A few factors come into play when you're determining how to measure your swimming pool for a new liner, like whether you have an above-ground or in-ground pool and whether you still have the pool manual. Also consider how much time you want to invest in the project. If you still have your manual, that makes the project easiest, since it should specify your pool's dimensions and save you the trouble of measuring it.
If you have an above-ground pool and you want to measure it yourself, determine what type of liner you need: overlap, beaded, unibead or expandable. When dealing with a round pool, use a tape measure to determine the inside distance from one side of the pool to the other along the bottom of the pool. It's best to measure in more than one place to make sure everything is even. If you have an oval pool, measure from the center of the long end to the other side of long end and then from the middle of the short side of the pool to the middle of the other short side. Again, take the measurements from the inside and along the bottom. The measurements for a rectangular pool are taken similarly to those for an oval pool: Measure from one end to the other and from each side to the other. Plus, you need to note the corners; are they square, rounded or angled? Measure the height of the pool, too. Take your measurements to a pool supply company to help you choose the right liner for your pool.
The measurements for an in-ground pool are similar, but you have to take more things into account, like the steps, slope and deep end. Use a worksheet to keep track of all the possible dimensions that your retailer might need from you. If there's water in the pool, take the net off your skimmer and drop a plumb bob through the pole so that you can measure the various depths of your pool.