Textron's line of Torx-head bits and screwdrivers are designed to be used specifically with Torx-head screws. The bits are six-point splines, the individual sizes of which are measured in a standard known as "T-sizes." If you ever come across a Torx bit that's mislabeled or doesn't have a T-size on it, you can figure out the exact T-size with a ruler and a chart from Textron.
Things You'll Need
Line up the ruler on the star-shaped screw tip to measure from one point to the point directly opposite.
Examine the markings on the ruler and write them down.
Compare the measurement with the "P to P" (point to point) column of the first chart on the Wiha website to determine your Torx bit's exact T-size. You can measure in inches or millimeters. The Wiha link can be found in Resources.