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The measurement for steel or wrought iron pipe under 12 inches in diameter is based on the inside diameter. When referring to such pipe sizes, the industry standard is to measure the hollow portion of the pipe, which is the inside diameter. This measurement is critical for ensuring compatibility with fittings and other piping systems, as it directly affects flow capacity and fitting connections.

Using the inside diameter allows for uniformity and standardization across various applications, providing a reliable means of sizing pipes for plumbing and other installation purposes. The outside diameter and nominal diameter are often used in different contexts, but for pipes under 12 inches, industry practice dictates that the inside diameter is the reference point for measurement.