Sizing a pump: the number on the box is not the number you use

The two flow rates, and which one lies

Every cartridge pump in this category advertises a flow rate in gallons per hour, and the larger, more prominent number is the pump flow — what the motor moves with nothing in its way. The number you actually need is the system flow rate, which is what comes out once the water has been pushed through the filter. Intex publishes both for the Krystal Clear range: the C530 is a 530 GPH pump with a 475 GPH system flow, and the C1000 is a 1,000 GPH pump with a 750 GPH system flow.

That gap is not a rounding error. On the C1000 it is a quarter of the advertised capability, and sizing off the headline figure means believing your water turns over faster than it does. When only one number is published, assume it is the optimistic one and treat the pump as smaller than advertised.

Why oversizing is not free

The instinct when in doubt is to buy the bigger pump. It is the wrong instinct here for a reason that is purely financial: a pump runs for hours every day of the season, and the electricity is proportional to its draw. Buying more pump than the pool needs means paying for capability you cannot use, every day, for the life of the pump.

The numbers are in our pump electricity guide: at the EIA national average of 18.34 cents per kWh, every 100 W of pump run 8 hours a day for a 120-day season costs about $17.61. A pump that draws 200 W more than it needed to is roughly $35 a season of nothing.

Undersizing is worse in a different way — the water genuinely never turns over, the filter never gets a fair chance, and you spend the summer chasing cloudy water with chemicals. That is why the arithmetic matters in both directions rather than only one.

The specification nobody publishes

Here is the frustrating part. You can size a pump from published information, but you cannot cost one, because Intex and Bestway publish flow rates and do not publish wattage. Flow tells you about turnover; watts tell you about your bill. The label on the unit or the motor nameplate is the only reliable source, which means the running cost of a pump is effectively unknowable until it is in your hands.

Treat that absence as information when you shop. Our pump roundup flags which manufacturers publish what, and where a figure is missing we say so rather than filling the gap.

Filter type changes the sizing conversation

A sand filter and a cartridge filter of nominally similar capacity behave differently over a season. A cartridge loads up progressively and the effective system flow falls as it does, which is why a pump that seemed adequate in June struggles in August. A sand filter is backwashed rather than replaced, so its flow recovers each time.

That means sizing a cartridge system with no margin is optimistic: you are sizing for a clean cartridge you will not have for most of the season. Our sand versus cartridge comparison covers the trade-off, and our cartridge guide covers the replacement interval that keeps the flow honest.

Matching the fittings, not just the numbers

One practical trap. Above-ground pool inlets and outlets are not standardized across brands, and a correctly sized pump with the wrong hose fitting is a return. Check the hose diameter and the fitting type against your pool before ordering, particularly if you are replacing a pump on a pool from a different manufacturer.

And check the electrical side. Pool pumps must be on GFCI protection, and any timer you add has to be rated for the pump’s amperage and for outdoor use without defeating that protection. Our run-time guide covers fitting the timer safely.