Here is a great tool to calculate the volume or capacity of your pool. Important!
Remember to measure from the depth of the water rather than the height of your wall.
Capacity calculations involve calculating surface area and volume of the pool or spa. This article describes in detail how to calculate the size and capacity of the pool you are planning. Calculating a pool’s area in square feet is the first step in determining information including pool gallons, maximum capacity of persons and other important information about your pool.
The cubic volume can be calculated by including the depth of the pool with the surface area. For accurate calculations, the pool should be divided into various areas according to the depth.
Constant Depth Pools: Square or Rectangular
Length x width x depth x 7.5 = volume (in gallons)
Length times width gives the surface area of the pool. Multiplying that by the depth gives the volume in cubic feet. Since there are 7.5 gallons in each cubic foot, multiply the cubic feet of the pool by 7.5 to arrive at the volume of the pool, expressed in gallons.
Variable Depth Pools: Square and Rectangular
Length x width x average depth x 7.5 = volume (in gallons)
Length times width gives the surface area of the pool. Multiplying that by the average depth gives the volume in cubic feet. Since there are 7.5 gallons in each cubic foot, multiply the cubic feet of the pool by 7.5 to arrive at the volume of the pool (expressed in gallons).
The formula: 3.14 x radius squared x average depth x 7.5 = volume (in gallons)
The number 3.14, refers to pi, which is a mathematical constant. The radius is one-half the diameter, so measure the distance across the broadest part of the circle and divide it in half to arrive at the radius. Squared means multiplied by itself, so multiply the radius by itself. For example, if you measure the radius as 5 feet, multiply 5 feet by 5 feet to arrive at 25 feet.
Use the hot tub to calculate the volume of a round container. Let’s do the tricky part first. The diameter of the tub is 10 feet. Half of that is 5 feet. Squared (multiplied by itself) means 5 feet times 5 feet equals 25 square feet. Knowing this, you can return to the formula:
3.14 x radius squared x average depth x 7.5 = volume (in gallons)
3.14 x 25 ft x 3 ft x 7.5 = 1766.25 gallons
In measuring the capacity of a circular spa, you might need to calculate two or three areas within the hot tub and add them together to arrive at a total volume. An empty circular hot tub looks like an upside-down wedding cake, because of the seats. Therefore, you might want to treat it as two separate volumes-the volume above the seat line and the volume below. In the wooden hot tub, where there is actually water above and below the seats, the tub can be measured as if there are no seats because this difference is negligible.