The diameter of the thread is calculated metric


When naming the diameter of a thread are in principle two values ​​in question. The diameter measured at the crests and the value indicating the spacing of the recesses. The international standard metric ISO thread specifies the diameter at the thread crests as the nominal value.

Metric rule thread

For so-called standard metric threads, the denomination is given as an M number. Except for three small-diameter, 1.2, 1.6, and 2.5-millimeter nominal sizes, all other sizes are natural numbers. The ratio to the thread diameter in the notch or thread determines the thread pitch.

Like the nominal value determination and the thread pitch, the flank angle of the metric control thread is firmly defined. The angle of two adjacent thread flanks is sixty degrees. The nominal value minus the assigned gradient difference gives the theoretical diameter of the fitting object, nut or screw.

Non-metric thread types

In addition to the standard threads, there are other types of threads with different pitch angles. The thread flanks in pipe threads, usually with sealing function, are 55 degrees. Metric trapezoidal threads have flank angles of thirty degrees. Saw threads have depending on the design flank angle of thirty to fifty degrees.

All thread types are equal to the nominal value of the diameter at the thread tips. However, the calculation of the diameter for bolting tools such as nuts and bolts must be done using other mathematical formulas.

motion difference

There must be a difference between two and three tenths between the determined nominal value of the diameter of a thread and the screwing means. This "play" allows the mobility and turnability of the screw in the thread or the nut on the thread.

Tips & Tricks

Extensive lists and configurators are provided on the websites of many manufacturers. They allow the determination of secondary diameters assigned to the nominal values. For example, after the nominal value measurement, you can assign the diameter of suitable bolts and nuts.


Video Board: Screw thread geometry features