How to Measure a Bicycle Thru-AxleMost thru-axles are described by four measurements, and you might see them described something like this: 15 x 100mm, 127 length, 1.5.
In this example, 15mm is the diameter, 100mm is the O.L.D., 127mm is the overall length and 1.5mm is the thread pitch. You can see more details about these measurements below.
The Diameter of the Thru-AxleThere are two common sizes. 15mm and 12mm. It's best to measure with a caliper in millimeters, but you should be able to tell with a ruler. 12mm is .47" - less than 1/2". 15mm is .59" - more than 1/2".
The Overall LengthThe overall length is important, because it is often confused with the O.L.D. measurement. The best way to know what to get is to measure the axle you are replacing. The overall length is measured from the base of the head to the tip of the threads.
If you can't measure your existing thru-axle, try checking with the manufacturer to see if they list the spec. It is usually about 20 to 30mm more than the O.L.D. It's not exactly the outside-to-outside, but is pretty close. The thru-axle needs to be long enough to go thru the thickness of the frame or fork. You don't want to be too narrow or the threading may not be long enough to hold securely.
Thread Pitch and Thread LengthThe thread pitch for a thru-axle is the distance from the tip of one thread to the tip of the next, measured in millimeters. A 1 mm thread pitch is much finer than a 1.5 mm pitch. This can be hard to measure without a caliper. In a half inch section of a 1mm threading, you'll have about 13 peaks. For a 1.5 mm threading, you'll have about 8 peaks in a half inch section. It's harder to see the difference between a 1.5 and 1.75 threading - a 1/4 of a millimeter is pretty small. Try to find the manufacturer specs for your fork or frame if you're unsure of your measurements.
Some thru-axles will also list the length of threaded section.
MRP publishes this nice PDF, which if printed to the proper scale can be used to check your current thru-axle.