Rectangular Pools

• 1). Measure the pool's length and width in feet.
  
  
• 2). Multiply the length times the width to calculate the pool's surface area. For example, a pool that measures 8 feet by 12 feet would have a surface area of 96 square feet.
  
  
• 3). Measure the depth of the pool. If you have a pool that gets gradually deeper, measure the depth in the shallow end and the depth in the deep end and take the average. For example, if your pool starts at 4 feet deep and goes to 8 feet deep, you would add 4 to 8 to get 12 and divide 12 by 2 to find an average depth of 6 feet.
  
  
• 4). Multiply the average depth by the surface area by 7.5 to find the pool's volume in gallons. In this example, you would calculate 96 square feet times six feet times 7.5 to get a volume of 4,320 gallons.
  
  

Circular Pools

• 1). Measure the distance across the pool -- also known as the diameter -- in feet.
  
  
• 2). Measure the depth of the pool. If you have a pool that gets gradually deeper, measure the depth in the shallow end and the depth in the deep end and take the average. For example, if your pool starts at 4 feet deep and goes to 8 feet deep, you would add 4 to 8 to get 12 and divide 12 by 2 to find an average depth of 6 feet.
  
  
• 3). Divide the diameter by 2 to find the radius. For example, if your pool is 20 feet across, you would divide 20 feet by 2 to get a radius of 10 feet.
  
  
• 4). Square the radius and multiply the result by 3.14 to find the surface area of the pool. In this example, you would square 10 feet to get 100 square feet and then multiply 100 square feet by 3.14 to get 314 square feet.
  
  
• 5). Multiply the surface area by the average depth by 5.9 to find the pool volume in gallons. In this example, you would multiply 314 square feet by 6 feet by 5.9 to get 11,115.6 gallons.