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To do this, you need to write down the measurements of the deep end and the shallow end of the pool and divide the result by two. Let's say the shallow end of the pool measures 3 feet and the deep end measures 8 feet. (3 + 8)/2 = 5.5. The average depth of the pool is 5.5 feet. Now, just take this number and multiply it by the length and width of the pool. You're really just multiplying the area of the bottom of the pool by its depth. Let's say that the width of the pool is 20 feet and the length is 40 feet. So, 5.5 x 40 x 20 = 4,400 cubic feet. To convert the answer to gallons, simply multiply the answer by 7.48. 4,400 x 7.48 = 32, 912 gallons of water. You're all done.
Find the average depth of the pool. Multiply the result by the length and width of the pool. Convert the answer to gallons.