How to measure the threads of bolts

Written by christopher john | 13/05/2017
How to measure the threads of bolts
Measure bolt threads to make sure you're using the right bolts. (Bolt image by krotex from

When working on a project that requires bolts, it's important to measure a bolt's threads to make sure you are using the right bolts for the job. The thread count is one of the measurements used to describe the size of a bolt. In measuring the threads, you also need to measure the diameter. Bolts are available with U.S. threads or metric threads. The two are measured differently, with U.S. threads measured in threads per inch and metric threads measured according to pitch.

Measure the diameter of a bolt directly underneath the bolt head. Measure a U.S. bolt with a ruler, in fractions of an inch. Measure a bolt with metric threads in millimetres, using a metric ruler.

Lay a ruler alongside the U.S. bolt. Count the number of threads on the bolt within a 1-inch section. Each peak along the bolt is considered one thread. This number is the thread per inch count, expressed as TPI. For instance, if the measurement is 8, express it as 8 TPI.

Calculate the TPI number on a U.S. bolt less than 1 inch long by counting the number of grooves over a 1/2-inch or 1/4-inch length of the bolt. Multiply by 2 (for the 1/2-inch length) or 4 (for the 1/4-inch length) to get the TPI number.

Lay a metric ruler along the length of a bolt with metric threads. Measure the distance from one thread, or peak, to the next thread, or peak. If the threads are too fine, count the number of peaks in 1cm along the metric ruler, and divide by 10. Express this measurement in millimetres.

Write the measurements for U.S. bolts as: "diameter"-"thread count." An example might be 1/4-8. Express the combined measurements for metric bolts as: "diameter" preceded by an "M" x "thread pitch" or for example, M8 x 1.5.

Things you need

  • Foot ruler
  • Metric ruler

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