Fastener Tech Data|Understanding Series Introduction
The size of a metric screw or bolt is specified as diameter, pitch and length, in millimeters (millimeters is abbreviated "mm"). For nuts, size will appear as diameter and pitch. There is an exception though—if pitch isn't specified, the fastener is coarse threaded. 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)
This example illustrates how a coarse threaded screw may appear…
M12 x 25 = 12 mm diameter, coarse thread is assumed (1.75 mm), 25 mm long
M12 is a matching coarse threaded nut
(To avoid confusion, we always include the thread pitch for both coarse and fine metric fasteners.)
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 table 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, as we've mentioned, coarse is assumed when pitch isn't specified. See the table 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 table 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 table 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 heads, length includes head height. Measure oval heads from the largest diameter point—where the dome top meets the conical bearing surface—to the tip.
Metric fasteners are commonly manufactured to meet standards set by DIN - German Institute for Standardization or ISO - International Organization for Standardization. Consequently, they are referred to by the applicable standard number. You may also see JIS - Japanese Industrial Standard and ANSI - American National Standards Institute specified. Following is a table of common DIN and ISO numbers and a brief description of the fastener…
| 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 fastener
- 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 fastener
- Standard: the DIN, ISO, JIS or ANSI standard number that applies to the fastener
Below is a partial list of metric sizes (diameter-pitch) Fastener Mart 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 |
The differences between inch and metric thread sizes are summarized in these tables…
- Thread Size Comparison Chart 1
- Thread Size Comparison Chart 2
Some metric fasteners are available in different strength grades, which is referred to as "property class" or just "class." See Understanding Fastener Grades and Classes for a comparison between metric and inch series strength grades. Also, visit Understanding Fastener Materials and Understanding Fastener Platings and Finishes for more information relating to metric fasteners.
Fastener Tech Data|Understanding Series Introduction
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