If you've ever tried to nest one piece of pipe inside another, you know how important it is to differentiate between inside diameter and outside diameter. The outside diameter of a pipe or any cylinder is exactly what it sounds like – the distance from one outside edge of the pipe to the opposite outside edge, measuring straight across the face of the pipe or across a cross-section that runs perpendicular to the pipe's long axis. If you have access to the end of the pipe, you can simply measure its outside diameter with a ruler or measuring tape. If you can't reach its ends or if the pipe's diameter changes, you can calculate the outside diameter based on the pipe's outside circumference.
TL;DR (Too Long; Didn't Read)
Measure or calculate the outside circumference of the pipe. Then divide that amount by pi, usually rounded to 3.1415. The result is the outside diameter of the pipe.
Calculating Outside Diameter Based on Circumference
If you can measure the outside circumference of the pipe with a flexible measuring tape, calculating its diameter requires just one easy step: Divide the circumference by pi. To date, pi's exact value has been calculated to more than 22 trillion digits. For a construction problem like this, rounding pi to 3.1415 usually gives you plenty of precision. So, if you're dealing with a pipe that has an outside circumference of 10 inches, its outside diameter is 10 ÷ 3.1415 = 3.18319274232 inches. By rounding to the fourth place, the outside diameter is 3.1832 inches. Remember to include your unit of measure – in this case, inches – when you write down your answer.
If you're asked to supply the radius of the pipe, the radius is simply half the diameter. So, to continue the example, a pipe with diameter 3.1832 inches has a radius of 3.1832 ÷ 2 = 1.5916 inches.
An Easy Way to Measure Outside Diameter
Although this is a quick and easy calculation, if you find yourself frequently figuring the outside diameters of real-world pipes, all that calculating can add up. Consider investing in calipers that fit around the outside of the pipe. You can use these to measure outside diameter directly instead of calculating diameter based on circumference.
There is another option for measuring outside diameter directly: Pi tape, sometimes also known as diameter tape or outside diameter tape. The markings on this flexible measuring tape automatically convert the pipe's circumference to its outside diameter, so all you have to do is read the diameter measurement off the tape. Pi tape is expensive and relatively obscure, but it gives quick, accurate results with absolutely no intermediate steps required.