In carpentry, the rule of thumb is to measure twice and cut once. When it comes to auto repair/maintenance, the mantra can now be to measure the bolt head or nut first rather than guessing and running back and forth to the toolbox until you find the right wrench or socket. The Wrench iD+ calibration tool from Zuther Xtreme technologies accurately indicates the size of any bolt head or nut as well as pipe or hose diameters. It shows the info in inches or millimeters (up to 2 1/8-inches and to 54 millimeters) on the built-in LED screen. The Wrench iD+ can also store the size for future use. Purchase for $30 from zxtremetech.com.
