What size pool pump do I need?
You need a pump that can move your entire pool volume in 8 hours or less (one full turnover). Divide your pool volume in gallons by 480 to get minimum required GPM. For an 18,000-gallon pool: 18,000 ÷ 480 = 37.5 GPM. A 1½ HP variable-speed pump typically handles pools up to 25,000 gallons with standard 1½" plumbing.
How do I calculate the GPM my pool pump needs?
Required GPM = Pool volume (gallons) ÷ (Turnover hours × 60). For 8-hour turnover: GPM = Volume ÷ 480. A 20,000-gallon pool needs at least 41.7 GPM. Your pump must deliver this flow against your system's total dynamic head (TDH): the resistance from pipe friction, filter pressure, and elevation.
What is total dynamic head (TDH) and why does it matter?
TDH is the total resistance the pump must overcome, measured in feet of head. It includes static head (vertical lift, typically 3–8 ft), filter resistance (15–25 ft depending on type), pipe friction (10–30 ft depending on diameter), and fittings. A pump rated at 60 GPM open-flow may only deliver 40 GPM at your actual system TDH. Always size for your TDH, not rated open-flow GPM.
Should I get a variable-speed pump?
Yes. U.S. federal law (DOE 10 CFR Part 431, effective July 19, 2021) requires all replacement residential pool pumps rated at 1 HP or greater to be variable-speed. VS pumps also use 50–80% less electricity than single-speed pumps and qualify for utility rebates in many states.
Is 1.5 HP too big for my pool?
With a variable-speed pump, oversizing is less critical than with single-speed pumps. A VS pump runs at lower speeds to deliver less GPM, saving electricity. The key risk is excessive flow velocity in smaller pipes (above 6 ft/sec in suction lines), which can damage equipment and create dangerous vacuum. Verify your GPM requirement with this calculator before purchasing.