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While measuring a pool’s depth may not seem complicated, the different sizes of pools require many calculations and adding them together. To learn how to achieve these calculations, look below at the detailed information provided.

To calculate the capacity, it will involve the volume and the surface area of the pool or spa. The information on this page will help you calculate the size and capacity of the pool you are looking to fill. The first step you will want to take is to calculate the pool’s area in square feet. This will help you determine the gallons, maximum capacity of people and other information about the pool.

Geometric Formulas

By using these geometric formulas, you will be able to calculate your pool’s size. Below are the basic formulas and calculations to determine surface areas:

A Area

L Length

W Width

H Height

r Radius

d Diameter

Pi 3.14 constant

Area of a square or rectangle: A = L x W

Area of a right triangle: A = (L x W)/2

Area of a circle: A = Pi x r x r

Calculating Volume

Including the depth of the pool with the surface area will determine the cubic volume. For precise calculations, it is recommended that pool 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)

To reach the surface area of the pool you will need to multiply the length and the width. Then multiplying that area by the depth will give you 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 determine the volume of the pool, stated in gallons.

Variable Depth Pools: Square and Rectangular

Length x width x average depth x 7.5 = volume (in gallons)

To reach the surface area of the pool you will need to multiply the length and the width. Then multiplying that area by the depth will give you 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 determine the volume of the pool (expressed in gallons). Measure the length, width, and average depth of the pool, rounding each measurement off to the nearest foot or percentage of one foot. One inch equals 0.0833 feet. Therefore, multiply the number of inches in your measurements by 0.0833 to get the appropriate percentage of one foot.

Example: 25 ft, 9 in. = 25 ft + (9 in. x 0.0833)

= 25 + 0.75

= 25.75 ft

If the shallow end is 3 feet and the deep end is 9 feet, and assuming the slope of the pool bottom is gradual and even, then the average depth is 6 feet. Average depth = (Depth at the shallow end + Depth at the deep end) / 2 Average depth = (3 + 9) / 2 = 6 feet.

If most of the pool is only 3 or 4 feet and a small area drops off to 10 feet, you will have a different average depth. In this case, you may need to divide the pool into two separate parts. First, measure the length, width, and average depth of the shallow section. Then use the same measurements for the deeper section. Lastly, calculate the volume of the shallow part and add that to the volume you calculate for the deeper part.

Take extra precaution to ensure that you use the actual water depth in your calculations (not the container depth).

For example, the hot tub depicted in Figure 2 is 4 feet deep, but the water is only filled to about 3 feet. Using 4 feet in this calculation will result in a volume 33 percent greater than the actual amount of water. This could mean serious errors when adding chemicals for example, which are administered based on the volume of water in question. There might be a time when you want to know the potential volume, if filled to the brim. Then, of course, you would use the actual depth (or average depth) measurement. In the example, that was 4 feet.

Length x width x average depth x 7.5 = volume (in gallons)

25.75 ft x 10 ft x 6 ft x 7.5 = 11,587.5 gallons

Circular Pools

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. Measure the distance across the broadest part of the circle and divide it in half to get the radius. Squared means to be multiplied by itself, so you will multiply the radius by itself. For example, if you measure the radius as 5 feet, then multiply 5 feet by 5 feet to arrive at 25 feet.

Use the hot tub to calculate the volume of a round container. We will do that hard part first. The diameter of the Jacuzzi 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

If you are measuring the capacity of a circular spa you will have to follow these steps. First, you will need to calculate two or three areas within the hot tub, and then you will add them together to reach the total volume. Because the seats in a Jacuzzi cause different levels, you may want to treat the area as two separate volumes – the volume above the seats and below.

Kidney or Irregular Shapes

The capacity of irregular shapes can be measured by two different methods. The first uses a combination of smaller, regular shapes in the hot tub. These various areas will be measured by the previously stated calculations for each square, rectangular or circular area. Then you will add these volumes to find the total capacity.

0.45 x (A+B) x length x average depth x 7.5 = volume (in gallons)

The total of measurement A plus measurement B multiplied by 0.45 multiplied by the length gives you the surface area of the kidney shape. (A + B = 18 feet). The rest of the calculations you are now familiar with. Try this volume calculation: 0.45 x (A+B) x length x average depth x 7.5 = volume (in gallons) 0.45 x 18 ft x 25 ft x 5 ft x 7.5 = 7593.75 gallons