It is important to know the volume of your pool. This is used to determine the dosage of every chemical that is added and if it’s incorrect you could damage the water by adding too much of a chemical.
The key to determine how many gallons of water are in your pool is:
Area x Average Depth x 7.5 = Gallons
If you have a more complex pool, you can just separate the pool into sections, run the above formula on each section, and then add them back up at the end. Determining the area and average depth will vary depending on what kind of pool you have.
First determine what kind of pool you have. Two questions can be asked to determine that. In most cases, this will be broken down into two categories: 1) What is the shape of the pool? and 2) What is the depth like? See below for help determining that.
- Constant Slope: Most common pool type. This means the pool has a shallow end and a deep end and not much variance. It might start at 3 feet and increase to 8+.
- Multiple Depths: The shallow end might be 3 feet and that might slope down to 5 and then slopes down to 8 feet before returning to 5 feet. Not as common.
- Shapes: Rectangle, circular, oval, oblong, etc.
- Circular
- Oval
- Oblong
Determining gallons in a constant slope rectangle pool:
Gallons=area x Average Depth x 7.5
Area=Width x length
Average Depth=Shallow end + Deep end ÷ 2
Example A: You have a simple rectangle pool. The shallow end is 3 feet, and the deep end is 8 feet. The pool is 20 feet wide and 50 feet long. Start by finding out what the area is. Measure the pool’s width and length. The area is 1000 and the average depth is 6. So it would look like:
1000 x 6 x 7.5 = 450,000 gallons
Determining gallons in a circular pool:
Gallons=area x Average Depth x 7.5
Area=Radius x Radius x 3.14
Average Depth=Shallow end + Deep end ÷ 2
Example: You have a simple circular pool. It measures 30 feet across. At the edge of the pool the depth is 3 feet and the center of the pool is 6 feet. Start by finding out what the area is. The radius is 15 feet and the average depth is 4.5 feet. So it would look like:
706.5 x 4.5 x 7.5 = 23,844 gallons (Rounded)
Determining gallons in an oval pool:
Gallons=area x Average Depth x 7.5
Area=A x B x 3.14
Average Depth=Shallow end + Deep end ÷ 2
Example: You have a simple oval pool. Measurements determine that A is 10 feet and B is 40 feet. Area is 1256 Sq Ft. At the edge of the pool it’s 3 feet and it slopes down to 7 feet at the center. So it would look like:
1256 x 5 x 7.5 = 47,100 gallons
Determining gallons in an oblong pool:
Now we’re getting into the more complex pool shapes, and we’re going to break the pools down into sections. For oblong, we’re essentially going to break it down into two shapes. Look at the above image. We’re going to take the rectangular section of the pool and then push the two sections left into a circular pool. We’re determining the area of rectangle and a circle and adding them together for the area of the whole pool.
Finding the area of an oblong pool is:
Radius x Radius x 3.14 + (L x W) = Area of the pool
Use markers set on the ground around the pool to section it off. Measure based on the image above to determine the numbers you need.
So if you the radius on both ends is 10 feet and the length of the middle section is 30 and the width of the middle section is 20 then the area of the pool is 914 Sq Ft.
Gallons=area x Average Depth x 7.5
Area=Radius x Radius x 3.14 + (L x W)
Average Depth=Shallow end + Deep end ÷ 2
Example: You have a simple oblong pool. The area is 914. The shallowest part of it is 3 feet and the deepest is 8. So it looks like:
914 x 5.5 x 7.5 = 37,703 gallons (Rounded)
Determining gallons in multiple depth pool:
The easiest way to determine volume in a pool with various depths is by splitting it into sections. Below is an image that we’ll break down. We are just going to take the complex pool, separate it into 3 simple rectangle sloping pools, and then add it all back together at the end.
Since we’re just breaking it down into three separate rectangular pool, you can refer to the very first item in this article on determining volume:
Gallons=area x Average Depth x 7.5
Area=Width x length
Average Depth=Shallow end + Deep end ÷ 2
Separate the pools into sections where the depth will just be a constant slope. In the above example, that means 3 sections. You can separate it into however many sections you need.
Section A: This is a simple rectangle pool. Length is 30 feet and width is 25 feet. Shallow end is 3.5 feet and deep end is 5 feet. By running the formula we’ve ran many times now it looks like this:
750 x 4.25 x 7.5 = 23,906 gallons (Rounded)
Section B: This is a simple rectangle pool. Length is 10 feet and width is 25 feet. Shallow end is 5 feet and deep end is 10 feet. So it looks like this:
250 x 7.5 x 7.5 = 14,063 gallons (Rounded)
Section C: This is a simple rectangle pool. Length is 10 feet and width is 25 feet. Shallow end is 8.5 feet and deep end is 10 feet. So it looks like this:
250 x 9.25 x 7.5 = 17,344 gallons (Rounded)
Now finally add them together and the volume of the multiple depth pool is: 55,313 gallons
Final Note:
By using the methods discussed here, you can calculate how many gallons are in most pools. Remember that if you have a complex pool, separate it into less complex shapes with a constant slope and just add it all up at the end for the volume.