What This Calculator Does
This tool estimates the total volume of water, in US gallons, needed to fill a rectangular swimming pool. Knowing your pool's gallonage is essential for dosing chemicals correctly, sizing a pump or heater, and estimating water bills. Just enter the pool's length, width, and average depth in feet.
How to Use It
Measure the inside surface length and width of your pool at the waterline. For the average depth, add the shallow-end depth and deep-end depth and divide by two. Enter all three values in feet and the calculator returns the volume in cubic feet and the equivalent in US gallons.
The Formula Explained
A rectangular pool's volume in cubic feet equals length \(\times\) width \(\times\) average depth. One cubic foot of water holds approximately 7.48 US gallons, so the full formula is:
$$\text{Gallons} = \text{Length (ft)} \times \text{Width (ft)} \times \text{Avg Depth (ft)} \times 7.48$$
The 7.48 factor comes from 231 cubic inches per gallon divided into 1,728 cubic inches per cubic foot.
Worked Example
Suppose your pool is 32 ft long, 16 ft wide, with a shallow end of 3 ft and a deep end of 8 ft, giving an average depth of 5.5 ft. The volume is $$32 \times 16 \times 5.5 = 2{,}816 \text{ cubic feet}.$$ Multiplying by 7.48 gives about 21,064 US gallons.
FAQ
Is this for round or oval pools? No — this version assumes a rectangular shape. Round pools use \(\pi \times \text{radius}^2 \times \text{depth} \times 7.48\) instead.
Why use average depth? Most pools slope from a shallow to a deep end, so averaging the two depths approximates the true volume of an evenly sloped floor.
Are these US or imperial gallons? The 7.48 factor yields US gallons. For imperial gallons, use a factor of about 6.23.
