# M – Metric Thread (ISO)

Figure 1: Demonstration of ISO Metric thread standard. The flank angle is 60°, and both male and female threads are parallel.

The ISO Metric Thread is one of the first internationally approved general-purpose thread types. ISO 68-1 defines the design principles of metric screw threads. ISO metric threads consist of symmetric v-shaped threads. The v-shaped thread form has a 60° flank angle, and male and female threads are both parallel (Figure 1).

## Metric standard

Metric threads come in different pitch sizes for a given diameter: coarse and fine pitches.

• Coarse pitch: Coarse threads have a default pitch size, and they are identified by only their diameter. These threads have a default pitch size according to metric thread dimensions in Table 1 and are the most commonly used thread types. They adhere to ISO 724 (DIN 13-1).
• Fine pitch: Fine threads have a smaller pitch size, and they are used less often. They are recognized by their diameter and pitch size. They adhere to ISO 724 (DIN 13-2 to 11).

Table 1: ISO metric thread size chart (Coarse)

 Thread size (mm) Major diameter (mm) Minor diameter (mm) Pitch (mm) M 3 2.98 2.459 0.5 M 4 3.978 3.242 0.7 M 5 4.976 4.134 0.8 M 6 5.976 4.917 1 M 8 7.974 6.917 1.25 M 10 9.968 8.376 1.5 M 12 11.97 10.106 1.75 M 16 15.96 13.835 2 M 20 19.96 17.294 2.5 M 24 23.95 20.752 3

Table 2: ISO metric thread size chart (Fine)

 Size - Nominal Diameter (mm) Pitch (mm) Tap drill (mm) M 1.0 x 0.2 0.20 0.80 M 1.1 x 0.2 0.20 0.90 M 1.2 x 0.2 0.20 1.00 M 1.4 x 0.2 0.20 1.20 M 1.6 x 0.2 0.20 1.40 M 1.8 x 0.2 0.20 1.60 M 2 x 0.25 0.25 1.75 M 2.2 x 0.25 0.25 1.95 M 2.5 x 0.35 0.35 2.10 M 3 x 0.35 0.35 2.60 M 3.5 x 0.35 0.35 3.10 M 4 x 0.5 0.50 3.50 M 4.5 x 0.5 0.50 4.00 M 5 x 0.5 0.50 4.50 M 5.5 x 0.5 0.50 5.00 M 6 x 0.75 0.75 5.20 M 7 x 0.75 0.75 6.20 M 8 x 0.75 0.75 7.20 M 8 x 1.0 1.00 7.00 M 9 x 0.75 0.75 8.20 M 9 x 1 1.00 8.00 M 10 x 0.75 0.75 9.20 M 10 x 1 1.00 9.00 M 10 x 1.25 1.25 8.80 M 11 x 0.75 0.75 10.20 M 11 x 1 1.00 10.00 M 12 x 1 1.00 11.00 M 12 x 1.25 1.25 10.80 M 12 x 1.5 1.50 10.50 M 14 x 1.0 1.00 13.00 M 14 x 1.25 1.25 12.80 M 14 x 1.5 1.50 12.50

Note: ISO 724 standard specifies threads up to M 300; hence Tables 1-2 shows only the most common thread sizes in use.

## Labeling examples

• M4 x 0.5: This indicates a fine thread with a diameter of 4 mm and a pitch size of 0.5 mm
• M8: This refers to a metric coarse thread with a diameter of 8 mm (which, with reference to Table 1, corresponds to a pitch size of 1 mm)