Regardless of the type of filter in use on a specific pool or spa, it is essential that it be of proper size and used for an adequate duration to provide clean water. A properly sized filter is essential for adequate water filtration. But knowing how to select the proper filter requires doing a little math. To accomplish this, it is necessary to know the pool’s water volume, the desired flow rate, and the desired filter type that can be matched to the other parameters.

The first step in finding the proper filter is to calculate the pool’s volume. For some pools, this is fairly straight-forward elementary school math. But for irregularly shaped pools, the calculations can be a bit more daunting. The following are more simple formulas for volume calculation.

Rectangular pools : length x width x average  depth: L x W x ( D1 + D2 /2)

Oval: Radius x Radius x Pi x Average   depth: R1 x R2 x (D1+D2/2)

Circle: Radius x Radius x Pi x Average  depth: R1 x R2 x (D1 + D2 /2)

Free Form Pool: Using square grid graph paper, make a drawing of the pool, letting each square represent one square foot. Count the squares to get the surface area. Then multiply by the average depth. After the pool or spa’s volume has been calculated, it must be converted to gallons of water, using the constant of 7.5 gallons in a cubic foot.

V x 7.5 = gallons of water

For example, consider that we have a standard 40-foot-by-20-foot rectangular residential pool with a constant slope where the shallow end is 3 feet and the deep end is 6 feet. The volume of the pool is calculated: 40 x 20 x ( 6+3 /2) = 3,600 feet to the 3rd power.

3,600 ft3 x 7.5 gal/ft3 = 27,000 gal.

Once we know the number of gallons of water in the pool. we can decide on a good turnover rate. In most jurisdictions, public pools require four turnovers a day. Continuing from the preceding example that was applied to a residential pool, we need 2 to 3 turnovers a day. The turnover rate is calculated as the volume of water in gallons multiplied by the number of required turnovers per day.

Turnover Rate = Volume x # required  turnovers/day

For a 27,000 gallon pool with 3 required turnovers per day, we need a turnover rate of 81,000 gallons per day. Next, we must calculate the design flow rate. This is the flow rate expected from the pump given the desired turnover rate. The flow rate is monitored by the flow meter and is usually measured in gallons per minute. Pool operators should ensure the the observed flow rate meets the turnover requirements.

To calculate the flow rate, simply convert the turnover rate to gallons per minute. Flow rate = Turnover rate / (( 24 hour/day) x ( 60 min/hour))

Continuing the preceding example, with a 27,000-gallon pool and 3 desired turnovers per day resulting in a turnover rate of 81,000 gallons per day, requires a flow rate of 56.25 gallons per minute. Flow rate = Volume (gallons) / Turnover time (min)

Continuing the example, for a 27-000 gallon pool and a desired 8-hour turnover time, the flow rate is 27,000 gallons divided by 8 hours  times 60 minutes in an hour.

The pool’s recirculation system should operate at 56 GPM. This is what the flow meter should read to meet the requirements of 3 turnovers per day. This is the flow rate at which the filter needs to operate. So now the goal is to find which model of filter can handle 56 GPM resulting in the desired 3 turnovers, or 8-hour turnover time. Filters are rated by their area and their filter rate. Filters are rated by their filter area and their filter rate. The filter area is the area of the filter media through which water flows, The filter rate is the rate at which water will flow through one square foot of the media. All types of filter media have a maximum quantity of water that it can effectively filter per area of this filter. Each type has an optimum flow rate where it performs best and provides the greatest filter performance. Manufacturers have developed specifications for the amount of water that should flow through a specific filter.  https://coronabrooksidepoolservice.com/category/blog/