Pool pumps
Pump sizing, without the guesswork
Pump sizing has two limits, not one. Too little flow and the water never turns over; too much and you blow past your filter's rated flow, wreck filtration, and burn power. The right pump lands between the floor your pool sets and the ceiling your filter sets.
Flow calculator
Find the flow your pool needs. That number is the floor your pump has to clear.
This is the flow floor. The ceiling is your filter's maximum rated flow, which you read off the filter, not calculate. Your pump should sit at or just above the floor and never above the ceiling.
Two numbers decide the pump
The floor is the flow your pool needs to turn over, straight from the calculator above. The ceiling is your filter's maximum rated flow, printed on the filter or in its manual. A sand filter, for instance, has a design flow its media bed is built for; push more than that and you channel water through the sand instead of filtering it. The correct pump delivers at least the floor and never more than the ceiling.
The oversizing trap
Bigger is the instinct, and it is usually wrong. An oversized pump forced through a filter rated for less runs at high pressure, pushes water through the media too fast to filter it well, strains the filter, and draws more power for worse results. It also raises the odds of pulling air on the suction side. More horsepower is not more clarity; matched flow is.
Single speed or variable speed
A single-speed pump runs at one flow, so you size it to hit your turnover in a set number of hours. It is cheaper to buy and the simplest to service. A variable-speed pump changes the calculation: because power draw rises steeply with speed, running slower for longer moves the same water for a fraction of the energy. Over a season that difference often pays back the higher purchase price, and many regions now require variable speed on new installs. Size it to reach turnover comfortably at a low, quiet speed rather than at full tilt.
Plumbing sets the real flow
A pump's rating assumes ideal conditions. Long pipe runs, narrow fittings, and a dirty filter all add resistance and cut the flow you actually get. If your calculated need sits near a pump's rated limit, size up a step so real-world flow still clears the floor once the plumbing takes its cut.
The sizing sequence
- Get your flow floor from the calculator.
- Read your filter's maximum rated flow. That is your ceiling.
- Pick a pump that delivers between the two through your actual plumbing.
- Prefer variable speed, sized to reach turnover at a low, efficient speed.
Related topics
- How Long Should You Run Your Pool Pump?Calculate daily runtime from pool volume and actual flow rate, with guidance on single-speed vs variable-speed, summer, cost, and when to adjust.
- Filter SizingMatch filter and pump to your pool volume and flow.
- Variable-Speed PumpsWhere the higher upfront price pays itself back.
- Single-Speed PumpsWhen the simple, cheaper pump still makes sense.
- Why Your Pool Pump Loses PrimeA pump that keeps losing prime is almost always pulling air somewhere on the suction side. A diagnostic sequence, the O-rings to check first, drain-back explained, and when the pump itself is actually the problem.