Above Ground Swimming Pool
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Take the measurements of the pool from side to side, straight across the center. This will give you the diameter of the swimming pool. Write it down.
Measure the depth of the sides of the pool from the top edge to the bottom edge. This is to figure the maximum amount of water that could be added to the pool. If you only want to figure how much water you are actually putting in, then just measure the depth of the water.
Determine the volume of space by using the formula, 3.14(r)squared, where r is the radius, which is half of the diameter found in Step 1. So 3.14(4)squared where 4 is the radius of a 8-foot-wide pool is roughly 158 square feet. This is the surface area.
Multiply the surface area by the depth of the pool. So if your depth is 4 feet, you multiply the example used in Step 3, 158 square feet, by 4 to get 632 cubic feet of space or water.
Calculate how much water fits into that space by using the fact that a cubic foot of space will hold 7.5 (technically 7.48) gallons of water. If we have 632 cubic feet of space and each cubic foot holds 7.5 gallons, then the pool holds 632 x 7.5, or 4740 gallons of water.
Figure the amount in a square pool by multiplying the two sides by the depth of the pool. Multiply the answer by the 7.5 gallons per cubic foot to get your volume of water.