Swimming Pool Blog

How to Size a Pool Filter

How to Size a Pool Filter
by Rob Cox June 7, 2013

Picking the Best Filter Size for Your Pool

sand filters for swimming pools



I've never understood why people buy the biggest pump they can afford, and then go small on the pool filter. I call it the "V-8 mentality." They want the biggest engine.

To save you time in your valuable day, I'll give you the moral of the story upfront. The main point of this post is to "GO BIG."




Choosing a larger pool filter makes sense, and it has many benefits.

  • Longer filter cycles, or time between backwashing
  • Less bypass of fine particles
  • Greater filtration of the water
  • Faster clean-up of water issues
  • Longer lasting filter media

Sizes of Residential Pool Filters

Filters are commonly sized by the square footage of surface area. It makes sense: the more square feet of filter media you have, the better the filter performance will be. But it gets tricky when you try to compare the three different types of pool filters to each other.

how to choose a swimming pool filterThat's because they all trap dirt and debris in different ways - they're like apples & oranges!

Don't let this confuse you, though. Within each filter type, there are typically four or five sizes.

Of these sizes, the smallest are suitable for above ground pools, the mid-range ones can be used for small pools around 15,000 gallons, and the big filters work best for pools above 25,000 gallons.

To add to the confusion are the manufacturer's listed turnover rates for their pool filters. These are usually listed on product information pages, and they show the maximum flow rates for the filters.

This snippet below, from our Hayward Star-Clear filter page, seems to suggest that a 25 sq. ft. filter can take care of a 12,000 gallon pool, and I can tell you that is not the case. Not even close.

Sand Filters

For sand filters on inground pools, I like to use a 30" tank diameter, which has a 4.9 sq. ft. filter surface area and holds 600 lbs of sand. For small pools (under 20,000 gallons), I might advise the next size down (24" tank) if the budget is tight. But never go smaller than a this for an inground pool.

Cartridge Filters

I have a rule of thumb for cartridge pool filters: 100 sq. ft. of filter surface area per 10,000 gallons of pool water, as a minimum. I like to see the 400 sq. ft. models on inground pools above 25,000 gallons. These can go an entire season without cleaning in most cases.

D.E. Filters

Because diatomaceous earth filters offer the most superior type of filtration, people tend to go small and buy a 24 or 36 sq. ft. D.E. filter. Ugh...no, GO BIG! Don't use anything below a 48 sq. ft. filter, and if your pool is over 25,000 gallons, go with the big boy (the 60 sq. ft. filter).

For the average inground pool, if money is available, buy a big D.E. filter and be done with it. If your budget is squeezed thin this year, buy the 30" sand filter.

Pump Size to Filter Size

With a big pool filter, there's no worry about the pump being too large for the filter. Also, don't worry if your pump seems undersized for a larger pool filter - this is rarely the case. When the pump is too large however, it can mean all sorts of trouble. Dirt can be pushed through the filter, internals can become crushed or cracked, and high pressure makes for a slow, inefficient and potentially hazardous situation.

Filter Size to Pool Size

Another rule of thumb I have for pool filter size is to look at the swimming pool size in gallons. If you've never calculated this before, take some measurements. The formula is Length x Width x Average Depth x 7.5, for rectangular pools. For oval pools, change the multiplier from 7.5 to 5.9.

Here's some pool filter size recommendations by type. Match your pool size in gallons (or less) with the filter model number. And, don't be afraid to go one size bigger!

compare pool filters

 You can find dozens of swimming pool filters at POOLCENTER.com