Extend the tape measure from one wall of the pool to the wall on the opposite end, lengthwise. Measure the pool across its longest length, if the pool is oval. Extend the tape measure across the width of the pool. Measure across the widest middle point, if the pool is oval. Calculate these measurements in feet. For example, on a rectangular pool, the length may be 32 feet and the width may be 16 feet.
Measure the pool's diameter, if the pool is round. Take 2 measurements, if you are not certain that the pool is a perfect circle. Basically imagine creating an invisible cross-shaped pattern with the tape measure as you measure across the pool twice. Calculate this measurement in feet.
Calculate the depth of the pool. Measure from the water line mark down to the pool bottom, using the tape measure, if you are unsure of the depth. Calculate the average depth of a pool that has an evenly sloping bottom by measuring the depth of the shallow end, then measuring the depth at the deep end. Add the results of these two measurements together, and divide that number by 2. This gives you the pool's average depth. For example, if the shallow end is 3 feet and the deep end is 9 feet, the average depth is (3+9)/2, or 12/2, which is 6 feet.
Multiply the pool's length times the width times the average depth. For round pools, multiply diameter times diameter times average depth. With a rectangular pool, for example, multiply 32 x 16 x 6, which equals 3,072. This means the pool holds 3,072 cubic feet of water.
Multiply the number from step 4 by the appropriate multiplier for the shape of the pool, to obtain how many gallons of water are in the pool. For a pool that has a rectangular or square that multiplier is 7.5. For pools with a round shape, the multiplier is 5.9. For oval-shaped pools, the multiplier is 6.7. So, for a rectangular pool, multiply 3,072 x 7.5, which gives 23,040 gallons of water as the volume of water in the pool.