- 1). Turn off the pipe system. If you are determining the size of a black gas pipe system, call a trained professional if you are uncomfortable working around gas pipe.
- 2). Examine the connections of the black pipe. The connection will have either a threaded connection or a rubber coupling surrounded by a metal sleeve.
- 3). Set one pipe wrench on either side of a threaded black pipe connection. Hold one pipe wrench in place while you turn the second pipe wrench counterclockwise to loosen the pipe connection.
- 4). Loosen each hex screw of a rubber coupling connection with a 5/16-inch hex head screwdriver. Slip the coupling away from the connection to expose an open end of the black pipe.
- 5). Align the 1-inch mark of a tape measure blade with one inside wall of the black pipe. Pull the tape measure across the pipe opening and read the number aligned with the other inside wall of the pipe. Subtract one inch to determine the pipe diameter. Round the number to the nearest hole number to determine the nominal size of the pipe. For example, a measurement of 4-1/16-inches would equal a 4 inch diameter black pipe.
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