If you are unable to identify the screw through its callout, you may need to measure it yourself. The easiest way to measure a screw is with a thread gauge or screw checker.
For example, on a #4-40 UNC-3A x . 5 screw, #4 represents the diameter. It has a diameter of 0. 112 in (0. 28 cm). The diameter is sometimes listed as a fraction, such as ¼. It has a diameter of 1⁄4 in (0. 64 cm). To help decipher the number ratings, look up a conversion chart online.
A #4-40 UNC-3A x . 5 screw has 40 threads per inch. A ¼-20 screw, by comparison, only has 20 threads.
If you see a callout like #4-40 UNC-3A x . 5, the UNC tells you that you have a coarse screw. UNC or coarse screws are the most common type and used for general purposes. UNF or fine screws have a smaller pitch, making them stronger and more resistant to damage, such as from vibration. You may also see a J, like UNJC or UNJF. These screws are bigger and even stronger than regular screws.
Note that the tolerance class is also divided into A and B. For instance, you might see 2A and 2B screws, which are slightly different sizes. A represents the external threads on the screw shaft and B represents the internal ones. For instance, a callout of #4-40 UNC-3A x . 5 corresponds to a 3A screw. This kind of screw is used for tight fits. If you plan on securing a screw with a nut, find one with a matching tolerance class.
For example, #4-40 UNC-3A x . 5 is not a left-handed screw. A left-handed version would be labeled like #4-40 UNC-3A-LH x . 5. Most screws you use at home will be right-handed screws. However, if you find a screw marked with LH, you will know to turn it in the opposite direction!
For unified threads, the length is given in inches. A #4-40 UNC-3A x . 5 screw is 0. 5 in (1. 3 cm) long. The length can be written as a fraction or a decimal, so you will also see screws labeled like #4-40 UNC-3A x ½.
For example, a metric screw callout looks something like M12 x 1. 75 x 85. If you aren’t able to find the callout, measure the screw yourself. If you have a nut or another fastener with a known size, you could also try securing the screw in it.
If you use different labeling systems at the same time, you may end up with screws that look similar but are a slightly different size. Always figure out the labeling system the callout uses.
For instance, on a M12 x 1. 75 x 85 callout, M12 is the diameter. It means the external threads are 12 mm (0. 47 in) wide.
A M12 x 1. 75 x 85 screw, for instance, has threads every 1. 75 mm (0. 069 in). The second number, 1. 75, indicates the pitch. The metric labeling system doesn’t label coarse or fine screws, so you have to pay attention to the pitch. Coarse screws are used in general applications, but fine screws are more resistant to damage. If you don’t see a pitch size listed, assume that you have a coarse screw. Sometimes the pitch isn’t listed because coarse screws are used much more frequently than fine screws.
For example, a callout of M12 x 1. 75 x 85 corresponds to an 85 mm (3. 3 in) screw shaft. Keep in mind that most screws are measured from the end of the shaft to the bottom of the head. The exception is flathead screws, which are measured from the top of the head.
