A standard pillow is 20 x 26 inches, and a pillowcase is at least 4 inches longer than the length of the pillow and 1 to 2 inches wider. This brings the standard pillowcase size to 21 x 30 or 22 x 30 inches.
When shopping, pillowcases are usually listed by the sizes standard, king and queen, making it easier to pick the correct size. The exact measurement of the pillowcase is not necessary, only the exact measurement of the pillow itself.
If sewing a pillowcase, 40 x 40 inches or 1 yard of fabric that is 45 inches wide is needed. King and queen-size pillowcases are typically longer than standard pillowcases by 2 to 4 inches, and one can fit the width of a standard pillow.