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.

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 3.1416 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:

By rounding to the fourth place, the outside diameter is 3.1831 inches. Remember to include your unit of measure – in this case, inches – when you write down your answer.

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.