Screw Size Table: Standard Reference Guide For All

Selecting the right screw size is crucial for the stability and longevity of woodworking projects. Screw sizes are typically measured in inches or millimetres, with the numbers on screws (#6, #8, etc.) indicating their diameter. The length and thickness (or gauge) of the screw are key factors to consider, as well as the type of wood and the presence of pilot holes. The thicker the wood, the longer and thicker the screw required. Screws that are too short may not hold boards together adequately, while screws that are too long can pierce through the wood. Pilot holes are generally recommended for hardwoods to prevent splitting, but they can reduce the grip of the wood around the screw threads. When attaching two boards across the grain, screws generally don't need to be as long as when inserting into end grain.

Screw length

When choosing a screw length, it is recommended to follow the 2/3 rule, which suggests that approximately 2/3 of the screw should be threaded into the secondary piece. This rule can be adjusted for thicker materials, where only 1/2 of the screw length may be threaded into the secondary piece.

The available screw lengths vary depending on the screw type and size. For instance, #4 screws, suitable for small crafts and jewellery boxes, typically range from 3/8 inch to 3/4 inch in length. In contrast, #10 screws, commonly used for general construction and outdoor projects, are available in lengths from 3/4 inch to 4 inches.

It is worth noting that some screws, known as countersinking screws, can be driven completely into the surface, including the head. In such cases, the screw length includes the head of the screw. On the other hand, non-countersinking screws, like truss-head and round-head screws, do not go all the way into the surface, and their length is measured from beneath the head to the tip.

When reading screw packaging, the screw length is typically specified at the end of the callout. For example, in the callout "#3-48 UNC-2B-LH x .5", the screw length is 1/2 inch, as indicated by ".5" at the end.

Screw head

The head of a screw is the top part that is driven by a tool in order to thread the screw into a material. Screw heads come in a variety of shapes, including flat, round, oval, and square. The shape of the screw head determines the type of tool needed to drive it. For example, a flat-head screwdriver is used for flat-head screws, while a square-head driver is needed for square-head screws.

The size of a screw head is important, as it must be proportionate to the shank and the rest of the screw. Generally, round-head wood screws have smaller heads than flat-head or oval-head screws. For example, the largest wood screw produced by Nettlefolds had a head diameter of one inch and a shank diameter of half an inch. As the head size decreases, the shank size decreases as well, with the largest size reduction occurring between screw sizes 1 and 2.

When selecting a screw, it is important to consider the size and shape of the head in relation to the application. For instance, in tight spaces, a flat-head or oval-head screw may be more suitable than a round-head screw, as the latter may be too large to fit in the space. Additionally, the size of the screw driver or bit that will be used to drive the screw should be considered, as it must fit the screw head properly for optimal torque transfer and to avoid stripping the head.

The screw head can also vary in thickness, which is important to consider when selecting a screw. A thicker screw head may be necessary for applications that require a strong hold, as it provides more surface area for the driving tool to grip and reduces the likelihood of the head being stripped. On the other hand, a thinner screw head may be preferred for aesthetic reasons or in applications where a low-profile screw is needed.

Screw thickness

When joining two pieces of wood, it is essential to use screws with the correct thickness. Screws that are too thin may pull out of the wood, while screws that are too thick can split the wood. Therefore, selecting the appropriate screw thickness is vital to ensure the stability and integrity of the joint.

The standard table for screw sizes typically includes the gauge, the threads per inch (TPI), and the length of the screw. For example, a screw labelled "14-10 x 25mm" indicates a 14-gauge screw with 10 threads per inch and a length of 25mm.

It is worth noting that the length of a screw is measured from the tip of the point to the underside of the head. This means that for screws that need to be flush with the surface, such as a countersunk head, the stated length includes the full length of the screw, including the head.

Additionally, when working with metal, the gauge, TPI, and screw tip will determine the thickness of metal that can be fixed. Coarse thread screws with up to 16 TPI are suitable for thin steel up to 2.4mm, while fine thread screws with more than 16 TPI can accommodate steel up to 12mm thick.

Screw material

When choosing the right screw for your project, it's important to consider the material it is made of. The most common screw materials are steel, copper, aluminium, and titanium. Here's a detailed breakdown of each material and its characteristics:

Steel Screws

Steel is the most common material for manufacturing screws due to its low cost. Carbon steel, an iron and carbon alloy, is often used, with Grade 2 being the most common type as it is highly workable. Steel screws are sturdy and strong, making them suitable for various tasks such as construction and woodworking. They are typically used indoors, as they may rust in moist environments if they are not coated for protection.

Stainless Steel Screws

Stainless steel screws are also widely used due to their inexpensive material and versatility. They are long-lasting and highly resistant to corrosion, making them suitable for outdoor, marine, and coastal applications. They are perfect for projects where moisture exposure is expected. However, they are softer than other types of screws and may lead to potential stripping if overtightened.

Copper Screws

Copper screws are known for their excellent corrosion resistance, making them suitable for both indoor and outdoor projects. They are also good conductors of electricity, which is crucial in plumbing and electrical applications. Copper screws can provide an attractive visual effect with their golden appearance, making them popular for decorative and ornamental projects.

Aluminium Screws

Aluminium screws are lightweight and highly resistant to corrosion. They are often used in situations where weight reduction is essential, such as in the electronics, automotive, and aerospace industries. However, aluminium is not as durable as other materials and may not be suitable for demanding applications.

Titanium Screws

Titanium screws offer a blend of strength and lightness, making them ideal for applications that require both robustness and low weight. They are extraordinarily robust and lightweight, with a high level of corrosion resistance even in challenging conditions. Titanium screws are often used in aerospace, medical implants, and high-performance racing cars. However, they tend to be more expensive than other types of screws.

When choosing screw material, it's important to consider the specific requirements of your project, including durability, weight, cost, and whether it will be used indoors or outdoors. Additionally, coatings can be applied to screws to enhance their strength, protection, or aesthetic qualities.

Screw pilot holes

Pilot holes are drilled into wood or other materials before you fasten them with a screw. They are an excellent way to prevent mistakes and avoid unnecessary damage to your materials. Pilot holes are particularly useful when you need to screw through the end grain or near the edge of a piece of wood, when screwing through dense or thick materials, and when you need to screw in precise locations.

When choosing your drill bit, select a size that is appropriate for your screw size and wood type. The pilot hole size for wood screws should be approximately the same diameter as the screw's shank, minus the threads. Softwoods generally require a pilot hole that is a little smaller than the screw shank diameter, while hardwoods need one that is a little larger since they are less likely to compress under the screw's pressure. If you are unsure, choose a drill bit that is 1/64 inch larger than the diameter of the screw's shank.

The length of your screw determines how deep your pilot hole should be. The hole should be as long as the screw so you can drill it in without too much effort. However, be careful not to make the pilot hole too long, as this can weaken your project by removing extra wood. You can add a piece of tape to your drill bit to mark the desired depth, or use a drill stop to produce the exact length you need.

To drill a pilot hole, first select the right drill bit and measure and mark the location of the pilot hole. Install the drill bit in the chuck of your drill, tightening it completely and centring the bit. Set the drill direction to forward, put on eye protection, and align the drill perpendicular to the surface. Start drilling slowly and then accelerate by squeezing the trigger harder.

Frequently asked questions