Plumbers don't use galvanized steel pipe as much as they used to, but they do use it extensively for outdoor pipe systems, and many older homes still have galvanized plumbing. If you have to make a repair to a system of galvanized pipes, you need to know the diameters of the pipes so you can purchase galvanized pipe fittings and replacement pipes with the same diameters. Assuming the pipes have standard wall thicknesses, which most residential plumbing pipes do, this is easy to do.
The nominal pipe size by which a pipe or fitting is identified at the store is a standardized approximation of the inside diameter of the pipe. Galvanized pipe size starts at 1/2 inch, increases in increments of 1/4 inch to 1 1/2 inches, then in increments of 1/2 inch to 4 inches, and from there in 1-inch increments to 12 inches. It's easy to measure inside diameter when the end of the pipe is exposed, but if the pipe is installed in a network, you have to measure the outside diameter or circumference of the pipe and use a chart to find the nominal size.
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Measuring Galvanized Pipe Size Directly
If the end of a galvanized pipe is exposed, you can measure inside diameter directly. The most accurate way to do this is to use a caliper. Most calipers have jaws that fit around the outside of a pipe and sharp tongs that fit inside. Insert the tongs into the pipe, open the caliper until the tongs touch the side walls, and note the reading. If the reading isn't a multiple of a half inch, round it down to the nearest multiple. For example, if you measure 9/16 inches, the nominal size is 1/2 inch.
Although a caliper gives you the most accurate reading, you can make the measurement without one. You could, for example, stretch a tape measure or ruler across the pipe opening, but remember that you want the distance between the insides of the pipe walls, not the outsides.
Converting From Outside Diameter
When the pipe you need to measure is connected at both ends, you have to extrapolate the inside diameter and pipe size from a measurement of the outside diameter. For this, you'll need a chart, such as the one provided by PlumbingSupply.com.
The easiest way to measure outside diameter is to use a caliper, but this time, you'll want to use the jaws that fit on the outside of the pipe, not the tongs that fit inside it. Close the jaws onto the pipe until they are just touching it and then note the reading. Use that along with the chart to determine nominal pipe size. For example, if the outside diameter measures 27/32 (0.84) inches, the nominal pipe size is 1/2 inch.
Converting From Pipe Circumference
Not every DIY plumber keeps a caliper in the toolbox, so if you don't have one and you need to take a quick measurement so you can go to the hardware store to buy fittings, you can do it with a sewing tape measure or even a piece of string. If you have sewing tape, wrap it around the pipe and note the measurement at the point where the end of the tape intersects. That's the pipe circumference. If you only have string, wrap it around the pipe, make a mark on the string where the end touches, and measure the distance from the end of the string to that mark.
Once you have the circumference, you calculate the outside diameter by dividing that number by π (3.1415). You can then look up the nominal pipe size on a chart. You might not even have to make that simple calculation because many charts directly correlate nominal pipe size with pipe diameter.