Understanding Screw Sizes: Diameter Table Explained

Selecting the right screw size is crucial for any construction project. Screw sizes vary depending on the region and industry, with the US using the Imperial system and most other countries using the Metric system. Conversion charts are available to help you find the correct screw size, which is essential for ensuring the integrity of the connection and preventing structural failures. Screw size also affects load-bearing capacity, with larger diameters typically offering higher load-bearing capacities. This paragraph introduces the topic of screw size diameter tables and highlights some key considerations for their application.

Conversion charts: US to metric

The US uses the Imperial system for screw sizes, while most of the world, including the UK, uses the Metric system. This can make it challenging to ensure you have the right screw size for your project.

Understanding Screw Sizes

Screw sizes are determined by their gauge and length. The gauge refers to the diameter of the screw, with larger numbers indicating a larger diameter. The length is the part of the screw that is buried in the wood or other material, and does not include the head of a raised or domed screw.

Metric System

In the metric system, both the diameter and length of screws are measured in millimetres. Metric screw diameters usually start with an "M", for example, M3, M4, or M5. To convert to the equivalent measurement in millimetres, simply drop the 'M' and add an 'mm' (e.g., M3 = 3mm).

Imperial System

In the Imperial system, the first number is the gauge of the screw, which refers to its diameter. Similar to the metric system, a larger number indicates a larger diameter. The length is represented similarly but is denoted in inches.

Conversion Chart Examples

• #10 x 2 1/2 = M5 x 80
• 1" x 8 = 25.4mm x 203.2mm

Please note that these conversions may not be exact, but the margins of error are generally acceptable and less than your own measurements.

Measuring screw size

Length Measurement

The simplest screw measurement to understand is length. Screw length is the distance from the flat part of the screw head to its tip or blunt end. However, the length measurement can vary depending on the type of screw head:

• For screws that are flat on the bottom of the head (non-countersunk), measure from the flat bottom of the head to the tip.
• For flat-topped, countersunk screws, measure from the flat top of the head to the tip.
• Oval-headed screws are an exception, as there is no flat side. Measure from the widest point of the head to the tip.
• For flat-headed, countersunk screws that rest flush with a surface, measure from the top of the flat head.
• For round-headed, countersunk screws, measure from where the oval top and the countersunk half meet in the middle.
• For other types of round-headed screws that aren't countersunk, measure from the flat underside of the screw head to the tip.

Diameter Measurement

Now, let's move on to measuring the width or diameter of the screw. There are four types of diameters you should know:

• Major Diameter: This is the largest diameter of the screw's thread. It's measured from the crest (top) of the thread on one side to the crest of the thread on the other side.
• Minor Diameter: This is the smallest diameter of the screw's thread. It's measured from the root (bottom) of the thread on one side to the root of the thread on the other side.
• Pitch Diameter: This is the diameter of a theoretical cylinder that exists halfway between the major and minor diameters. Practically, you'd use this information for creating new threads rather than measuring finished parts.
• Nominal Diameter: This is a shorthand term for "nominal major diameter." It's an estimated major diameter, rounding up to the nearest whole number.

To measure the diameter accurately, use a digital caliper or a ruler/measuring tape. Measure the distance from the outer thread on one side to the outer thread on the other side to get the major diameter. You can also measure the minor diameter by assessing the distance from the bottom cavity of the threads from one side to the other. Ensure you refer to conversion charts to find equivalent sizes, as needed.

Thread Pitch Measurement

Finally, let's discuss measuring the thread pitch, which refers to the distance between adjacent threads on a screw. You can measure the thread pitch using a thread pitch gauge or a calibrated ruler. For metric parts, the pitch is measured in millimetres, with larger pitches indicating coarser threads and smaller pitches signifying finer threads. For inch parts, measure the thread pitch in terms of the number of threads per inch (TPI). Generally, screws with fewer TPI have coarser threads, while those with more TPI have finer threads.

Matching screw and pilot hole sizes

Matching screw sizes with appropriate pilot hole sizes is crucial to prevent the material being fastened from splitting or getting damaged. Using an incorrectly sized pilot hole can compromise the integrity of the connection and lead to structural failures over time.

Pilot holes are necessary for most screw types, except for self-drilling screws, which can drill their own pilot holes. Screws such as Type B Tapping, Type F Thread Cutting, and Thread Forming Screws for Plastics or Metals require the correct sizing to enable insertion into the material. Even sharp-pointed screws like Type A or Type AB Self-Tapping Screws can split wood or distort metal if the pilot hole is not properly sized.

The process of drilling pilot holes for wood screws is often done in one pass using tapered drill bits. This process requires precise depth control, as drilling too deep can cause the threads to fail to hold, while drilling too shallow can result in the screw becoming jammed. Traditional wood screws require tapered drill bits to achieve a tight fit, maximizing holding power. In contrast, modern production screws have a reduced shank diameter, allowing for the use of straight drill bits, which do not require the same critical depth control.

When selecting the appropriate drill bit size for square drive screws or other fasteners, consider the material properties. Generally, a harder material or a softer screw calls for a larger pilot hole. As a rule of thumb, the drill bit should match the size of the screw shank between the threads. For hardwoods, opt for a slightly larger bit, and for softwoods, go with a slightly smaller one.

Thread pitch

On the other hand, the metric system measures thread pitch in millimetres. To calculate the thread pitch, you need to measure the distance between two adjacent thread peaks. For instance, a screw with an M6x1 thread has a diameter of 6mm and a 1mm distance between the threads.

It is worth noting that standard fasteners, such as those in the Imperial system, are designated by diameter and TPI, whereas metric fasteners include an additional measurement of thread pitch after the diameter. For example, a standard fastener might be 1/4-20, while a metric fastener would be M6x1.

The choice between using Imperial or metric measurements depends on the application and the region. The Imperial system is predominantly used in the United States, while the metric system is more common in other parts of the world, including an increasing trend in the US.

Screw standards and regional variations

The variety of screw standards and sizes that exist today is a result of the Industrial Revolution in the 19th century, which brought about the expansion of railway lines. This expansion created a need for compatible nuts and bolts that could be used in any location. Prior to this, fixings were not standardised, and there were no regulations defining screws and their characteristics. As a result, different regions and countries had their own set of standards and regulations, which caused confusion and difficulties for manufacturers.

To address this issue, different standards organisations emerged in various parts of the world. In Germany, the DIN (Deutsches Institut für Normung) standards were established, while in the United States, the ASME (American Society of Mechanical Engineers) set standards. The UK and its colonies, including India and Australia, adopted the Whitworth and BS (British Standard) standards, while Japan implemented the JIS (Japanese Industrial Standards).

However, these regional standards were not always compatible with each other, creating challenges for manufacturers operating in multiple countries. This led to the creation of the International Organization for Standardization (ISO) in the mid-20th century. The ISO committee worked to define international standards that could be adopted across different sectors, and these standards have become the benchmark for the fixing industry.

Today, the ISO, JIS, and DIN standards are based on the metric system and are closely related. These standards are common in most parts of the world and may even become a global hardware standard. On the other hand, North American countries, including the US, follow the Imperial system, also known as the Unified Thread Standard.

The regional variations in screw standards can be confusing when selecting the right fastener for a project. It is important to use a digital caliper to accurately measure the diameter and length of the screw and refer to conversion charts to find the equivalent sizes if needed.

Frequently asked questions