Identify a screw thread
Two caliper readings are usually enough to name an unknown thread. Measure the outside diameter and the pitch, enter them below, and the tool returns the closest thread designation with its nominal dimensions — then links you to every fastener built to that thread.
Enter at least one measurement. Diameter alone narrows the list; diameter with pitch usually identifies the thread outright.
Threads in the database
browse if you would rather match by eyeISO metric (37 sizes)
Unified National Coarse (25 sizes)
What the numbers mean
A thread is defined by four numbers. The major diameter is what a caliper reads across the crests. The pitch diameter sits about halfway down the flank, and is the dimension that actually controls fit — it is what a thread micrometer measures and what the tolerance classes in ISO 965 and ASME B1.1 are written against.
The pitch is the axial distance between crests. Fine threads raise the minor diameter and the stress area, which is why they hold better in thin walls and shake loose less readily, at the cost of being slower to assemble and easier to cross-thread.
For tapping, the tap drill is the pitch subtracted from the nominal diameter — 6 mm minus 1 mm pitch gives a 5 mm drill, the familiar rule of thumb that turns up across every metric size.
Identification is only as good as the caliper reading. Plated and worn fasteners measure under nominal, so treat a match within about 0.1 mm as confirmation rather than proof.