How many gallons does a 18ft 48 pool hold?

Knowing the gallon capacity of your swimming pool is important for several reasons. First, it helps you determine how much water is needed to initially fill the pool. Additionally, it allows you to calculate the amount of chemicals needed to properly sanitize and balance the water. Finally, understanding pool volume aids in estimating how much water will be lost due to evaporation over time.

In this article, we will walk through the steps to calculate the total gallons held by a typical 18ft x 48in residential swimming pool. We will look at the formula used and discuss the factors that determine overall pool capacity. Read on to learn how much water your 18ft x 48in pool needs!

What Size is an 18ft x 48in Pool?

When determining pool size, there are two key measurements – length and width. For a rectangular pool, length and width are usually sufficient to describe the overall dimensions.

In this case, we are working with an 18ft x 48in pool. This tells us that:

– The length of the pool is 18ft
– The width of the pool is 48in

To put the width into the same units as the length, we need to convert the 48in into feet:

– 48in = 4ft (since 1ft = 12in)

So our overall pool size is:

18ft x 4ft

This means the pool is 18ft long by 4ft wide. Knowing the precise length and width will allow us to calculate the surface area, which is needed to determine the gallon capacity.

Pool Surface Area Formula

The pool capacity formula relies on first calculating the surface area. Surface area is a measure of the total area taken up by the pool within its length and width dimensions.

For a basic rectangular pool, surface area can be calculated using the simple formula:

Surface Area = Length x Width


Length = the pool’s length in feet
Width = the pool’s width in feet

Let’s plug our dimensions into the formula:

Surface Area = 18ft x 4ft
= 72 sq ft

Therefore, the surface area of an 18ft by 4ft rectangular pool is 72 square feet.

Pool Gallon Capacity Formula

Now that we know the surface area of the pool, we can use that value to estimate the total gallon capacity.

The standard formula for determining pool gallons is:

Gallons = Surface Area x Average Depth x 7.5


Surface Area = total surface area in sq ft (length x width)
Average Depth = the pool’s average depth in feet
7.5 = the number of gallons in 1 cu ft of water

This formula calculates the total cubic feet of water held by the pool and then converts it to gallons. The 7.5 conversion factor is used because there are 7.5 gallons in every 1 cubic foot of water.

Let’s break this down step-by-step for our 18ft x 48in pool:

1) Surface Area = 72 sq ft (as calculated above)

2) Average Depth = Let’s assume the average depth is 4ft

3) Plug into formula:
Gallons = Surface Area x Average Depth x 7.5
= 72 sq ft x 4ft x 7.5
= 2,160 gallons

Therefore, the total estimated gallon capacity of an 18ft x 48in pool with an average depth of 4ft is 2,160 gallons.

Factors Affecting Pool Capacity

While the above calculation provides a good estimate for planning purposes, the actual amount of water needed may vary slightly in practice. Some of the factors that can impact overall capacity include:

Shape – Irregularly shaped pools require separate length/width measurements for each segment and adding together the total surface area.

Depth – Using the deepest and shallowest spots to find true average depth gives a more accurate value than a single estimate.

Slopes and Contouring – Gradual slopes reduce capacity compared to steeper pool walls.

Obstructions – Built-in steps, seating, plumbing and other features take up space that could otherwise hold water.

Dimensional Precision – Measurements must be as precise as possible, since even small discrepancies get magnified when calculating volume.

Displacement – The amount of water displaced by swimmers needs to be added on top of total capacity to account for when the pool is being used.

While you can’t account for every inch, starting with accurate length and width measures will get you very close. And you can always tweak the average depth or other assumptions to refine your estimate.

Key Takeaways

Calculating the number of gallons needed for a pool involves a few simple steps:

1. Measure length and width precisely for surface area

2. Determine average depth

3. Apply formula: Gallons = Surface Area x Average Depth x 7.5

4. Factor in other considerations like shape, slope and obstructions

5. Adjust estimate accordingly to refine as needed

For a basic 18ft x 48in rectangular pool with an estimated 4ft average depth, the total gallon capacity comes out to around 2,160 gallons.

Knowing the approximate volume and gallonage is crucial for filling up the pool, adding chemicals properly, and understanding how much evaporation takes place over time. This helps you maintain safe and sanitary pool water.

Frequently Asked Questions

How much does an 18ft x 48in pool weigh when filled?

An 18ft x 48in pool holding 2,160 gallons of water weighs approximately 18,000 lbs or 9 tons when full. This is calculated based on water weighing 8.34 lbs per gallon. Always consult a structural engineer to ensure the surrounding patio or lawn can support this weight.

How much does it cost to fill an 18ft x 48in pool?

The cost to fill an 18ft x 48in pool depends on your local water rates. At an average price of $2 for every 1,000 gallons used, it would cost around $4.32 to completely fill 2,160 gallons. This does not account for sewer charges in some areas or chemical costs. Best to check your latest water bill for the actual rate.

How many chemicals do I need for an 18ft x 48in pool?

Most pool chemical dosages are based on the number of gallons. An 18ft x 48in pool holding 2,160 gallons would need:

– 2.16 lbs of chlorine stabilizer
– 21.6 lbs of pool shock
– 8 gallons of liquid chlorine
– 4.32 quarts of algaecide

Always follow manufacturer instructions for exact dosing guidelines. A pool professional can recommend the right chemical plan for your needs.

How often should I change the water in an 18ft x 48in pool?

The water in an 18ft x 48in pool should be partially drained and refilled every 3-5 years on average. Draining 25-30% of the water helps control chemical buildup and contaminants. More frequent partial drains may be needed for heavily used pools or those with imbalanced chemistry.

What filter size do I need for an 18ft x 48in pool?

An appropriately sized filter is crucial for proper water circulation. For an 18ft x 48in pool, a filter with a flow rate of at least 45 GPM is recommended. Cartridge and DE filters in the 150-200 sq ft range would be sufficient for this size pool. Always match the pump and filter flow rates for optimal efficiency.


Determining the number of gallons in your pool provides invaluable insight into maintenance requirements. While an 18ft x 48in pool will theoretically hold around 2,160 gallons, real-world conditions my alter the volume slightly. Measure carefully, account for all design factors, and adjust chemical dosing accordingly over time. Proper sizing of the filter, pump and other equipment based on total gallon capacity ensures the pool operates safely and efficiently all season long.

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