Determine the purpose of the sheet that you'll be measuring for. The point here may seem obvious, but the function of a fitted sheet or covering will ultimately be different depending on the intent. For example, you will need to consider functionality when measuring for a sheet that will be slept on as opposed to one meant solely for decoration.
Remove everything from the mattress or cushion to be measured. Every piece of covering should be removed before measuring in order to attain the closest, most accurate measurement. In addition to any sheets or blankets, remove any other pieces of covering that might hinder accurate measurement, such as mattress padding.
Measure the width of the mattress or cushion. Twin sheets will measure 39 inches wide; doubles will be 54 inches. Queen-sized sheets are 60 inches and king-sized are 78 inches in width. These measurements are standard and will thus be the same at any retailer.
Measure the length of the mattress. Both twin-sized and double-sized mattresses are 75 inches in length, while a queen is 80 inches, and a king is 80 inches long.
Measure the depth of the mattress. This measurement can be as significant as any other. While standard named sheet sizes reflect length and width, they don't necessarily take the depth of a mattress into account. Knowing this measurement will ensure a perfect fit when it comes to selecting your sheets at the store.