How to Determine What Sizes of Cartridge Filters Fit Which Swimming Pools
- 1). Calculate your pool water capacity in gallons. You may already know this number, but if you don't, calculate it before you buy a filter. You need to know the shape, length, width as well as the depth of your pool. The formula is Length x Width x Average Depth x Multiplier = Gallons. The Multiplier is 7.5 if you have a square, rectangle or free-form pool. For oval and round pools the multiplier is 5.9. For example a 12-by-18, 4-foot-deep oval pool would be calculated as follows: 12 x 18 x 4 x 5.9. Average depth for a pool that gradually gets deeper is calculated by adding the shallowest and deepest depths and then dividing by two.
- 2). Calculate the flow rate for your pool. The flow rate is measured in gallons per minute (GPM). Divide the number of pool gallons by the desired turnover rate. Most pool manufacturers use 8 hours. For example divide 8,000 gallons by the turnover rate of 8 hours. This gives you a 1,000-gallon flow rate. Take your flow rate number with you when shopping for filters.
- 3). Calculate the filter flow rate of the desired cartridge filter model you would like to purchase. Obtain a cartridge filter with a filter flow rate slightly higher than your needs to accommodate the effect of dirt and debris on the filter's performance. Multiply the specific filter's design flow rate by the effective filtration area as listed on the product information tag or web page. This number should exceed the flow rate you previously calculated. If the number does, that means the filter is suitable for your pool.
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