A good filtration system for a pool or spa depends on your ability
to properly size and select the filter.
To accomplish that, you must calculate the vessel’s volume,
capacity and flow rates, and some other key factors —
including the type of pump. After all, the filter fits into the
equipment sequence after the pump, so regardless of whether
you’re replacing an old filter or installing a filter on a
brand-new pool or spa, you’ll need to match the filter to the
pump and the size of the vessel.
1. FIGURE OUT THE VOLUME
The first step in finding the correct filter model is to determine
how much water has to be filtered. Here are some simple formulas
and techniques to use when calculating the volume of a swimming
- Rectangular pool: length x width x average depth = pool
- Circular pool: radius x radius x 3.14 (pi) x average depth
= pool volume
- Oval pool: 1/2length x 1/2width x 3.14 (pi) x average depth
= pool volume
- Irregular shapes: For pools that are not truly rectangular,
circular or oval, calculate the volume using the following
Make a scale drawing of the pool on a piece of square-grid graph
paper, with each square representing one square foot. (Note: The
drawing can be made to any scale you want as long as you keep it
Now count the number of squares. Don’t forget to estimate how
many complete squares all of the partially filled squares would
amount to. That gives you a close estimate of the pool’s area
in square feet.
Multiply the area by the approximate average depth of the pool, and
you will find the volume in cubic feet.
2. CALCULATE THE CAPACITY
Capacity is defined as the number of gallons of water that the pool
will hold (as opposed to volume, which is a spatial
Capacity formula: Pool volume x 7.48 = pool capacity in
Example: Say you have a rectangular pool that is 40 feet long and
20 feet wide, with an average depth of 5 feet. First you’d
need to figure out the pool volume, so simply plug these numbers
into the volume equation and multiply: 40 x 20 x 5 = 4,000 cubic
feet (pool volume).
Now plug the volume into the capacity equation and multiply: 4,000
x 7.48 = 29,920 gallons (the pool’s capacity).
3. FIND THE FLOW RATE
The flow rate is the volume of water flowing past a given point
during a specific period of time, measured in gallons per minute
(gpm) or gallons per hour (gph).
Flow rate formula: Capacity ÷ turnover time (in hours) = flow rate per
hour Flow rate per hour ÷ 60 = flow rate per minute
Example: Say you want to set up the pool to have an 8-hour turnover
rate. The equation to find the flow rate for a 29,920-gallon pool
(regardless of its shape) is: 29,920 ÷ 8 = 3,740 gph.
To calculate the flow rate per minute in this case: 3,740 ÷ 60
= 62.3. That’s the rate at which you’d want the filter
Stating this situation another way, the calculations for your
29,920-gallon pool show that you require a flow rate of 62.3 gpm to
filter the pool’s capacity in eight hours. Therefore, your
goal is to determine which model of filter can handle 62.3 gallons
of water per minute — resulting in the desired complete
turnover every eight hours.
4. DETERMINE THE FILTER FLOW RATE
Next, determine if the filter you want to use can handle the flow
rate you require. The filter flow rate is defined as the amount of
water filtered over a given period of time, expressed in gallons
Filter flow rate formula: Filter area x filter rate = filter flow
rate in gpm.
The “filter area” is the surface area of the filter
medium. It is measured in square feet. The “filter
rate” is the number of gallons of water that flows through
one square foot of effective filter medium per minute during the
operation of the circulation system.
Example: Say you have a filter area of 5 square feet and a filter
rate of 12.5 gpm. (These numbers are based on a model of sand
filter.) To calculate the filter flow rate: 5 x 12.5 = 62.5 gpm.
That’s awfully close to the desired flow rate of 62.3 in the
example of Step 3.
Tip: If determining the filter flow rate seems a bit daunting, just
get both figures — the filter area and rate — from the
manufacturer. With their figures, you’ll be better able to
select the correct filter to suit your needs.
5. MAKE YOUR FILTER CHOICE
Do you prefer sand, diatomaceous earth or cartridge filters?
Pressure or vacuum filters? If you have narrowed your filter choice
down to a preferred manufacturer, it will make it even easier to
perform your calculations and select a filter that will do the
The example we’ve been using refers to characteristics of a
sand filter. It’s important to note that both DE and
cartridge filters have notably lower filter rates. (Typically,
high-rate sand models filter water at a rate of 20 gpm per square
foot, DE models work at 2 gpm/sq.ft. and cartridge units at 1
gpm/sq.ft.) Nevertheless, the mathematical relationships between
filter area, filter rate and filter flow rate will remain the
6. OVERSIZE THE FILTER
That’s right. When matched with the proper pump, you can and
should oversize the filter. Why? Because a larger filter area will
lower the flow rate per square foot of media, enabling the filter
to capture even more debris and thus increase the filter’s
Filters that are larger than indicated by our calculations of flow
requirements are particularly necessary for pools that get heavy
use (such as commercial pools) and for backwashing (because the
filter requires additional size for backwashing).
How oversizing works: Referring back to our earlier example, you
could either select a model with a larger filter area or a model
with an increased filter rate. To allow for debris buildup and
backwashing, you might select a model with a filter area of 5
square feet and a filter rate of 20 gpm. That would yield a filter
flow rate of 100 gpm — well beyond the desired rate of 62.3.
Alternatively, you could choose a filter with 4 square feet of
filter area and a filter rate of 25 gpm per square foot.
Note: If you size your sand or DE filter too large, there
won’t be enough pressure to backwash it clean. So, be sure to
maintain the necessary backwash flow.
7. LIMIT THE FILTER RATE AND ADJUST THE FILTER AREA
In many cities and counties, filter rates on pools are regulated.
NSF International sets maximum filter rates to ensure effective
filtration because the faster the water passes through the medium,
the less effectively it is cleaned.
Filter rate ceilings are imposed most often for commercial
facilities. If this is the case where you live, you may have to
compensate by selecting a model with a larger filter area. By doing
so, you can achieve the same flow rate without exceeding the
maximum filter rate.
8. SELECT THE CORRECT FILTER
Taking all the above calculations and factors into consideration,
you’re now ready to select the proper filter for your