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. There are two common calipers: manual (or Vernier) calipers and digital calipers.
If budget is a concern, then perhaps a Vernier caliper is preferable, since they are generally less expensive than their digital counterparts. These calipers operate purely mechanically, but have a clever trick that allows them to be just as accurate if read properly:
Open the jaws to take a measurement and tighten them over the object to be measured. Then take a close look at the little window:
The leftmost tick is between 4.5 and 4.6, so this object is between 4.5 cm...