In order to select the right sheet size, it can be important to first measure the bed for which you are buying sheets. There are a number of standardized mattress sizes, from twin to king, though the precise measurements can vary depending on the brand. The most important measurement to take is thickness, since that dimension tends to have the most fluctuation from one manufacturer to another. Some general rules of thumb are to look for deep pocket sheets if your mattress has a pillow top, and to make sure not to purchase regular king size sheets for a California king model. If you use an air mattress, futon, or other non-standard mattress, then precise measurement can be even more important in selecting the right sheet size.
There are a number of factors to consider when choosing sheets, from the material to the thread count, but sheet size is one of the most important. Improperly sized sheets often fail to cover an entire mattress, or they may shift excessively when someone tries to sleep on the bed. The first step in choosing the right size is to determine what type of mattress you have, since most sheet sets are designated that way. In many cases, you can end up with the right size by simply selecting twin sheets for a twin mattress, double sheets for a double mattress, and so on.
It can also be important to measure your mattress before buying sheets though, since there are often variations in size between different manufacturers. Each standard mattress size has a general set of dimensions, but some mattresses can be slightly smaller or larger. The width and length measurements tend to be fairly uniform, but the thickness can vary quite a bit depending on the mattress design. Pillow top mattresses are typically much thicker than normal, and foam mattresses can be either thinner or thicker. If you have a mattress with a pillow top, or an especially thick foam mattress, you will typically need to select deep pocket sheets.
Measuring your mattress can be even more important when looking for the right sheet size for non-standard mattresses. Air mattresses are typically referred to by the same designations that are used for regular mattresses, such as double and queen, but the thicknesses can vary quite a bit. Some air mattresses are also designed to be about the same thickness as a combined mattress and box spring, so you may also need to take that into account when looking for the right sheet size.