What size pool pump do I need?

Choose a pump that delivers your required flow at your system's total dynamic head (TDH). Pool volume gives you a starting flow; the heater, salt cell, skimmers and water features can raise it. The filter and plumbing set limits that the pump must respect. Start with your saved volume, or use the pool volume calculator to measure it.

Gallons alone cannot identify the right horsepower. Horsepower describes power; a performance curve describes the flow a particular pump can deliver against resistance. Actual operation is where that curve meets the system curve, as the Hydraulic Institute explains. A box's maximum GPM is not a promise of installed flow.

What pump does a 15,000- or 20,000-gallon pool need?

For one turnover during a 8-hour run window, a 15,000-gallon pool needs 31.25 GPM. A 20,000-gallon pool needs 41.7 GPM under the same assumption. Those are starting flows, not horsepower recommendations. Add the equipment requirements above before shopping.

The engine multiplies volume by your chosen turnover target and divides by the available running minutes. The run window is your assumption, not a universal daily rule. For choosing daily hours, use the pump run-time calculator.

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Do I add the heater, salt cell and waterfall flows together?

Equipment in the same pipe shares the same flow. Our example heater requires 40 GPM and its downstream cell requires 25 GPM. Sending 40 GPM through that pipe satisfies both. These are illustrative inputs; enter your own manuals' requirements. Hayward's heater manual, Table 14, shows why the model matters.

Engine results for 15,000 US gallons, one turnover in 8 hours. Filter, heater and cell are upstream of the return split; both branches return to this pool. Skimming requirements are omitted.

A branch is different: reserving 20 GPM for ordinary returns while sending 30 GPM to a feature requires 50 GPM overall. Feature water still counts toward turnover. This follows conservation of flow; the calculator does not determine how your valves will divide it. Confirm branch flows in service. Separate feature pumps, heater bypasses and equipment on individual branches need their own hydraulic assessment.

How do I find total dynamic head for pump selection?

TDH includes losses through pipes, fittings, valves and equipment, plus applicable static lift. It must be stated at the flow you want and the valve arrangement you will use. Higher flow changes friction; an old reading at a different speed is not automatically your design head. These distinctions follow the Hydraulic Institute's system-curve method. The total dynamic head calculator does the work either way: from gauge readings on a running system, or added up from the plumbing and equipment at your target flow.

For an existing system, a technician can use pump suction and discharge measurements with the gauge elevations and pipe diameters accounted for. Filter pressure alone omits part of that measurement. For new plumbing, obtain a hydraulic estimate at the target flow: the pipe size calculator gives the friction loss of each pipe section and its fittings, to be added to the equipment losses at the same flow. A primed loop returning to the same pool does not acquire permanent static lift simply because its pad is higher; an open waterfall above the pool can add it.

Leave head blank until you have it. The resulting GPM is a preliminary requirement. With verified head, compare manufacturers' curves at the intended speed, then confirm installed flow and equipment limits.

Can a pool pump be too big for the filter?

Yes: its operating flow can exceed the filter rating. Enter the actual model's maximum flow, and any stricter heater, cell or plumbing limit. A blank limit means unverified. A connection diameter or filter pressure reading cannot certify the permitted flow of the whole installation. Backwashing may have a separate flow requirement to check against the filter manual.

Do weak returns mean I need a bigger pump?

Measure the shortage before replacing the pump. Debris, air leaks and valve settings can reduce flow. Jandy's pump troubleshooting manual checks restrictions and suction leaks before concluding that equipment needs replacement.

• If flow dropped recently, check baskets, filter condition and the current valve arrangement.
• If flow falls when a feature starts, measure how the return branches divide it.
• If clean-system flow remains below the calculated requirement, compare the pump curve with measured head.

Cloudy water alone does not establish a pump-size problem; the cloudy-water diagnostic guide covers chemistry and filtration checks.

Should I choose a variable-speed or above-ground pump?

Match the operating range and installation, then compare costs. Variable speed lets one pump serve routine circulation and higher-flow equipment periods; Pentair describes this use of adjustable speeds. Check each operating mode and priming program against the equipment limits. Use our variable-speed savings and electricity cost calculators for the budget.

For an above-ground replacement, the same flow calculation applies, but the pool/filter manufacturer's permitted pump combinations, hose and fitting compatibility, voltage and required pump position also matter. Gallons do not override those installation requirements. Confirm them with the equipment manuals before treating any candidate as a match.