Metric ISO threads are the international standard for bolts and screws. Knowing the nominal diameters, pitches (standard and fine) and pre-drilling diameters is essential for anyone working in the workshop, laboratory or prototyping. This guide is a practical reference to keep handy at all times.
How to read a metric thread
The standard designation follows the format M[diameter]×[pitch]. For example, M8×1.25 indicates a metric thread with a nominal diameter of 8 mm and a pitch of 1.25 mm. When the pitch is not specified (e.g. M8), the standard (coarse) pitch is meant. Fine pitches are always explicitly stated.
Standard metric thread table
|
Thread |
Nom. diameter (mm) |
Standard pitch (mm) |
Pre-drilled hole std (mm) |
Fine pitch (mm) |
Fine predrilled hole (mm) |
|
M2 |
2,0 |
0,4 |
1,6 |
0,25 |
1,75 |
|
M2,5 |
2,5 |
0,45 |
2,05 |
0,35 |
2,15 |
|
M3 |
3,0 |
0,5 |
2,5 |
0,35 |
2,65 |
|
M4 |
4,0 |
0,7 |
3,3 |
0,5 |
3,5 |
|
M5 |
5,0 |
0,8 |
4,2 |
0,5 |
4,5 |
|
M6 |
6,0 |
1,0 |
5,0 |
0,75 |
5,25 |
|
M8 |
8,0 |
1,25 |
6,75 |
1,0 |
7,0 |
|
M10 |
10,0 |
1,5 |
8,5 |
1,25 |
8,75 |
|
M12 |
12,0 |
1,75 |
10,25 |
1,5 |
10,5 |
|
M14 |
14,0 |
2,0 |
12,0 |
1,5 |
12,5 |
|
M16 |
16,0 |
2,0 |
14,0 |
1,5 |
14,5 |
|
M18 |
18,0 |
2,5 |
15,5 |
1,5 |
16,5 |
|
M20 |
20,0 |
2,5 |
17,5 |
1,5 |
18,5 |
|
M22 |
22,0 |
2,5 |
19,5 |
1,5 |
20,5 |
|
M24 |
24,0 |
3,0 |
21,0 |
2,0 |
22,0 |
Standard pitch vs. fine pitch: when to use which
Standard pitch (coarse)
Standard pitch is the default choice for most applications. It offers a thread that is stronger, easier to manufacture and more tolerant to dirt. It is the standard in 3D printing, hobbies and most general mechanical applications.
Fine pitch
Fine pitch threads offer greater resistance to spontaneous unscrewing due to the reduced helix angle, more precise adjustment (each turn of the screw corresponds to less feed), and greater tensile strength for the same diameter. They are used in automotive, aeronautics and all applications where tightening accuracy and vibration resistance are critical.
Corresponding hexagonal (Allen) spanners
|
Thread |
Allen key (mm) |
|
M2 |
1,5 |
|
M2,5 |
2,0 |
|
M3 |
2,5 |
|
M4 |
3,0 |
|
M5 |
4,0 |
|
M6 |
5,0 |
|
M8 |
6,0 |
|
M10 |
8,0 |
|
M12 |
10,0 |
Strength classes
Carbon steel screws are classified according to their mechanical strength. The most common classes are: 4.8 for low load general purpose screws, 8.8 for standard structural and mechanical applications, 10.9 for high strength applications, and 12.9 for maximum mechanical performance.
The first number indicates 1/100th of the minimum tensile strength in MPa, the second the ratio of yield strength to tensile strength.
Screws and threaded rods at DHM-online
At DHM-online you will find the entire range of metric fasteners: socket head (Allen), countersunk, pan head, hexagon head screws, grub screws, nuts, washers and threaded rods. Available in sizes M2 to M24 in steel and stainless steel.
Browse the Hardware and Fasteners section to find everything you need.
