Screw Size Conversion Chart

In the metric system, both the diameter of screws and the length are measured in millimeters. The first number in the imperial system is the gauge of the screw which refers to the diameter – the larger the number, the larger the diameter. The length is represented similarly in the imperial system but instead of millimetres, it is denoted in inches.

Metric Screws and Their Approximate Imperial Equivalents

Metric Imperial
Diameter (mm) Length (mm) Nominal Size (Gauge) Length (in)
3 12 4 1/2
3 20 4 3/4
3 25 4 1
3.5 12 6 1/2
3.5 16 6 5/8
3.5 20 6 3/4
3.5 25 6 1
3.5 30 6 1 1/4
4 20 8 3/4
4 25 8 1
4 30 8 1 1/4
4 40 8 1 1/2
4 45 8 1 3/4
4 50 8 2
4 60 8 2 1/2
4 70 8 2 3/4
5 30 10 1 1/4
5  40  10  1 1/2
5  50  10  2
5  60  10  2 1/2
5  70  10  2 3/4
 5  80  10  3
 5  90  10  3 1/2
5 100 10 4
6  50  12  2
6  60  12  2 1/2
6  70  12  2 3/4
6  80  12  3
6  90  12  3 1/2
6  100  12  4

Example how to read the table: M5 x 80 equals approximately to #10 x 2 1/2

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