Screw sizes can be confusing, especially when they are listed in a variety of ways depending on where and when they were produced. In the UK and Europe, metric formats are used, whereas in the US, imperial units are still in use. Imperial screw sizes are normally presented as the gauge by the length, with the length given in inches and the gauge representing the diameter of the thread of the screw. For example, a #4 screw is designed for small crafts, jewellery boxes, and attaching hinges, and is available in 3/8 inch to 3/4 inch lengths. On the other hand, metric screws are listed by the diameter of the screw thread in millimetres and the length of the screw is also measured in millimetres. So, a 4 x 40mm screw has a thread diameter of 4mm and a length of 40mm.

No. 4 through 7 Screw Size Characteristics

Screw length: long enough to hold boards together, but not too long that they pierce through

Choosing the right screw length is crucial to ensure that your woodworking project holds together securely. If the screws are too short, they may not adequately hold the boards together, but if they are too long, they will pierce through to the other side.

When joining boards, the screw should always go through the thinner piece and thread into the thicker piece. As a general rule, aim for about 2/3 of the screw to be threaded into the secondary (thicker) piece. However, as the thickness of the lumber increases, this ratio can be reduced to 1/2.

The type of joint you are using is an important factor to consider when choosing screw length. Face joints, which bring together the face grain of two boards, are typically stronger than end joints that join the end grain of boards. Face grain is the strongest part of a board and grips the threads of a screw more effectively. Therefore, shorter screws can be used for face joints. For end joints, longer screws are needed to compensate for the weaker, less dense wood fibres that are more prone to splitting and breaking.

For example, when attaching 3/4" plywood to 1x boards, a #8 x 1-1/4" screw is perfect for a face joint, while a #8 x 2" screw is needed for an end joint. Similarly, when attaching 1/2" plywood to 1x boards, a #6 x 1" screw is suitable for a face joint, while a #6 x 1-1/4" screw provides extra gripping power for an end joint.

In addition to joint type, the orientation of the wood grain in the receiving board affects screw length. Screws inserted into end grain require a greater length than those used for attaching boards across the grain. For screwing into the end grain, aim for at least two inches of thread to be embedded in the board.

When choosing screw length, it is also important to consider the forces that will be applied to the boards. If lateral or sheer pressure will be applied, such as in kitchen wall cabinets, thicker screws may be needed to withstand the sliding motion. For withdrawal loads, where boards are pulled straight apart, ensure that at least 1 to 1-1/2 inches of thread is inserted into the receiving board, without poking through the opposite side.

By selecting the appropriate screw length and considering factors such as joint type, wood grain orientation, and applied forces, you can effectively hold your boards together without the screws piercing through.

Screw thickness: thick enough to grab boards, but not too thick that they split wood

#4 to #7 screws are used for a variety of applications, from small crafts and jewellery boxes to attaching hinges and children's furniture. When selecting the appropriate screw size, it is crucial to consider the thickness of the screw in relation to the boards being joined.

Screw thickness, also referred to as gauge, is a critical factor in ensuring the screw effectively grabs the boards without causing damage. If the screw is too thin, it may pull out of the wood, resulting in a weak joint. Conversely, if the screw is too thick, it can split the wood, rendering it useless. Therefore, selecting the appropriate screw thickness is essential.

To achieve the desired balance, the screw should be thick enough to securely grip the boards. Generally, wood screws must be long enough to hold the boards together securely. As a rule of thumb, the screw should penetrate at least half the thickness of the bottom piece of wood. For example, when joining two 1-inch thick boards, a screw with a length of at least 1.5 inches is recommended.

However, it is important to avoid using a screw that is excessively thick. The screw should not be so thick that it splits the wood. To prevent this, it is advisable to pre-drill pilot holes, especially when working with hardwoods that are more prone to splitting, such as oak or walnut. By creating a pilot hole, you can ensure a smooth and precise fit for the screw without forcing or splitting the wood.

When selecting the appropriate screw thickness, it is also crucial to consider the type of wood being used. Softwoods, such as pine or cedar, work best with coarse-threaded screws, as their broader threads provide a stronger grip. On the other hand, hardwoods like oak or maple require fine-threaded screws, which allow for a tighter fit without the risk of splitting.

In summary, when selecting screws for your project, aim for a thickness that is adequate to hold the boards securely without being too thick. By following these guidelines and considering the type of wood and the presence of pilot holes, you can effectively grab the boards with screws while avoiding the issue of splitting.

Screw head-bore size: diameter of the screw head

The head-bore size of a screw refers to the diameter of the screw head. This is distinct from the shank diameter, which is the diameter of the unthreaded portion of the screw's body.

For traditional wood screws, the shank diameter matches the outside dimensions of the threads. Modern production screws, on the other hand, have a smaller shank diameter than the major diameter (the outside of the thread) but larger than the minor diameter (the body of the screw between the threads).

The head-bore size of a screw can vary depending on the type and size of the screw. For example, #4 screws, which are designed for small crafts, jewellery boxes, and attaching hinges, will have a smaller head-bore size than #10 or #12 screws, which are used for heavy-duty construction and hanging solid-core doors.

The head-bore size of a screw can also depend on the shape of the screw head. For instance, round head screws tend to have smaller heads than flat head or oval head screws. As a general rule, the head diameter is about 15% smaller for round head screws.

When selecting a screw, it is important to consider the size of the screw head in relation to the application. If the head size is critical, it may be necessary to contact the supplier for specific dimensions.

Screw shank-hole size: diameter of the smooth part of the screw above the threads

When selecting a screw for a project, it is important to consider the shank-hole size, which refers to the diameter of the smooth, unthreaded portion of the screw above the threads. The shank-hole size is crucial for ensuring a secure and stable fastening.

For wood screws, the shank-hole size should be smaller in diameter than the threaded portion. Ideally, the shank hole should be around 80% of the screw's diameter. For example, if using a #8 wood screw with a diameter of 0.164 inches, the recommended shank hole would be approximately 0.131 inches.

It is worth noting that traditional wood screws have a shank diameter that exactly matches the outside dimensions of the threads. In contrast, modern production screws have a reduced shank diameter, resulting in a deep thread profile that offers superior holding power.

When working with hardwoods, such as oak or walnut, which are prone to splitting, it is advisable to drill a pilot hole before driving the screw. On the other hand, softwoods like pine or cedar are less likely to split, so a pilot hole may not be necessary.

By selecting the appropriate shank-hole size and considering the characteristics of the wood, you can ensure that your screws provide a strong and secure hold for your project.

In addition to the shank-hole size, the length and thickness (gauge) of the screw are also critical factors. The screw must be long enough to securely hold the boards together without piercing through the other side. Regarding thickness, the screw should be thick enough to grip the boards firmly without being too thick, which could result in the wood splitting.

Screw pilot holes: drilling a pilot hole in hardwoods before driving the screw

#4 through #7 screw sizes are typically used for small crafts, jewellery boxes, attaching hinges, etc. When working with hardwood, it is recommended to drill pilot holes before driving the screw. This will prevent the wood from splitting and ensure that you do not round or snap off the screw head due to friction.

Choosing the Right Drill Bit Size

The drill bit you choose for your pilot hole should be slightly smaller than the screw threads. The pilot hole should be large enough to allow the centre of the screw to pass through easily, but small enough that the threads have enough wood to grip onto. For a screw with a dome head, you can use a countersink drill bit to create a round, V-shaped groove that will accommodate the tapered bottom of the screw head. This will prevent the wood from splitting as you bury the screw head and create a neater finish.

Preparing the Wood

Before drilling your pilot hole, mark the spot where the hole will go with a pencil or tape. Then, use the tip of a nail or an awl to create a slight indentation over the spot, which will prevent your drill from slipping. If you are working with two pieces of wood, you can clamp them together or use a bit of wood glue between the boards to keep them steady.

Drilling the Pilot Hole

Place the tip of your drill bit into the indentation you created and hold the drill so that the bit is perpendicular to the wood. Start drilling at a low speed and slowly increase the speed once the drill has grabbed the wood. Drill the pilot hole slightly deeper than the length of the screw to allow for excess material at the end of the hole.

Driving the Screw

Once your pilot hole is drilled, replace the drill bit with the correct bit for driving in your screws. Line the screw up with the hole and slowly press down on the trigger. The screw will enter the hole with light pressure and can be embedded all the way into the wood.

Frequently asked questions