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 threads
• How to determine screw thread sizes using UTS
• How to measure screw thread sizes using the metric system
• How to choose screw thread size (with imperial and metric size charts)

How to measure screw threads

First, here is an explaination of some basic screw thread terminology:

External thread: Also known as male threads, external threads are found on the outside of a screw or bolt

Internal thread: Also known as female threads, internal threads are on the inner surface of a hollow cyclinder, such as a cap or nut

Thread angle: The angle between the thread flanks

Major diameter: The the largest diameter of the screw

Pitch: The distance from a point on the screw thread to the point on the next thread.  

View more parts of a screw thread, including major and minor diameters, in our guide, Parts of a screw thread.

Sizes of screws can be identified by screw diameter and pitch. 

There are several different systems of measurement used for screw thread sizes. These include the Unified Thread Standard (UTS) – generally known as Imperial – which is used in the United States, and the metric system, which is used in most other countries and is growing in popularity in the U.S. 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?

Fastener thread sizes can either be left or right handed. Understand the differences in our guide, Left-handed and right-handed fastener threads: Everything you need to know.

How to determine screw thread sizes using UTS

The UTS (Unified Thread Standard) 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. 

How to measure screw thread sizes using the metric system

The metric system uses millimeters (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 caliper 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.

Machine screw threads are measured using metric threads. View our range of machine screws.

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

How to choose screw thread size

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

Imperial screw thread size chart

Metric threads 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 dispatch. Request your free samples or download free CADs now.

Questions?

Email us at sales@essentracomponents.com or speak to one of our experts for further information on the ideal solution for your application 800-847-0486.