Determining the amount of water in the pool is essential when adding pool chemicals and sizing pumps and filters. Chemicals are added in specific amounts according to the number of gallons in the pool, raising or lowering the pH or adding more chlorine. Too much chlorine can damage your eyes and skin, while too little puts you at risk of bacterial infections and algae infestations. To find the number of gallons in a pool, first you must find the pool's measurement in cubic feet and then multiply by the number of gallons in a cubic foot.

Swimming pool chemical testing guage.
credit: CaronB/iStock/Getty Images
Before adding chemicals, measure the pool's chlorine and pH levels.

Square or Rectangular Pools

Step 1

Measure the pool's length and width in feet. Write the measurements down in your notebook.

Step 2

Measure the deepest part of the pool in feet. Then measure the shallowest part of the pool in feet. Add the two numbers together, and then divide that number by two to find the average depth of the pool.

Step 3

Multiply the length by the width, then multiply by the average depth. Finally, multiply by 7.5 to find the number of gallons in the pool.

Round or Oval Pools

Step 1

Measure the length and the width of the pool. While a round pool's numbers are the same, an oval pool has two different numbers. Write the numbers down in feet.

Step 2

Find the average depth of the pool by measuring the deep and shallow ends. Add the two numbers together and divide by two for the average depth.

Step 3

Multiply the length by the width, then by the average depth. Finally, if the pool is round or oval multiply by 5.9. If it is rectangular with rounded ends, multiply by 6.7. This provides the number of gallons in the pool.

Kidney or Custom-Shape Pools

Step 1

Measure the length of the pool in feet and write the number down. If the pool is a custom shape, you may need to measure in more than one location and find the average by adding the measurements together, then dividing by the number of measurements used.

Step 2

Measure the width of the pool at several points, including the widest and narrowest points. Find the average width in feet and write it down.

Step 3

Multiply the average length by the average width of the pool. Then multiply by the average depth. Finally, if the pool is a kidney shape, multiply by 7.0 and if it is a custom shape, multiply by 7.5 to find the number of gallons in the pool.