Following on from our other post, Your Complete Guide To Machine Screws. If you want to know how to measure a machine screws size, you have come to the right place. This post will guide you through the correct way to measure the size of screws.
Being able to correctly measure the size of a screw the first time and every time is essential to your success. Proper measurements ensure the correct fit, secure assembly, and optimal performance. I will walk you through the steps and tools needed to measure screws with confidence.
By the end, you will (hopefully) be more confident in how to measure a screw thread.
What are Metric Machine Screws?
Metric machine screws are threaded fasteners that follow the metric measurement system. Unlike their imperial counterparts, these screws use millimetres for measurement, offering a precise and universally understood system.
Although people generally use machine screws to screw directly into a tapped hole, they can however also use them in combination with a nut.
Common Terminology
- Thread Pitch: The distance between threads, measured in millimetres.
- Diameter: The external width of the screws threads.
- Length: The distance from the head to the tip of the fastener.
- Head Type: The shape and design of the head (e.g, countersunk or pan head).
Tools Needed To Measure Machine Screws
Digital Callipers
Callipers
A calliper, also sometimes called a caliper, is an essential tool to measure machine screws diameter and length. People prefer using digital callipers for their ease of use and precision. Further to that, digital callipers that measure to 100th of a mm are good for measuring the diameters of small screws because they give a more accurate measurement.
Thread Pitch Gauge
This tool helps determine the thread pitch of a machine screw, which is the distance between threads. They are also useful for identifying if larger screws or bolts are the standard metric coarse pitch, or are fine pitch thread.
Ruler or Measuring Tape
While less precise than a calliper, these can be useful for quickly measuring larger machine screws lengths and diameters.
How Do You Measure Machine Screws Thread Diameter?
Looking at the images below, you will see the thread diameter of each. To measure the thread diameter, you measure the external size of the threads, not the internal part.
Additionally, do not measure the end of the thread as it tapers down to allow for easy screwing and will likely measure smaller than the actual thread size.
If you are measuring the diameter with callipers, you should take care not to insert the tips between the threads, they should sit on the outside of the thread.
When measuring the diameter, especially with callipers that measure in hundredths of a mm. Metric thread diameters will likely measure very slightly under the stated size, this is perfectly normal. You can find more info on this in our post about metric thread tolerances.
What Is The “M” Size Of A Screw?
The “M” size is simply the diameter of the screw with an M in front. “M” signifies that the screw is metric and the number after is the diameter in millimetres.
For example, If the screw measures 6mm in diameter, then the screw size would be an M6.
How do you measure the length of a screw?
When measuring machine screws with a raised head type like a pan head or a cheese head, you would measure these differently from a machine screw with a countersunk head. I will explain in more detail below. For more information on different types of machine screws, read our post Your Complete Guide To Machine Screws.
How Do You Measure The Length Of Countersunk Screws?
First, we will look at how to measure the length of a countersunk screw. If you look at the image, you will see that to measure a countersunk screw, you will see that you measure the entire length including the head. This reason is that countersunk screws are designed to be screwed in and sit flush with the material surface.
If your measurement is 30mm from the top of the head to the end of the thread, therefore, the screw length would be 30mm.
How Do You Measure Machine Screws With A Raised Type Head Length?
Want to know how to measure machine screws with a raised head type? Let me explain in simple terms how to do this. If you are measuring a machine screw length that is not countersunk, you would measure from underneath the head to the end of the thread. Looking at the image, you would sit the measuring tape against the underside of the head and measure to the tip of the screw.
If for example, the total length was 30mm, then this is the length of your screw.
Putting The Sizes Together
Now let’s put the 2 sizes together after taking the 2 measurements.
First speaking about the thread diameter in the example above, the thread measured 6mm, giving us an M6 machine screw.
Next, on measuring the thread length of each of the 2 types, countersunk and pan head machine screws, they both measured 30mm.
If we put the 2 sizes together, the diameter and the length. This gives us a size of M6 x 30.
What Is The Thread Pitch Of A Machine Screw?
A machine screw thread pitch is simply the measurement in mm between the tips of each thread.
The smaller the machine screw is the harder it becomes to measure the pitch. This is where it can be handy to use a thread pitch gauge to help identify what it is. You would fit the thread pitch gauge in between the threads to give you an accurate reading on the thread pitch..
The standard thread pitch for metric bolts and screws is known as metric course pitch. However virtually all metric machine screws are manufactured in course pitch
In other types of bolts etc, there are also metric fine pitch available. You can learn more about thread pitches in our post, understanding metric thread pitches.
Looking back at our example above of our M6x30, the standard thread pitch for an M6 is 1.0mm. Adding this all together, the size of the screw would be, m6 x 30 x 1.0, or maybe labelled as m6 x 1.0 x 30.
However this is not always the case and they will often just be labelled as m6x30, if the thread pitch is not mentioned then it is usually safe to assume they would be the standard course pitch.
What Is The Thread Length Of A Machine Screw?
Machine screws are typically all threaded the full length. a raised head type like a pan head, the thread length would be the overall length excluding the head.
If you needed to know the actual thread length of a countersunk screw, you would need to deduct the height of the head from the overall length of the screw.
FAQ’s
Q. How are screws measured?
A. Screws are measured by measuring the diameter of the thread and the length of the thread and combining the 2 sizes. You can find a full explanation can be found above on how to measure bolts.
Q. Can you tell me how to measure the size of screws?
A. This post is a full in-depth guide on the correct way of measuring metric screws. It will show you how to correctly measure bolts and screws and explain the differences like measuring a raised head screw or how to measure a countersunk screw.
Q. Does a countersunk screws length include the head?
A. Yes, if you are measuring the length of a countersunk screw, then unlike other types of screws and bolts, you will include the head in the measurement. You can find a full explanation further up in this guide.
Conclusion
By accurately measuring your machine screws, you can be confident of a correct fit every time. Hopefully, this guide has given you a better understanding of how to measure machine screws confidently.
For more detailed information on other types of fasteners, please refer to our other related guides on how to measure a hexagon bolt and how to measure socket screws.
If you have any questions or need clarification on how to measure screws, you can get in touch by calling 01259 218830, or you can drop us an email on sales@boltbase.co.uk.