver 1,500 GPH at 10 feet. Understanding this relationship is essential for choosing the right pump.
Overview
Pump performance is measured in GPH (gallons per hour) or GPM (gallons per minute) at a specific head pressure (vertical lift). A pump rated at 3,000 GPH at 0 feet of head may only deliver 1,500 GPH at 10 feet. Understanding this relationship is essential for choosing the right pump.
What to Know
GPH/GPM at Head Pressure
- Every pump has a performance curve: flow rate decreases as head pressure (vertical lift) increases.
- The nameplate rating is usually at 0 feet.
- What matters is the flow rate AT YOUR specific head pressure.
- Measure the vertical distance from the pit bottom to where the discharge exits the house.
HP Selection
- 1/3 HP: moderate groundwater, up to 8-10 feet of head.
- 1/2 HP: frequent heavy water or longer discharge runs.
- 3/4 HP and 1 HP: severe water tables, very long runs, or commercial applications.
- Always check the performance curve, not just HP.
Calculating Your Needs
- Measure pit inflow rate: mark the water level, time how long it takes to rise 1 inch, calculate gallons per hour.
- The pump must remove water faster than it enters.
- Add 50% safety margin.
- Then check pump performance curves at your specific head pressure to find the right model.
Buying Tips
- Always check the performance curve at YOUR head pressure, not the nameplate maximum.
- Size up if in doubt. An oversized pump runs shorter cycles. An undersized pump runs constantly and burns out.
- Include horizontal run in head calculation: roughly 1 foot of head per 10 feet of horizontal pipe.
- For well pumps, get a professional well yield test before sizing.
Common Mistakes
- Buying based on HP alone. A high-HP pump with a poor impeller design may underperform a lower-HP quality pump.
- Ignoring head pressure. A pump rated for 3,000 GPH delivers much less at 10-15 feet of lift.
- Undersizing. A pump that barely keeps up during moderate rain will fail during heavy storms.
- Forgetting to include friction loss from elbows and long horizontal runs.
Bottom Line
Measure your head pressure, calculate your inflow rate, then find a pump that delivers 50% more flow than needed at your specific head pressure. Size up, not down.
Frequently Asked Questions
What should I look for when buying sizing a pump: gph, head pressure & hp?
Always check the performance curve at YOUR head pressure, not the nameplate maximum. Size up if in doubt. An oversized pump runs shorter cycles. An undersized pump runs constantly and burns out. Include horizontal run in head calculation: roughly 1 foot of head per 10 feet of horizontal pipe.
What are common mistakes when buying sizing a pump: gph, head pressure & hp?
Buying based on HP alone. A high-HP pump with a poor impeller design may underperform a lower-HP quality pump. Ignoring head pressure. A pump rated for 3,000 GPH delivers much less at 10-15 feet of lift. Undersizing. A pump that barely keeps up during moderate rain will fail during heavy storms.
What is the bottom line on sizing a pump: gph, head pressure & hp?
Measure your head pressure, calculate your inflow rate, then find a pump that delivers 50% more flow than needed at your specific head pressure. Size up, not down.
What's the bottom line on sizing a pump: gph, head pressure & hp?
Measure your head pressure, calculate your inflow rate, then find a pump that delivers 50% more flow than needed at your specific head pressure. Size up, not down.
What are the top tips for sizing a pump: gph, head pressure & hp?
Always check the performance curve at YOUR head pressure, not the nameplate maximum.. Size up if in doubt. An oversized pump runs shorter cycles. An undersized pump runs constantly and burns out.. Include horizontal run in head calculation: roughly 1 foot of head per 10 feet of horizontal pipe..
What mistakes should I avoid when buying?
Common mistakes: Buying based on HP alone. A high-HP pump with a poor impeller design may underperform a lower-HP quality pump.. Ignoring head pressure. A pump rated for 3,000 GPH delivers much less at 10-15 feet of lift.. Undersizing. A pump that barely keeps up during moderate rain will fail during heavy storms..
What are the best pumps brands?
Top brands include Armstrong, Bell & Gossett, Flotec, Franklin Electric, Goulds, Grundfos. Each serves different price points and needs.