The size of a metric fastener (screw, bolt, etc.) is specified as diameter, pitch and length, all in millimeters (millimeters is abbreviated “mm”). An exception involves coarse threaded fasteners, in which case only the diameter and length may be listed (coarse threads is then assumed). Consider this size…
M8-1.0 x 20
Here’s what the description means…
M = Metric thread designation
8 = Nominal diameter, in millimeters
1.0 = Pitch (distance from thread to thread), in millimeters
20 = Length, in millimeters (see below)
If you normally work with inch fasteners, pitch can be a little confusing—pitch is the distance between adjacent threads. In the example above, a pitch of 1.0 means that from one thread to the next thread, the distance is 1.0 millimeter. Inch fasteners are described as threads per inch, which is sometimes mistakenly called pitch. If you need to know the threads per inch of a metric fastener, first convert pitch from millimeters to inches (multiply by 0.03937), then divide 1 by that number. Or, use the chart below…
Pitch (mm) |
Pitch (inches) |
Threads per Inch |
---|---|---|
0.2 | 0.0079 | 127.00 |
0.25 | 0.0098 | 101.60 |
0.3 | 0.0118 | 84.67 |
0.35 | 0.0138 | 72.57 |
0.4 | 0.0157 | 63.50 |
0.45 | 0.0177 | 56.44 |
0.5 | 0.0197 | 50.80 |
0.6 | 0.0236 | 42.33 |
0.7 | 0.0276 | 36.29 |
0.75 | 0.0295 | 33.87 |
0.8 | 0.0315 | 31.75 |
1.0 | 0.0394 | 25.40 |
1.25 | 0.0492 | 20.32 |
1.5 | 0.0591 | 16.93 |
1.75 | 0.0689 | 14.51 |
2.0 | 0.0787 | 12.70 |
2.5 | 0.0984 | 10.16 |
3.0 | 0.1181 | 8.47 |
3.5 | 0.1378 | 7.26 |
4.0 | 0.1575 | 6.35 |
4.5 | 0.1772 | 5.64 |
5.0 | 0.1969 | 5.08 |
5.5 | 0.2165 | 4.62 |
6.0 | 0.2362 | 4.23 |
Metric fasteners are usually classified as either coarse or fine referring to their thread pitch, although some sizes are available in an extra fine thread. Coarse metric fasteners, when compared to standard “inch” fasteners (those commonly used in North America), tend to be less coarse—their threads are more closely spaced—for a comparable size. Fine metric fasteners can be more or less fine, depending on size. Coarse is the most common metric thread and is assumed when pitch isn’t specified. See the chart below…
Size | Series | Thread | Pitch (inches) |
Threads per Inch |
---|---|---|---|---|
M14-2.0 | Metric | Coarse | 0.079 | 12.7 |
9/16-12 | Inch | Coarse | 0.083 | 12 |
M14-1.5 | Metric | Fine | 0.059 | 16.9 |
9/16-18 | Inch | Fine | 0.056 | 18 |
M3.5-0.6 | Metric | Coarse | 0.024 | 42.3 |
6-32 | Inch | Coarse | 0.031 | 32 |
M3.5-0.35 | Metric | Fine | 0.014 | 72.6 |
6-40 | Inch | Fine | 0.025 | 40 |
The threads of an M14-2.0 screw, which is coarse, are spaced 0.079 inches (2.0 mm). A 9/16-12 screw, which is comparable in diameter and is also coarse, has its threads spaced 0.083 inches, which are farther apart, so there will be fewer threads per unit of measure (we used an inch in the chart above). The same goes for a coarse M3.5-0.6 and a 6-32 screw—the threads of the 6-32 are more widely spaced so, again, it will have fewer threads for the same length. (M3.5-0.6 and M3.5-.6 are exactly the same—the “0” is included to make sure that the decimal point isn’t missed.)
When you look at fine M14-1.5 and 9/16-18 fasteners, however, the 9/16-18 is finer—it has more threads per inch. But the M3.5-0.35 has close to double the number of threads per inch of a 6-40, so its threads are very closely spaced (0.014 inches, or 0.35 mm).
The chart below compares various metric sizes to the closest inch equivalent…
Metric | Closest Inch |
---|---|
M1.4-0.3 | 0-80 (Larger) |
M2-0.4 | 1-64 (Smaller) |
M2.5-0.45 | 3-56 (Larger) |
M3-0.5 | 4-48 (Smaller) |
M3.5-0.6 | 6-40 (Same) |
M4-0.7 | 8-36 (Larger) |
M5-0.8 | 10-32 (Smaller) |
M6-1.0 | 1/4-28 (Larger) |
M7-1.0 | 1/4-28 (Smaller) |
M8-1.25 | 5/16-18 (Smaller) |
M10-1.5 | 3/8-16 (Smaller) |
M12-1.75 | 1/2-13 (Larger) |
M14-2.0 | 9/16-12 (Larger) |
M16-2.0 | 5/8-11 (Smaller) |
M18-2.5 | 3/4-10 (Larger) |
M20-2.5 | 3/4-10 (Smaller) |
M22-2.5 | 7/8-9 (Larger) |
M24-3.0 | 1-8 (Larger) |
M30-3.5 | 1 1/8-7 (Smaller) |
M36-4.0 | 1 3/8-6 (Smaller) |
Length is measured the same as inch fasteners, except in millimeters. For button, cheese, hex, pan, socket and low socket heads, measure length from under the head. For flat and oval heads, length includes head height.
Metric fasteners are also manufactured to meet standards set by either DIN – German Institute for Standardization or ISO – International Organization for Standardization. Consequently, they are referred to by the standard number to which they apply. Following is a table of common DIN and ISO numbers and a brief description of the fastener…
The chart below compares various metric sizes to the closest inch equivalent…
No. | Description |
---|---|
DIN 84 | Slotted Cheese Head Machine Screw |
DIN 85 | Slotted Pan Head Machine Screw |
DIN 94 | Cotter Pin |
DIN 125A | Flat Washer (without Chamfer) |
DIN 125B | Flat Washer (with Chamfer) |
DIN 127B | Split Lock Washer |
DIN 137B | Spring Wave Washer |
DIN 315 | Wing Nut |
DIN 439B | Machine Screw Hex Nut, Finished Hex Jam Nut |
DIN 911 | Long Arm Hex Key |
DIN 912 | Hex Socket Head Cap Screw |
DIN 916 | Hex Socket Set Screw |
DIN 931 | Hex Head Cap Screw Partially Threaded |
DIN 933 | Hex Head Cap Screw Fully Threaded |
DIN 934 | Finished Hex Nut |
DIN 939 | Double End Stud |
DIN 960 | Hex Head Cap Screw Partially Threaded |
DIN 961 | Hex Head Cap Screw Fully Threaded |
DIN 963 | Slotted Flat Head Machine Screw |
DIN 964 | Slotted Oval Head Machine Screw |
DIN 965 | Phillips Flat Head Machine Screw |
DIN 966 | Phillips Oval Head Machine Screw |
DIN 975 | Threaded Rod |
DIN 980V | Hex Steel Prevailing Torque Lock Nut |
DIN 985 | Hex Nylon Insert Lock Nut |
DIN 6334 | Rod Coupling Nut |
DIN 6797A | External Tooth Lock Washer |
DIN 6797J | Internal Tooth Lock Washer |
DIN 6912 | Hex Low Socket Head Cap Screw |
DIN 6921 | Hex Flange Head Screw |
DIN 6923 | Hex Flange Nut |
ISO 7379 | Hex Socket Head Shoulder Screw |
ISO 7380 | Hex Button Socket Head Cap Screw |
DIN 7985 | Phillips Pan Head Machine Screw |
DIN 7991 | Hex Flat Socket Head Cap Screw |
Note: If you know the DIN or ISO number of the fastener you’re looking for, you can use our search function to locate it quickly.
Remember these tips about metric fasteners…
- Diameter: the larger the number, the larger the screw or bolt
- Pitch: the larger the number, the more coarse the thread (the fewer the threads per unit of measure); coarse is the most common metric thread and is assumed when pitch isn’t specified
- Length: the larger the number, the longer the screw or bolt
- Standard: the DIN or ISO standard number describes the fastener
Below is a partial list of metric sizes (diameter-pitch) ABCO Fasteners offers (be sure to search our store for other sizes)…
Coarse | Fine | Extra Fine |
---|---|---|
M1.4-0.3 | ||
M2-0.4 | ||
M2.5-0.45 | ||
M3-0.5 | ||
M3.5-0.6 | ||
M4-0.7 | ||
M5-0.8 | ||
M6-1.0 | ||
M7-1.0 | ||
M8-1.25 | M8-1.0 | |
M10-1.5 | M10-1.25 | M10-1.0 |
M12-1.75 | M12-1.5 | M12-1.25 |
M14-2.0 | M14-1.5 | |
M16-2.0 | M16-1.5 | |
M18-2.5 | M18-1.5 | |
M20-2.5 | M20-1.5 | |
M22-2.5 | ||
M24-3.0 | M24-2.0 | |
M30-3.5 | ||
M36-4.0 |

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