Pool Pump Sizing Calculator

Sizing a pool pump is really two separate calculations chained together: how much water needs to move, and how hard the pump has to push to move it. This calculator starts with turnover — your pool volume divided by the number of hours you want to fully circulate that water (industry standard is 6-8 hours) — to get a required flow rate in gallons per minute. That GPM figure alone doesn't tell you what pump to buy, because the same flow rate through a narrow pipe demands far more pump power than through a wide one. So the second half runs a simplified version of the Hazen-Williams equation — the standard empirical friction-loss formula used throughout hydraulics engineering, here assuming PVC pipe with its accepted C-factor of 150 — using your required GPM, pipe diameter, and total run length to estimate how many feet of "head" the plumbing itself eats up.

That friction loss is added to your static head (the elevation change plus equipment resistance) to get Total Dynamic Head — the real number a pump has to overcome. From there, horsepower falls out of the standard hydraulic formula HP = (GPM × TDH) / (3960 × efficiency), assuming a typical 60% pump efficiency, and the annual energy estimate assumes 10 hours of daily runtime at $0.13/kWh. Use the GPM, TDH, and HP figures here to narrow your pump shopping list, not as a substitute for a manufacturer's pump curve: friction loss compounds fast with undersized pipe (note the exponent on GPM is nearly 1.85), so doubling flow through the same pipe roughly triples the friction loss. If your actual electric rate or typical runtime differs from the assumptions baked in, the energy cost will scale linearly with whichever number you correct.