Sizing a pool pump starts with turnover, not horsepower. You want to circulate the whole pool through the filter in a reasonable time — typically 8 hours — so the required flow is simply the pool volume divided by the turnover time. From there the plumbing size, run length, and filter set the head, and those together point to a pump. Bigger is not better: the most common mistake is a pump that pushes more water than the pipes can safely carry.
Worked example
A 32 × 16 ft rectangular pool averaging 5 ft deep holds about 19,000 gallons. For an 8-hour turnover that is roughly 40 gpm. Through 2-inch plumbing with a sand filter, the system head is around 25 ft, which needs only about a half to three-quarter true horsepower — far less than the “1 HP” or “1.5 HP” labels many pumps carry.
The over-sizing trap — and the guardrail
Pool pumps are usually sold by an inflated “uprated” horsepower, which tempts buyers into pumps far too big for their plumbing. Pushing too much water through undersized pipe drives the velocity up, which wastes energy, strains the plumbing, and can collapse a suction line or cavitate the pump. As a rule of thumb, a 1.5-inch line should not carry much past 40–45 gpm, a 2-inch line past about 80 gpm, and a 2.5-inch line past about 110 gpm. This calculator flags when your required flow exceeds the safe maximum for your plumbing so you can slow down the turnover or upsize the pipe instead of buying a bigger pump.
Single-speed vs variable-speed
Because a pool loop lifts almost nothing (it draws from the pool and returns to the pool), it is nearly all friction and very little static head — exactly the case where a variable-speed pump shines. Running a variable-speed pump at half speed for twice as long achieves the same turnover using roughly a quarter of the energy, often saving well over half the annual pumping cost. US Department of Energy rules for dedicated-purpose pool pumps now effectively require variable-speed on most pumps of about one horsepower and up, so for anything but the smallest pool, variable-speed is both the cheaper-to-run and the compliant choice.