When selecting the right screws for a project, knowing your screw size is vital. However, manufacturers often use different nomenclature when describing the size of their fasteners and this can make choosing the correct screw very confusing. A quick glance at a screw size chart can help you avoid getting the wrong type of screw and save you time and money.
A good rule of thumb is that the head diameter of a wood screw should be roughly double the shank (long section) diameter. This holds true whether the shank is threaded or not. Similarly, the length of a screw should be about half the thickness of the material that the screw will enter into.
When a screw has threads, the corresponding thread size is often listed in the callout. This information is usually found in the second number of the screw size callout, which is also referred to as the screw’s gauge. The diameter of a screw’s threads is measured in ‘treads per inch’ (TPI), and the more threads a screw has, the finer it is classified as.
The threads on a screw are a key factor in its strength, but the head of the screw is what creates most of the stress on the screw during usage. Because of this, it is important that the head of a screw has sufficient’stress area’. This is reflected in the size of the screw’s head, and can be measured using a screw gauge or a simple ruler.
While the screw’s gauge and length are fairly easy to determine, it can be more difficult to figure out the precise diameter of a screw. This can be especially challenging if the screw is being used in an application with a high tolerance. Screws that are oversized or undersized are more likely to damage the fastener and cause other problems such as looseness or failure.
Many manufacturers classify their screws by two numbers and a nominal length. The first number, which is often referred to as the screw’s major diameter, represents the screw’s overall diameter. The second number is the screw’s thread pitch and is typically represented as either a decimal or a fraction. For example, a screw with a #9-15 x 1″ head has a major diameter of 1.25 inches and a pitch of 40 threads per inch.
While a screw’s lead and pitch are closely related, they are sometimes confused by novice users. This is because many screw sizes are referred to by both the lead and the pitch, which can be extremely confusing. To distinguish these two dimensions, look for a screw with a callout such as “#9-15 x 1-1/2” or “#10-40 UNC-3A x 2”. The “UNC” in the callout indicates that this is a coarse screw, while the number represents the pitch, which is very similar to that of a regular screw. This makes it much easier to figure out the exact diameter of a screw. Likewise, metric screw sizes use a different system of measurement that is significantly more straightforward to understand. screw size chart