How Metric Screws Are Measured

Metric screws may have coarse or fine threads.

Metric screw measurement is the same as those measured in inches, except the measurements are in millimeters (mm). The measurements indicate the diameter, length and thread pitch of the screw or bolt.

The Designation

M8-by-1.25-by-30 is an example of a typical metric thread designation. It is always in the same order. The M indicates it is a metric screw.

Threads

The number 1.25 indicates the distance between threads. Measuring the thread from one point to the exact point on an adjoining thread is the thread pitch. A thread gauge is a tool used for this purpose and gives precise thread measurements

The Diameter

The number 8 indicates that the bolt is 8 mm in diameter. The unthreaded diameter of the bolt is this dimension. The outside dimension of the threads is the major diameter, and should be the same as the uncut bolt. Screws and bolts are available with a fine or extra fine pitch.

Length

The number 30 indicates the length under the head to the end of the screw or bolt. This example is 30 mm long, or a bit over 1 inch.

References

• Engineer's Handbook: Reference Tables--Metric Thread Size & Tap Drill Table
• Engineers Edge: Standard External Metric Thread and Fastener Sizes M1.6-M18
• The Engineering Toolbox: Threads--Metric Iso 724

Resources

• "Machinery's Handbook Toolbox Edition"; Oberg, Jones, Horton and Ryffel; 2008