Metric thread - chart, dimensions, sizes

What is a metric thread?

Metric thread is a type of thread that is characterised by a triangular outline. Metric thread’s dimensions are given in millimetres by the manufacturer.

The thread designation of the metric thread is the letter M, which stands for "Metric". Whereas, just after it, you will see a figure or number which is related to the nominal thread diameter. Its size is given in millimetres.

For example: an M2 metric thread means that the diameter “d” is 2 mm and the thread pitch is standard, which in this case is 0.4.

However, in addition to the above, there are also metric fine threads that have an additional designation. This is the number indicating the pitch dimension P.

Our metric fine thread would be designated as follows: M2x0.25.

What are the dimensions of metric threads? Metric threads chart

Main dimensions of metric threads are in the table below:

What are the additional parameters of metric threads?

Metric threads are also defined by several additional parameters that are worth knowing about:

• r - with this letter, the manufacturer designates the roundness of the tip and the bottom of the thread root. Here, the standard also applies, which is usually 1/10 of the pitch of the thread concerned. 
• a - this letter specifies the opening angle of the thread crest.

Here, roundness is especially important, as edges that are too sharp intensify stresses that can cause damage.

Metric threads vary in their workmanship, which is defined by two more parameters, namely class and tolerance.

The class indicates the accuracy of the workmanship and is divided into three types:

• accurate (threads made with high accuracy), 
• medium-accurate (general purpose threads), 
• rough (used rarely, in difficult cases, e.g. when the material does not allow higher accuracy).

What's more, each of these classes is further divided into three thread twist lengths:

• S, which is small, 
• N, which is medium, 
• L, which is large.

Only the combination of class and twist length gives us an insight into the thread tolerance, which we specify with the letters “h” and “H” (sometimes also “g” and “G”).

What is the difference between metric threads and inch threads?

The main difference between metric and inch threads is their system of measurement. Metric threads use millimetres for the nominal diameter and pitch, while inch threads use inches.

1 inch equals 2.54 centimetres.

However, when selecting threads, you are unlikely to use this information because manufacturers do not mix systems of measurement in their product descriptions. The conversion factor is only useful for the design of equipment and installations in which you will use screws.

An additional difference is the division of the various threads.

Inch threads are distinguished based on their series. Therefore, you can find regular, equal pitch, fine pitch, and extra fine pitch inch threads on the market. And metric - as you already know - there are regular and fine pitch versions. In addition, there are also metric multi-coil and left-hand threads, but these are rarely used.

Can you connect metric threads to inch threads?

Yes, it is possible to combine them using a reduction, also known as an “adapter”. It is useful in cases where you need to merge joints with different measurement systems into a single structure.

What are the dimensions for holes for metric threads?

Holes for metric threads may have different dimensions, we have summarized them in the table below:

Hole table for metric threads M

Hole table for metric fine threads