The size of the casing refers to the outside diameter (O.D.) of the main body of the tubular (not the connector). Casing sizes vary from 4.5″ to 36″ diameter. Tubulars with an O.D. of less than 4.5” are called Tubing. The sizes of casing used for a particular well will generally be limited to the standard sizes that are shown in Figure 3. The hole sizes required to accommodate these casing sizes are also shown in this diagram. The casing string configuration used in any given location e.g. 20” x 13 3/8” x 9 5/8” x 7” x 4 1/2” is generally the result of local convention, and the availability of particular sizes.