Identifying the correct screw size for an application starts with determining which of the screw characteristics is most important. Depending on the screw type this is usually the diameter or the quality.
Screw thread sizes are generally referred to by their first number; the imperial system uses a gauge figure and the metric system has a major diameter measured in millimetres.
Many screws in the metric system have their measurement listed as M followed by a number, for example M4. The first number indicates the screw diameter and the second number is the pitch. Usually there are two different pitches – coarse and fine – for each diameter but sometimes there is an extra-fine pitch which is useful for things like wood screws that require a very small pilot hole to be drilled. Coach screws have a hexagonal head which takes a hex key and are therefore more likely to need the smaller diameter.
Metric screw sizes can be determined from the major and minor diameters, or by counting the threads per inch (twists/pitch). If you’re buying in imperial it is best to use a handy conversion chart as it can be confusing! Also see our guide on Metric Imperial Screw Sizes. Many metric screws are self-tapping which means that they have a tip which can cut through the material it is being screwed into, so no need for a pilot hole!
When screw diameters are listed in imperial units, they are usually presented with the gauge size first followed by the length of the screw in inches. They do not typically show a number of threads per inch (though some screw manufacturers may include it somewhere on the packaging). For example, a screw with a gauge size of 4 would have a head size that is roughly twice the diameter of its shank.
The relationship between the gauge number and the screw head size is not as simple as a direct multiple (like 2 x head) of its diameter, especially for screws with a countersunk head, which have an oval top. However, it is a convenient relationship to remember as a rule of thumb.
Fortunately, the metric system is becoming more and more widespread around the world. Most manufacturers and fasteners now use it exclusively, so it is very important to know both systems in order to compare and search for products.
Choosing the right screw size is essential to a successful construction project. A wrongly chosen screw can split wood or even compromise the structural integrity of a building. It’s important to understand the three main measurements associated with screws and bolts – gauge, length and threads per inch (TPI).
Gauge is measured by counting the number of thread peaks across an inch. Then you divide that number by the number of threads per inch for that screw type. For example, a screw labeled #10 x 36 has a gauge of 10 and measures an inch and a half long.
Length is the distance from the end of the shank to the head. This is especially important for screw types that need to penetrate into a thicker material such as wood or metal. This can be measured using a ruler or a tape measure and a screw gauge. Another handy tool is a historic meter stick, which has one side marked in inches and the other in centimeters with alternate markings of black paint or wood.
While the major diameter is found on screw threads, there are also a number of other width measurements that are important to know for certain fasteners like hex, pan, button and socket cap screws. The width beneath the unthreaded portion of the screw is known as its root or minor diameter. The length of the screw, which is measured from the head to the tip of its threads, is another key measurement for non-countersinking screws like hex, pan, bugle, flat and trim heads.
In the imperial system, the shaft length is usually listed in inches and is followed by a decimal and a number or, in some cases, by an abbreviation (for example, #10×36 indicates that the screw has a #12 gauge with 36 threads per inch). Coach screws are listed with the head size first and the length second, such as 6-32 x 1 1/2″.
In the metric system, screw sizes are listed in millimeters and the diameter is identified by the first number. The pitch of a screw, which is the distance between adjacent thread peaks, is more complicated and requires precise tools like a Vernier Caliper to measure accurately.6 screw diameter