When you purchase a winter cover for your inground pool, you're going to need some way to secure it.  Water tubes are the most common method of securing your inground winter cover to your pool.  So instead of trying to figure out how many of what size tubes you are going to securely hold your cover in place, just go ahead and buy a water tube kit.  Water Tube Kits for InGround Winter Pool Covers have all of that figured out for you.  Just buy the kit based on the deminsions of your inground pool.