How to Calculate a Pool Filter Size?

Among the many pool calculations you’ll have to deal with when you do DIY pool care and maintenance; there is pool filter sizing. It might be that you’re buying a new pool filter, or you’re replacing your worn-out model. Whichever the reason, you need to know how to size it correctly for your swimming pool to get the best filtration. I will take you through the simple math involved to make the whole thing as simple as it can be.

1) How Much Water Does Your Pool Hold

The first step in sizing your pool filter is knowing its volume, which will help you determine the minimum flow rate and the turnover.

I have made a pool volume calculator that can help you with these calculations. But if you’re a hands-on kind of guy, you might be interested in knowing how to do the calculation yourself. Here are the formulas:

Constants used

Rectangular or Square Pool

There are two types of rectangular or square swimming pools:

a) With uniform depth

Here you need to measure the longest side (length), the shortest side (width), and how deep the pool is (depth).

Once you have all these measures, use the following formula:

Volume = length x width x depth

b) With two depths

Unlike most above-ground pools build with uniform depths, your swimming pool might have shallow and deep ends.

For such pools, you need to get the average depth by adding the two depths (shallow and deep) and divide them by 2.

Shallow end depth + deep end depth / 2 = average depth

Now with the average depth, things get easier.

Volume = length x width x average depth x 7.5

Circular Pool

If your swimming pool has a circular or cylindrical shape, you need to get its diameter (distance between two points through the center) and divide it by two. What you get is the radius.

Measure the depth of your pool in feet. Once you have the radius (r) and depth, you can use the following formula:

Volume = pi x r2 x depth
So
Volume = 3.142 x r2 x depth

Irregular or Kidney Shaped Pool

If your swimming pool has an irregular/kidney shape or combines multiple shapes, you have to divide it into regular shapes and calculate the volumes of these smaller pools. When you add all that up, you get the total volume of your swimming pool.

2) What’s the Minimum Flow Rate Required

Now that you know the amount of water your swimming pool can hold, you can now move to the next step of calculating the minimum flow rate required from your pool filter.

You will, however, start by determining the turnover rate, which is the number of cycles required to pass the water through the filter within a day (6-, 8-, 12-, or 24-hour cycles).

The turnover rate might also be set by your local swimming pool laws, with most cities requiring a swimming pool filter to complete a filtration cycle at least twice a day. Most people go for two 6-hour cycles or one 12-hour cycle.

Divide the pool volume in gallons with the turnover rate in minutes to get the minimum flow rate in gallons per minute. If you divide the pool capacity with the turnover rate in hours, you get a minimum flow rate in gallons per hour.

Min Flow Rate (GPM) = Pool Volume (Gallons) / Turnover rate (Minutes)
Min Flow Rate (GPH) = Pool Volume (Gallons) / Turnover rate (Hours)

Assuming your swimming pool is 24ft long and 15ft wide and a depth of 10.5ft, its volume would be:

24ft x 15ft x 10.5ft x 7.5 = 28,350 Gallons

The required minimum flow rate for 2 (6-hour) cycles is:

28,350 Gallons / (6 hours x 60) minutes = 78.75 GPM

Your swimming pool filter must supply 78.75 gallons per minute to meet the two 6-hour turnover cycles per day.

3) What is the Maximum Flow Rate Required

Now that you have the flow rate required from the pool filter, you need to determine the maximum flow rate your pool circulation requires.

You might over-size your swimming pool filter, which is not bad, but it might cost you extra that you could have avoided.

The place where you need to be more concerned with the maximum flow rate is buying a pool pump. Oversizing it could blow your plumbing out or even cause damages to your pool equipment.

However, matching the pool pump’s flow rate can help match the performance of the two. If your pool pump flow rate is 20GPM, your pool filter must have a higher flow rate than this.

Why? Your pool pump is responsible for pushing water through your pool filter, the plumbing system, and other pool equipment.

If the pool filter flow rate is lower than your pump’s, pressure will build upon the pump side and burst the pipes or even damage the unit.

4) Does Pool Filter Size Matter?

Matching the pool filter size with the pool pump size is crucial. Pool filters are available in surface area and pool pumps in horsepower. It’d be best to size the two correctly. The table below gives you estimates of how to size your swimming pool filter and pumps.

Optimal Filter Sizes to Fit Various Pump Sizes

Filter Size estimate

As you well know, pool filter sizes are estimated in gallons per minute, the minimum flow rate you calculated before.

And as I mentioned, your pool filter flow rate must be at least the same as your pump’s GPM or higher so that it can handle the pump’s power.

The rule of thumb that can help you here is one square foot per 10,000-gallon swimming pool.

Optimal Filter Sizes For Pool Sizes
(Gallons)
Pump Capacity (Gallons) Pump Size Rate(GPM) Intake lines (Minimum in Inches) Recommended Filter Size (Square Foot)
10000 0.75 1.0-1.5 Sand – 3.0
cartridge – 100 – 200
DE – 36
15000 1.00 1.0-2.0
20000 1.50 1.5-2.0 Sand – 3.9
cartridge – 300 – 400
DE – 48
25000 1.50 1.5-2.0
30000 2.00 1.5-2.0 Sand – 4.9
cartridge – 400 – 500
DE – 60
35000 2.00 1.5-2.0
40000 2.50 2.0-3.0 Sand – 6.9
cartridge – 500+
DE – 72
45000 2.50 2.0-3.0