Guide to Sizing your Filters

Beyond the decision as to which filter to choose,the most important consideration is that the filter be appropriately sized.
The size of your filter is based on the desired flow rate. This is defined as the as the amount of time that is required to move the pool or spa's water volume once through the system.
Obviously, this does not man that every drop of water will reach the filter; but rather, if we have 20,000-gallon pool, 20,000 gallons will experience the filter in a given amount of time.
Different areas of the country have different recommendations and/or regulations as to what the standard  quantity of time should be : Hourly intervals 12,8,and 6 are common.
Once the turnover rate is decided, the desired flow rate can be calculates. This calculation is done in gallons per minute (gmp) in relation to the volume of water that is in the pool Specifically the formula is



VOLUME (GAL) / TURNOVER RATE(HRS) X 60 (MIN) = DESIRED FLOW RATE (GPM)


For example, if we have 30,000 gallon pool and we want to be filtered once every 8 hours, then our desired flow rate would be 63 gallons per minute, or:

                               30,000 (GAL) / 8 (HRS) x 60 (MIN)= 63 (GPM)

From this information, we can now determine the appropriate-size filter to accomplish the goal of 63 gallons passing the filter per minute. One more piece of information will be necessary, and that is the rate at which water can be filtered through on- square foot of the filer medium. This is called the filter rate and varies from filter medium because of the intrinsic properties of the composition of the filter. Filter rates for common filters media are shown in the table below.
To determine the appropriate-sized filer, measured as the filter area, in square feet (ft2) we use the following formula:

   DESIRED FLOW RATE (GPM)/ FILTER RATE(GPM)FT2=  FILTER AREA

So for the given examples of a 30,000 gallon pool with a flow rate of 63 gallons per minute, if we choose a D.E filter with a filter rate of 2 fpm, we will need 32 square feet.


D.E. FILTER (RATE OF 2 GPM) 63 (GPM) / 2 (GPM) FT2 = 32 FT2

A cartridge filter of 1 fpm will require 63 square feet.

CARTRIDGE FILTER (RATE OF 1 GPM) 63 (GPM) / 1 (GPM) FT2 = 63 FT2

A sand filter 0f 20 fpm will need 3- square feet

SAND FILTER (RATE OF 20 GPM) 63(GPM)/ 20 (GPM) FT2 =3 FT2

Service professionals are always  advised to choose a filter somewhat larger than is needed as this tens to give longer filer cycles. Larger filters will also mean less manual labor in servicing the filter. With proper chemical balance, following- the simple formulas provided here could very well earn you a crystal clear pool.

Take from July issue of Service Industry News