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 pools, different sizes



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 up front. The main point of this post is 'Go Big'.



Choosing a larger pool filter makes sense, and 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. Makes sense, the more sq. ft. of filter media you have, the better the filter performance. But it gets tricky when you try to compare the 3 types of pool filters to each other.

That's because they all trap dirt and debris in different ways - they're like apples & oranges!

But don't let this confuse you. Within each filter type, there are typically 4 or 5 sizes.

Of these sizes, the smallest are suitable for aboveground pools, the mid-range for small pools of 15,000 gallons and the big filters, best for your pools above 25,000 gallons.

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

This snip 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 inch tank diameter, which is a 4.9 sq. ft. filter surface area, and holds 600 lbs of sand. For small pools (under 20,000 gals), I might advise the next size down (24 inch 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 gals of pool water, as a minimum. I like to see the 400 sq. ft. models on inground pools above 25,000 gals. These can go an entire season without cleaning in most cases.

D.E. Filters

Because they offer the most superior type of filtration, people tend to go small with DE, and buy a 24 or 36 sq. ft. DE filter. Ugh - go Big! Nothing below a 48 sq. ft. filter, and if your pool is over 25,000 gals, go with the Big Boy - the 60 sq. ft. filter.

For the average in ground pool, if money is available, buy a big DE filter and be done with it. If your budget is squeezed thin this year, buy the 30 inch 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 make for all sorts of trouble. Dirt can be pushed through the filter, internals can become crushed or cracked, and high pressure makes for slow, inefficient and a potentially hazardous situation.

Filter Size to Pool Size

Another rule of thumb I have for pool filter size is to look at the pool size, in gallons. If you've never figured it out before, take some measurements. The formula is L x W x Avg. 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!

 You can find dozens of swimming pool filters at



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