Using a caliper

The most common technique for taking accurate measurements is the use of a tool called a caliper, a must have for anyone who models for 3D printing. Calipers measure distance with a high degree of precision and can measure in three different ways; the outside diameter of an object with the outside jaws, the internal diameter with the inside jaws, or the depth with the depth probe at the far end. The easiest kind of caliper to use is the digital type.

Simply turn on the digital caliper, tare or zero the reading while closed, then open the jaws, put the object to be measured between them, clamp it down, and take the reading. It's fast and relatively accurate without much effort. Most models even have a port that can transfer the measurements directly to the computer. But digital calipers have the disadvantage of being more expensive and relying on batteries which when run out eliminate the ability to measure accurately.

If budget is a concern then perhaps a Vernier caliper is...