How Many Gallons Are In a Swimming Pool?

If you just purchased a home with a pool or you are in the process of building a swimming pool, like many you are probably wondering how many gallons of water should go in the pool. Well, the answer is – it depends.

Common Residential Swimming Pools

A residential in-ground swimming pool can hold anywhere from 1,000 gallons to 100,000 gallons of water assuming it is a rectangular-shaped swimming pool. The average size swimming pool is between 15,000 and 20,000 gallons. Smaller pools hold about 11,000 gallons while the larger ones can hold close to 85,000 gallons.

The most common size for residential pools is 18′ by 36′. This size pool will usually hold between 20,000 and 30,000 gallons of water.

Pool Size and Shape

As noted above, the average size pool is rectangular, but pools come in all different shapes and sizes. The size of your pool will play a role in how many gallons it can hold. For example, a kidney-shaped pool that is 18′ long and 9′ wide can only hold about 15,000 gallons of water.

Another factor that comes into play is the depth of the pool. A pool that is 4′ deep will obviously hold less water than a pool that is 8′ deep. In fact, a 4′ deep pool that is 18′ long and 9′ wide will only hold about 11,500 gallons while the same-sized pool that is 8′ deep will hold three times as much water – close to 34,000 gallons.

Don’t Forget About Chemicals & Debris

Another important factor to consider is that you will need to account for the addition of chemicals and debris when determining how many gallons to put in your pool. For example, if you are using a chlorine tablet dispenser, you will need to make sure there is room in the pool for the tablet dispenser as well as the water that it will displace. The same goes for things like ladders, slides, toys, and other pool accessories.

How Often Should You Fill Up Your Pool?

You should only need to fill up your pool once – when you first set it up. After that, you will only need to add water as it evaporates or if you have a leak.

To determine how much water you will need to add due to evaporation, you can use this formula:

For pools that are 18′ by 36′, you will lose about 1/4″ of water per day due to evaporation. This means that you will need to add about 6 gallons of water per day or 180 gallons per month just to keep the level where it needs to be.

Of course, if you have a leak, you will need to add more water as needed. A small leak (1/32″ diameter) can lose up to 250 gallons per day while a larger leak (1/8″ diameter) can lose up to 2,500 gallons per day.


In short, the answer to the question “How many gallons are in a swimming pool?” is that it depends on the size and shape of your pool as well as whether or not you have a leak. To be on the safe side, you should always have at least 1,000 gallons of water on hand to top off your pool as needed.