How to measure bolts size and thread ?Just a few simple steps.

When you want to know the size of the bolt you use, but you don’t know how to measure it. After reading this article, you can measure the various data of the bolt yourself.

Step 1. Prepare the measuring tools

The tools include vernier calipers and micrometer, Also prepare a notebook and pen to record our measurements.

Step 2. Measure the bolt head thickness

Whether it is hexagon head bolt, square head bolt, round head bolt or other types of bolts, to measure the thickness of its head, in addition to the unit of measurement with millimeter, head shape complex bolts need to be measured in many aspects. Today it starts with the simpler hexagon bolt.

Measure the bolt head thickness

Step 3. Measure the diameter of the bolt head

Use the vernier caliper to jam the head of the bolt, and measure its width and diagonal length. If you use the inner hexagon bolt, you need to measure the internal diameter of the inner hexagon, so as to ensure that your measurement results are more accurate, so as to avoid unnecessary losses caused by the size error.

Measure the diameter of the bolt head

Step 4. Measure the thread diameter

The full-threaded bolt only needs to measure a bottom diameter (d), and the half-threaded bolt needs to measure the diameter of the unthreaded part (ds) to record the values measured in the notebook.

Measure the thread diameter

Step 5. Measure the length of the bolts

Start measuring the length of the bolt bar (L), and then measure the length of the half-threaded bolt (b) Here you can write the specification of the bolt, specification composition = thread diameter (d) * length (L) For example, the diameter of the bolt is 8mm and a length is 50mm. Then the specification of this bolt is the M8 * 50

Measure the length of the bolts

Step 6. Measure the distance between the threads

Thread is divided into fine thread(UNC) and Coarse thread(UNF), generally used is a metric thread (Coarse thread), with a caliper or micrometer to measure the distance between each thread(P). You can also find out the thread distance through the table above. If it is a fine thread, you should accurately measure the spacing of the part of the thread and mark it to the book.

distance between the threads


1. The data of one measurement may not be accurate, so it should be measured several times more. The results were averaged several times.

2. When measuring the thread, if you can not see clearly, you can use a magnifying glass

3. Determine what materials are used in the bolts, such as common stainless steel, carbon steel, titanium alloy, aluminum alloy, brass and so on. Different metals have different strength grades. Like 3.6,4.6,4.8,5.6,6.8,8.8,9.8,10.9,12.9 are the grades of carbon steel bolts, and 45,50,60,70,80 are the strength grades of stainless steel bolts.

4. It is best to draw a drawing, so that we can more intuitively see the details of the product and thread data you want.


The above is the measurement method of the bolt size and thread size, with these data can accurately identify the size of the bolt. If you have other questions, you can contact us directly and will be happy to help you.


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