What Is An M8 Screw? Size Mean [Dimensions and Diameter]

Fasteners, like screws, are necessary to ensure that machinery, furniture, equipment, and other similar items are assembled securely. But there are still people who are confused about the screw sizes, including the M8 screw size.

In the majority of situations, you must utilize the correct size of the fastener. Remember, screws come in different sizes and are made of various materials to hold together multiple types of equipment.

Additionally, the benefits of screws are contrasted against nails while considering their use. They are strong and offer a long-lasting bond with their less rust susceptibility feature.

When opposed to nails, screws have the advantage of being more reusable due to their strength and extended life. Because they seldom flex during the removal process, you may use these screws for another purpose.

Hence, it is crucial to be knowledgeable about the proper usage of screws depending on their sizes. Let’s start this tight discussion with an introduction of the M8 screw for your next project.

What is an M8 Screw?

Metric screws are generally measured using the ‘M’ measurement system. In this matter, the millimeter (mm) measurements specify metric fastener sizes by diameter, pitch, and length.

Therefore, the ‘M’ on the M8 screw signifies that it is metric, and the number of millimeters the shank size is in diameter.

In line with this, the diameter of an M8 screw is 8mm. Plus, the M8 bolt’s default thread pitch is ranging from 1.25 or 1.0 mm.

More so, for the perfect size drill bitfor the #8 screw, you may use the following:

  • Tapered Bit (for the hardwood) – 11/64″
  • Sharp Bit (for the softwood) – 5/32″
  • Straight Bit (for the hardwood) – 1/8″
  • Straight Bit (for the softwood) – 7/64″

It is essential to remember that the United States used the International System of Units (Metric ISO) as of late. It has been the EU, Japanese, and Australian standard for more than 30 years.

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How to Read The M8 Screw Size Specification

The specifications of an M8 screw may appear as: M8-1.0 x 20. The following standard applies to the screw:

  • This bolt has a metric thread according to the stamp ‘M’ indicated on its specs.
  • The nominal bolt shaft diameter is eight millimeters.
  • The thread pitch of the bolt, measured in millimeters, is 1.0.
  • Furthermore, the last measurement, 20 millimeters, is the length.

However, it is quite usual to find metric screws described in this manner: M12-50.

This shortened format has been modified to exclude the pitch specification, thereby resulting in a coarse thread. The bolts always use coarse threads when the pitch measurement is missing from a metric bolt specification.

In this instance (M12-50), the diameter is 12mm, and the length is 50mm.

While working with inch fasteners is commonplace for you, the pitch may be rather perplexing. It is because of the distance between the threads.

The distance from one thread to the next in the example shown is 1.0 millimeters. Screws that are defined as threads per inch, or sometimes termed pitch, are called inch fasteners.

For you to measure the metric fastener threads per inch, first convert pitch from millimeters to inches (multiband by 0.03937), then divide one by that amount.

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Standard M8 Screw Sizes

M8 screws have non-interchangeable thread sizes because of the different thread angles and pitches. For your reference, here are the standardized sizes according to fine and coarse.

The standard size for the Metric Coarse is M8 x 1.25. Meanwhile, we have M8 x 1 for the Metric Fine.

Furthermore, when it comes to British Standard Fine or British Standard Whitworth, we have a standardized 5/16″ size.

In addition, for Unified National Coarse Thread, the size is 5/16 – 18. On the other hand, we have 5/16 – 24 standards for the Unified National Fine Thread.

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M8 Screws Standard Features and Requirements

Since metric screw units correlated to different sized nuts and drilled holes, manufacturers developed specific standards for screws. Additionally, the screws are made with thin or coarse threads that represent the pitch of the line of a screw.

Coarse-thread screws have a lower susceptibility to seizing, cross-threading, and galling. The opposite is true for fine-thread screws.

Fine-thread screws are more likely to come loose from movement and can be tapped and adjusted more easily. There is no need to identify the thread type of screws that cut their threads in a soft material.

For this reason, it is indeed essential to know the features and requirements of M8 screws before purchasing them.

Standard Thread Lengths

The definition of thread length is included in the metric bolt standard. However, if it is not mentioned, then a coarse pitch is assumed.

Metric bolts are offered in coarse or fine thread pitch with extra fine pitch choices on certain diameters.

Additionally, you have to remember that metric bolts have more threads per inch than equivalent imperial bolts. It is because they have a coarser thread pitch.

The standard thread length for an M8 screw is 125 mm, and the shorter size is 22 mm. Meanwhile, the standard for the M8 screws with a length of longer than 125 mm is 28 mm.

Metric Bolt Hole Dimensions and Drill Bit Sizes

The diameter of metric bolts is more than the shaft diameter. Because the shaft diameter of an M8 bolt is under 8mm, it should fit through an 8mm hole.

Nonetheless, it is typical to drill clearance holes somewhat bigger to provide a little variance in placement. When cutting a thread is required, the hole size should be a lower diameter to allow for tapping.

Calculating the size of the tapping hole required to tap a metric bolt is simple if you know the measured bolt diameter.

A typical coarse thread pitch of 1.25mm M8 bolt, for instance, requires a hole diameter of 6.75mm to be tapped. The hole would be 7mm if the M8 bolt had a 1mm thread pitch.

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Here are the list examples of hole dimensions and drill bit sizes of an M8 screw:

  • Clearance Hole (mm):9
  • Thread Pitch (std):25
  • Tapping Drill (std Pitch):75 mm
  • Thread Pitch (fine):
  • Tapping Drill (fine pitch):7 mm

Allen Key Sizes and Spanner

It is crucial to use the proper thing for the task. Remember, you should use the correct-sized spanner or Allen key to prevent sliding the screw.

Take note; when you use a spanner or Allen key that is too big, you may damage the nut or bolt. So, here are the standard sizes of these tools that you can use for the M8 screw.

You may use a spanner with a size of 13 mm for an M8 screw. On the other hand, a 6 mm Allen key is the standard size for you to tighten an M8.

Standard Proof Load

If a bolt is progressively loaded, there comes a moment when it starts to strain and. After that, a point when it breaks. The standards specify a proof load when determining how much weight a bolt should be permitted to bear.

 Remember, the proof load is usually about 90 percent of the weight, then the bolt starts to strain.  

Proof loads are usually provided in Newtons in manufacturers’ tables, but we’ve translated it into kg force. The proof load is generally regarded to be the absolute greatest load that a bolt should be required to bear.  

In today’s condition, manufacturers will form their machinery where the bolts carry a lot less than the proof load. This factor gives a good safety factor to the design.

Here is the list of a standard proof load of an M8 screw:

  • Grade 8.8 – 2 120 kg
  • Grade 9.8 – 2 380 kg
  • Grade 10.9 – 3 040 kg
  • Grade 12.9 – 3 550 kg

Maximum Fastening Torque

You may adjust the tension in the screw by performing a bolt up to the torque to get an 85% proof load. Remember, if the threads are gently lubricated, the maximum suggested torques will work.

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For your reference, here are the maximum fastening torque standards for M8 screws:

  • Grade 8.8 – 28.8 Nm
  • Grade 9.8 – 32.3 Nm
  • Grade 10.9 – 41.3 Nm
  • Grade 12.9 – 48.3 Nm

Take note; you may only use these recommendations if the manufacturer does not specify the torque in the manual.

Standard Bolt Sizes

To avoid complications while using your M8 screws, you should know the standardized sizes. You will see what you should buy from the hardware stores that suit your needs through this factor.

For an M8 screw, it should have a 1.25 mm size in pitch. Plus, the length of the threaded portion should be 22 mm.

Moreover, the screw must have a 5.3 mm head depth (k). In terms of across flats (s), it should be 13 mm.

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Frequently Asked Questions (FAQs):

Is 5/16 The Same As M8?

Screws in the sizes M8 and 5/16 are not the same and cannot be used interchangeably. They are, nevertheless, capable of functioning under specific conditions.

It is important to note that M8 is in metric and has an 8 mm bolt diameter. Also, it usually comes with an additional number, such as 1.0, which serves as the pitch (distance between threads) in mm. This factor is the main difference with the 5/16 bolt.

5/16 inch bolts do not indicate pitch. It gives the thread count per inch. While fine and coarse are the most frequent classifications, you may encounter just two types of materials.

What does M8 1.25 mean?

M8 1.25 is the thread pitch of the screw. The term thread pitch is used to measure the disparity between two threads.

When it comes to metric fasteners, the thread pitch is utilized instead of TPI. The distance is in millimeters, too.

The M mark means metric. While 8 implies 8mm nominal diameter (the thread will be shorter than 8 mm at the highest point). Then, 1.25 states 1.25 mm per thread (the standard coarse pitch for the line).

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Is M8 Bigger Than M6?

The M8 screw is bigger than the M6 screw. For this reason, you may use an M6 for the bike threadings while M8 for the wall mount.

M6 is a 6 x 1-millimeter thread. You may also use this for other items, like brake mounting bolts, threadless stems, and so forth.

On the other hand, M8 has a thread pitch of 8 x 1.25 mm. Also, common screws are 8 mm pitches.

Is 8mm The Same As M8?

The basic major dia of the M8 x 1.25 is 8 mm. Hence, 8mm is an equivalent of M8. However, you should remember that there may be other screws that have an 8 mm size.

Furthermore, the basic major dia in the inch of M8 x 1.25 is .3150 inches. Take note of these conversions when buying screws in your hardware stores.


If you use the appropriate screws for your tasks, you will avoid screwing things up. However, before you use your M8 screw, you should ensure that it meets all criteria and specifications.

This article should serve as a guide for you if you’re going to buy your screw. Remember, size does matter, but don’t let it get the best of you by being too concerned with it.

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