The Ultimate Guide To Measuring Screw Sizes

There are several ways to measure screw sizes. The three main screw measurements are gauge, length, and threads per inch (TPI). The gauge refers to the outside thread diameter, also known as the major diameter. The length is the distance from the head to the tip of the screw, and the TPI is the number of threads in a one-inch section of the screw. It's important to measure screws correctly to ensure you get the right size for your project. You can measure screws using either the imperial or metric system, depending on how the measurements are listed where you are shopping for screws.

Length of the screw

Measuring the length of a screw is a straightforward process, but it's important to note that the method can vary depending on the type of screw. The length of a screw is typically measured from the tip of the screw to the flat part of the head. Here are some specific guidelines to follow:

Flat-Head Countersunk Screws

Flat-head countersunk screws are designed to sit flush with the surface they are embedded in. To measure the length of this type of screw, start from the top of the flat head and measure to the tip of the screw. This includes the length of the head in the measurement.

Round-Headed Countersunk Screws

Round-headed countersunk screws only sink partially into the surface, so the rounded top will protrude above the surface. To measure the length, start from the bottom of the rounded top, where it meets the countersunk portion, and measure to the tip of the screw.

Non-Countersunk Screws with Round Heads

For screws with round heads that are not countersunk, measure from the flat underside of the screw head to the tip.

Non-Countersunk Screws with Flat Heads

For screws with flat heads that are not countersunk, measure from the bottom of the head to the tip of the screw.

Oval Head Screws

Oval head screws are an exception to the typical rules. Measure the length of these screws from the widest point of the head to the tip.

Self-Drilling Screws

Self-drilling screws are measured from the very tip of the drill point to the blunt end.

Grub Screws and Threaded Bars

For screws without a head, such as grub screws or threaded bars, measure the length from end to end.

Hex and Hex Washer Head Screws

Although hex and hex washer head screws have flat surfaces on both the top and bottom, the length is typically measured from the bottom of the head to the tip of the screw.

Nominal Length vs Overall Length

It's important to distinguish between the nominal length and the overall length of a screw. The nominal length is the standardised size used to define the length of the fastener, while the overall length refers to the actual physical length of the screw. Nominal lengths are often used in product names or catalogue listings to provide a simplified term for convenience. Nominal lengths typically relate to the functional length of the screw or are rounded to the nearest millimetre to avoid unwieldy decimals.

Diameter of the screw

To measure the diameter of a screw, you need to measure the width of the threaded part of the screw. This is also known as the screw size. Place the end of the screw against a ruler or measuring tape and measure across the widest part of the screw's threads. If you are using a metric system, the diameter of the screw will be listed in millimetres. For example, a 5 mm screw would be labelled as 5.0 on the packaging. For the imperial system, measure the width of a thread in fractions of an inch. For instance, a #0 gauge screw is 1/16 of an inch in diameter, #1 is 5/64 of an inch, and a #2 is 3/32 of an inch. You can also measure the minor diameter, which is the bottom cavity of the threads from one side to the other.

The major diameter is the largest diameter of a screw's thread and is measured from the crest (top) of the thread on one side to the crest of the thread on the other side. The pitch diameter is the diameter of a theoretical cylinder that exists halfway between the major and minor diameters. The nominal diameter is shorthand for the "nominal major diameter", which is an estimated major diameter.

Thread pitch

In the metric system, thread pitch is measured in millimetres. The distance between two adjacent threads is measured, usually with a caliper or thread gauge. For example, a metric screw with a pitch of 0.4 mm will have 0.4 mm between each thread peak. The higher the pitch number, the fewer threads there are.

Screw head types

The two basic screw head types are countersunk and non-countersunk. The former requires an angular shape underneath the head to prevent splitting wood when drilling or using force. The latter, meanwhile, is flat under the head and does not need countersinking. Here is a list of the different types of screw heads:

Countersunk Screw Head Styles

• Flat: These sit entirely flush with the material's surface, preventing things from catching on them. They are used in wood-worked consumer products such as cabinetry and woodworking applications.
• Oval: Also called raised screw heads, these have a similar angle to flat heads and require countersinking. This decorative dome-shaped head is often used for switch coverings.
• Bugle: Bugle heads are used primarily on drywall, wood decking and plasterboard screws. This shape reduces damage by distributing stress over a wider surface than flat heads.

Non-Countersunk Screw Head Styles

• Hex: Hex heads are designed to allow for greater torque and typically require a wrench or socket for installation.
• Modified Truss: Also known as wafer head screws, K-Lathe screws, and round washer head screws, these combine the benefits of the pan head design with an attached washer built onto the head to increase head diameter.
• Button: A rounded design typically used in socket-driven screws.
• Domed: This dome-shaped head features a flat inner part that helps it sit flush on the surface. It is a popular screw head type used to enhance aesthetics.
• Pan: The most common type of rounded head screw. They are used in many applications that require a flat-bottomed screw.
• Round: This design is becoming less common but offers a rounded appearance.
• Fillister: Fillister heads have a slightly rounded top and tall cylindrical sides. They have a deeper drive slot than round or pan head screws.
• Truss: Used for sheet metal, truss head screws have a wider and slightly rounded surface. They generally provide a lower profile and larger bearing surface than round or pan heads.
• Flange: This style features a circular flange directly underneath the head. The flange can even take the place of a washer for specific projects.
• Pancake: Distinguished by their round, low-profile heads, pancake head screws are ideal for fastening panel clips to wood.
• Socket Cap: These heads are unique to socket drive recesses and install flush against the surface.
• Square: Square heads are used in bolts and screws and typically require a wrench for installation and removal.

Metric vs imperial

When it comes to measuring screw sizes, you need to know whether you're working with the metric or imperial system. The metric system is the most common, with the majority of screws measured in millimetres. However, there are times when you may need to use the imperial system, which uses inches or fractions of inches.

Metric System

In the metric system, the diameter of screws and their length are measured in millimetres. For example, a screw marked 5.0 x 60 has a diameter of 5 mm and is 60 mm long. Metric bolts are designed according to the metric system, and it is easier to scale up and down compared to the imperial system.

Imperial System

The imperial system is standard in the United States. The first number in the imperial system refers to the gauge of the screw, which relates to the diameter. The larger the number, the larger the diameter. For example, a #0 gauge screw has a diameter of 1/16 of an inch. The length is also measured in inches. A package labelled 10 x 2" means the screw has a gauge of 10 and is 2 inches long.

Choosing Between the Two

Sometimes, the choice between the two is a matter of preference. However, if you are selling products in a market that predominantly uses one system, it would be wise to use that system. For instance, some US-based manufacturers selling more volume in Europe use metric fasteners in their products.

If you need a wide variety of sizes within a relatively small range, the metric system is better as it is easier to scale. The process of subdividing units of measurement is also more straightforward in the metric system.

Frequently asked questions