To ensure that your swimming pool stays clean, safe, and open, you need to ensure that your pump and filter are working properly.

The first step toward achieving that is using the right size of pump and filter for the pool. Though it’s not easy to determine the right size of the filter, after going through this post, you will be able to know what size filter is best for your pool.

To determine the right size of filter for your pool, you first need to know the size of your pool pump. This simply means that everything starts with the size of the pool pump.

**What size pump do you need?**

Due to fear, many pool owners buy larger pumps that are far more expensive. Doing so is a big mistake because it will increase your energy consumption compared to smaller pumps.

So, if your pool doesn’t require a large pump to function properly, you are just wasting money. The best approach is to find the right size pump for your pool needs. Then buy the appropriate size to help you reduce your energy bill.

Moreover, larger pumps can overpower the filter if the filter system has a small capacity.

On the other hand, if the pump is smaller than what is required, it may not be able to pump and power the filter system. This can cause algae growth and a build-up of contaminants.

**What size pump do you need?**

Choosing the right sized pump is one thing, choosing the right size of filter is another. If the filter is too small, the pressure from the pump will be too much for it and can break down the filter. This will render the filter inefficient.

Moreover, the pressure buildup can affect the pump as well, leading to serious damage and possibly an explosion.

On average, your pool pump and filter should be able to cycle your pool water every 6-8 hours. With this in mind, you will be able to determine the right size filter your pool requires.

**Choosing the best size of filter for your swimming pool**

**Determine your pool volume:**

Before now, I believe that you already know the volume of your pool. Knowing the volume of water your pool holds is all you need to determine the size of the filter to buy. So, it’s crucial that you know your pool volume. Without it, it is almost impossible with the method we are using.

However, if you don’t know, you can use simple elementary calculations to determine your pool volume. As we know, there are four dominant pool shapes: round, rectangular, kidney, and oval.

The rectangular shape will be easy to calculate. But others can be calculated with some simple formulas.

Before we go further, you need to first find out the depth of the pool. Usually, pools have a sloppy bottom, which means that the bottom is not level. So, we need to find the average depth.

To do that, measure the depth of the deepest and shallowest ends. Add the two depths together and find the average.

For example, if the deepest part is 11 feet and the shallowest part is 3 feet, the average depth will be D = (11+3)/2 = 7 feet.

With that, we can go further to calculate the pool volume.

For rectangular pools:

Measure the length and width of the pool. Then find the pool volume by multiplying the figures.

Pool volume V = L x W x D

If your pool has a rectangular shape, divide the shape into two and find the volumes separate. Then sum the values to know the total volume.

For round pools:

Measure the radius of the pool surface and find the area of the pool surface. To find the radius, measure the diameter of the pool through the center of the pool. Then divide the value by 2. r = diameter/2.

Then calculate the surface area with the radius using the formula A = πr²

Where A is the surface area of the pool, π is 3.14, and r is the radius.

To calculate the volume, multiply the surface area by the average depth (D) of the pool.

Volume = A x D.

For oval pools:

Measure the shortest diameter and the longest diameter. Then multiply the figure with the average depth.

Volume = Shortest Diameter x Longest Diameter x Average Depth

For kidney pools:

Measure the shortest width and longest width, and add the values. Measure the pool length. Then calculate the volume by multiplying the figures with the average depth.

Volume = (shortest width x longest width) x Average Depth x Length of Pool

Now that you know the pool volume in cubic feet, you need to convert it to gallons. This is necessary because a gallon unit is the standard unit used by many manufacturers.

To convert cubic feet to gallons, all you need to do is multiply the volume you calculated above by 7.48.

However, if you already know your pool volume you don’t need to go through this calculation stress.

**Determine the flow rate:**

If you notice, most pool pump capacities are measured in GPM (gallon per minute). Since we have converted the pool volume to the gallon, it will no longer be difficult to calculate the GPM.

As we stated before, the average time for a complete filter cycle is 6-8 hours. Most residential pools use 8 hours. So we are going for 8 hours.

For us to get the actual value of GPM, we need to convert the hours to minutes. So we multiply 8 by 60 to get the actual time in minutes instead of hours.

Time = 8 x 60 = 480 minutes.

Then, to find the value of the pump capacity, divide your pool volume by 480.

For example, if your pool volume is 10,000 gallons, the pump size required is 10000/480 = 21 GPM.

If your pool is for commercial purposes, you need to do the calculation with 6 hours instead of 8 hours.

**Find the Feet of Head**

The piping that connects the pump and the pool is an important factor to take into consideration. It is called the Feet of Head. It is very crucial to know that the higher the head, the more the GPM decreases.

You need to have at least a rough estimate of the feet of heat between the pump and the pool. The valves in the pipe and other features need to be considered as well.

**Find the right pump:**

Now that you know the right size (GPM) and feet of head, you can easily get the right pump for your pool. Most manufacturers and suppliers/retailers have a chart that shows pump size based on their specific foot of head.

However, since you know the required pump specifications, it’s better to go for something slightly larger than the calculated value.

**Find the right filter**

Remember, if your filter is too small, it can carry the pressure from the pump, and it can lead to the breakdown of both the filter and the pump. Also, note that it is less risky to use a filter that is larger than the required size than to get a smaller one.

Now that you know your pump size, it is much easier to determine the right size of filter to buy. As we pointed out for the pump, go for the size that is slightly higher than the calculated size.

The pool pump size is directly related to the filter size. So go for the size that matches or is slightly higher than your pool pump. For example, if the pool pump rating is 21 GPM, get a filter with a capacity of 21 GPM or more.

**Other features to consider**

When determining the filter capacity, you don’t only consider the pool volume and feet of head. There are other features you should also take into consideration, such as spas, waterfalls, and other water features that might require the pump and filter to function.

For example, if your pool is very large and the calculated GPM is up to 60, then you may need a larger pump and piping of at least 2 inches in diameter that can carry up to 100 GPM.

**So, what capacity of sand filter do you need for your pool?**

For a 27′ pool, a larger filter will work effectively. Since you can’t have too much filter for your pool volume, it is better to go for the larger ones, if you can’t do the calculations above. For instance, the 1.5 horsepower pool pump is big. However, if you use 2 hp, it will still work quite well.

#### What about the size of the filter?

Another guide that works well is to use a filter of 1.5 sq ft for every 10,000 gallons of water. For inground pools, a 24″ tank (3.1 sq ft) of sand filter is the standard. a 30″tank (4.9 sq ft) of sand filter for a larger pool size of 30,000 gallons or more.