What are screw thread sizes?

Screw thread size refers to the dimensions of the threads on a screw, which determines how it fits into a threaded hole. Understanding screw thread size will help you choose the right screw for a stable, secure connection. In this guide, we’ll explain how to tell screw thread size, covering:

• How to measure screw thread sizes
• How to determine screw thread size

How to measure screw thread sizes

Screw sizes can be identified by their diameter and pitch. The major diameter is the largest diameter of the screw, while the pitch is the distance from a point on the screw thread to the point on the next thread.  

There are several different systems of measurement used for screw thread sizes. These include the metric system, which is used in most countries and is growing in popularity in the U.S. Also, the Unified Thread Standard (UTS) – generally known as Imperial – which is used in the United States. How to tell screw thread size depends on whether or not you’re dealing with metric threads or unified screw threads.

You can learn more in our guide, What is the difference between metric and standard fasteners?

Metric: How to tell screw thread sizes

The metric system uses millimetres (mm) to measure screw thread sizes. To get your screw size, you’ll also need to measure the screw’s major diameter, but unlike the UNS screws, you’ll use thread pitch instead of threads per inch. While determining threads per inch is straightforward, thread pitch is trickier due to the minute spacing involved. 

Use a calliper for accuracy. To calculate thread pitch, measure the distance between two adjacent threads at their peaks, as shown here. Then take the major diameter of the screw. Let’s say it’s 6mm, which is represented as M6. Now let’s say the pitch is 1mm. Your screw size is M6 x 1mm.

UTS: How to determine screw thread size

The UTS system uses an inch-based measurement for screw threads, and includes different thread series, such as the coarse thread series (UNC) and fine thread series (UNF), which are finer, tighter threads. 

Threads are measured in threads per inch, or TPI. You simply count the number of thread peaks along a one-inch length. You’ll also measure the major diameter of the screw, as shown above. These two measurements will combine to give you the screw size you need. For instance, if your major diameter is ¼″ and you have 20 threads per inch, then your screw size is ¼″ x 20. 

If you want to learn more about how to determine screw thread size, see our guide, What are screw heads, drives and threads?

Screw thread size: determining your screw size

Now that you have your screw measurements, you can use our metric or imperial screw thread size chart below to determine the correct size that you need. Note: blacked-out areas in the imperial chart denote unavailable sizes.

Metric screw thread size chart

Imperial screw thread size chart

Download free CADs and try before you buy

Free CADs are available for most solutions, which you can download. You can also request free samples to ensure the solution you’ve chosen is exactly what you need. 

You might also find it helpful to read our Guide to machine screws. 

If you’re not quite sure which product will work best, or you need help in determining screw thread size, our experts are always happy to advise you.

Whatever your requirements, you can depend on fast despatch. Request your free samples or download free CADs now.

Questions?

Email us at sales@essentracomponents.co.uk or speak to one of our experts for further information on the ideal solution for your application 0345 528 0474.