Sizing a pump by water volume alone is how people end up with a pump that's too strong (wrecks seals, wastes power) or too weak (green pool). This tool estimates your Total Dynamic Head — the real resistance of your plumbing — and gives a flow-and-head target plus an honest HP range.
Total Dynamic Head (TDH) is the total resistance a pump must work against, measured in feet of head. It is the friction of water moving through your pipes and fittings plus the resistance of the filter, heater, and chlorinator — and that equipment resistance is often the single largest part, which is exactly what volume-only calculators miss. You size a pump by finding the flow you need (enough to turn the pool over in your target time) and the head that flow creates, then choosing a pump whose performance curve delivers that flow at that head.
This TDH figure is an estimate. Friction-loss formulas are empirical, the equipment-loss allowance is a typical value you can adjust, and a real system has details no calculator sees — so treat the number as a sizing guide, not a spec. That is also why the tool gives an HP range rather than a single horsepower: two pumps of the same horsepower can have very different performance curves, so the right way to choose is to match a pump's published curve to your flow-at-head target, not to buy a nameplate number. Watch the pipe-velocity warning too: if your pipe is too small for the flow your turnover needs, the fix is larger pipe, not a bigger pump — pushing water too fast (above about five feet per second) causes noise, wear, and hydraulic problems no pump can solve. Finally, on regulations: since the U.S. Department of Energy's Dedicated-Purpose Pool Pump rule took effect on July 19, 2021, most new or replacement in-ground filter pumps above roughly 0.711 hydraulic horsepower (around 1.0–1.15 total horsepower) effectively must be variable-speed, because single-speed motors cannot meet the required efficiency score. Existing pumps can keep running; the rule applies to new sales. Rules change and vary — verify current requirements before buying. (Regulatory detail current as of early 2026.)
A 21,600-gallon pool, 8-hour turnover, 1½-inch pipe: required flow ≈ 45 GPM. Friction ≈ 20.6 ft plus 25 ft equipment head ≈ 45.6 ft TDH. Velocity ≈ 7.1 ft/s flags the 1½-inch pipe as undersized for that flow.