Water volume is the most important aspect of pool water chemistry. The smallest change in pool dimensions  length, width or depth  can significantly alter the amount of gallons you have in your swimming pool. The number of gallons determines the amount of chemicals you put in your pool and the size of filter you will need to circulate all those gallons of water.
The Easy Way

Your 27foot pool can have a range of dimensions that alters the amount of gallons significantly. The easiest way to check the approximate pool water capacity for your exact pool is to visit the website Poolcalculator.com. Navigate down to the heading titled "Estimating pool volume." You will be prompted to enter your pool's length, width, depth measurements as well as shape. For pools with a range of depths, enter the average depth, which can typically be calculated by adding the amount of feet in the shallowest end and the deepest end, then dividing that by two. For example: 2 (shallowest) + 12 (deepest) = 14/2 = 7 feet average depth.
Why Gallons Matter

Pool chemistry requires a pool owner to know the amount of gallons their pool holds. All chemicals are added in term of gallons of water. For example, a 27foot round pool 4 feet deep measures 17,000 gallons approximately. Meanwhile a 27foot round pool with a depth of 5 feet holds 21,400 gallons. If you are adding too many chemicals or too few chemicals because you are unaware of the gallon capacity, it can cause problems in balancing your water chemistry.
Filter Size

Your filter size is important as well. If you overestimate the amount of gallons in your pool, you may waste money on a filter that is too powerful for your pool. Also, the motor is paired according to its filtration demands. Motors that are paired with appropriate filters can range from something as small as .5 horsepower to as large as 2.5 horsepower. The larger the pool, the stronger the motor must be.
Do Your Own Calculations

For those who don't want to rely on an online pool volume calculator, there's a formula available from Backyard City Pools. Measure the length and width of your pool. Calculate average depth in your pool. If you have only one pool depth, then that specific depth should be entered as the average depth. Multiply the length, width and average depth together followed by using a multiplier to calculate the final answer. Use 7.5 as the multiplier for rectangular, square or freeform pools and 5.9 for oval and round pools. For example, a 27foot, 4feet deep round pool is calculated as such: 27 (length) x 27 (width) x 4 (average depth) x 5.9 (multiplier) = 17,204.