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…

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…

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…

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…

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…

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…

Below is a partial list of metric sizes (diameter-pitch) ABCO Fasteners offers (be sure to search our store for other sizes)…