For each casing size there are a range of casing weights available. The weight of
the casing is in fact the weight per foot of the casing and is a representation of the
wall thickness of the pipe. There are for instance four different weights of 9 5/8″ casing:

casing weight

Although there are strict tolerances on the dimensions of casing, set out by the API, the actual I.D. of the casing will vary slightly in the manufacturing process. For this reason the drift diameter of casing is quoted in the specifications for all casing. The drift diameter refers to the guaranteed minimum I.D. of the casing. This may be important when deciding whether certain drilling or completion tools will be able to pass through the casing e.g. the drift diameter of 9 5/8” 53.5 lb/ft casing is less than 8 1/2″ bit and therefore an 8 1/2” bit cannot be used below this casing setting depth. If the 47 lb/ft casing is too weak for the particular application then a higher grade of casing would be used (see below). The nominal I.D. of the casing is used for calculating the volumetric capacity of the casing.

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