How to measure bolt size

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Measuring a bolt: the tools you&#;ll need

Are you measuring metric bolts or standard bolts? Standard bolt sizes are slightly different and are measured in inches while metric bolts are measured in millimeters (mm). 

Otherwise, bolt measurements are carried out the same way, except when it involves threads, which we&#;ll get to. For the majority of bolts and screws, right-hand thread is most widely used. Find out more in our guide, Left-handed vs right-handed fastener threads.

For measuring bolts with accuracy, you&#;ll find these tools helpful:

• Caliper: You&#;ll need this for measuring metric thread pitch. Standard uses threads per inch and doesn&#;t require a caliper for that task, but it&#;s helpful for determining if thread on a bolt is straight or tapered.

• Bolt size gauge: A handy tool that lets you fit bolts through bolt-sized holes. The bolt gauge is divided into standard and metric sections. Simply push the bolt into the holes on the bolt gauge and find the smallest hole the bolt will fit into and you&#;ll have the size you need. 

If you don&#;t have either tool at hand, you can use a ruler for measuring bolts.

Measuring a bolt correctly requires that you know the different areas of the fastener, as shown here.

A hex shape is perhaps the most common head associated with bolts, but it&#;s by no means the only one. Measuring u-bolts is of course very different, but we&#;re looking at measuring bolts that are more familiar. Thread length also varies according to the bolt. Some are fully threaded while others, such as the one here, are partially threaded. Thread length is an important part of your bolt dimensions. 

To learn more about bolts, bolt sizes, the different types, grades, standards and thread sizes, read our guide, What are the different types of nuts and bolts? 

Step 1: How to measure bolt diameter

Measuring metric bolts for diameter is carried out the same way as when determining standard bolt dimensions.  

• To determine bolt diameter, measure the distance from the outer thread on one side to the outer thread on the other side. This is called major diameter. 
• Alternatively, you can measure the bolt shank diameter. The shank is known as the body or sometimes, the shoulder. This is the threadless part of the bolt. The bolt shank is important, as it prevents radial movements. Without a shank, there's a greater chance of the bolt loosening. In some cases, the shank diameter and the major diameter may be the same, but in others, the major diameter may be larger than the shank&#;s diameter due to the threads. 

You also might find it helpful to check out our guide, How to keep bolts from loosening.

Step 2: How to measure bolt length

Just as with diameter, both standard and metric bolt lengths are measured the same way. How you measure bolt length depends on the bolt head dimension. 

• When the head protrudes above the surface: Determining hex bolt dimensions is a good example. Measure bolt length from the tip, or bottom, of the bolt to where the shank meets the head. 
• When you&#;re dealing with a countersunk bolt: A countersunk bolt will sit flush with the surface. For example, a carriage bolt must be countersunk to work properly, even though it has a dome shaped head. So you&#;ll measure its bolt length from the top of the head to the bottom of the threaded part. Another way to think of it: the length of a carriage bolt &#; or all countersunk bolts &#; includes head height. 

Step 3: How to measure threads

Measuring bolts involves thread length, specifically, determining the thread pitch for metric threads. For standard fasteners, you&#;ll use thread per pitch, called TPI. 

It&#;s absolutely critical to determine thread pitch or TPI. Both ensure that your nut and bolt are going to couple together as they should. If they&#;re dissimilar, they&#;ll seize or strip the threading. 

How is thread pitch measured?

Thread pitch is simply the difference between two threads. Threads per pitch, on the other hand, refers to the number of threads on a bolt. Specifically, it tells you how many threads are in an inch.  

How to find thread pitch

• Using your caliper, measure from the peak of one thread to the next, expressed in millimeters. That&#;s the thread pitch for your metric bolt. 

How to find threads per pitch

With TPI, you don&#;t measure thread. You simply count the number of peaks within a one-inch area. For example, in the illustration here, we count six threads in one inch.

Learn more about threads in our guide, Identifying threads and connectors.

Bolt dimensions explained

You need to know how to read bolt sizes. Let&#;s say that your metric bolt dimensions are:

Diameter: 2mm
Length: 5mm

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Your thread pitch measurement is .4mm.

Metric bolts are designated by the letter M. So a bolt with a 2mm diameter is 2M. The bolt size thus appears as 2M x .4 x 5mm. 

For standard bolt dimensions, let&#;s say you came up with these measurements:

Diameter: 1 1/2&#;
Length:    3 1/4&#;

For threads per inch, you counted 6 threads. Your bolt measurement is 
1 1/2&#; &#; 6 x 3 1/4&#; (or 3.250)

Standard bolts and screws are either Unified National Coarse Thread (UNC) or Unified National Fine Thread (UNT). To learn more, see our guide, What is the difference between metric and standard fasteners?

You&#;ll also find a metric and standard bolt size chart for comparing sizes. 

How to protect bolts

Bolt and nut protection caps serve several functions on your assemblies &#; especially if your application is outdoors &#; so don&#;t overlook their excellent benefits: 

1. Aesthetics: conceals unsightly threads, giving a finished look to the overall assembly.
2. Prevent injury: covers sharp threads to prevent contact
3. Prevent corrosion: protects bolt thread from exposure to moisture, chemicals, and other corrosive substances
4. Prevent damage: protects threads on a bolt from damage during shipping, handling, and installation, prolonging the life of the bolt
5. Protect against weathering: stands up to environmental factors to protect bolts
6. Provide security: discourages loosening or tampering

Examples of cap protection for both standard and metric bolt head sizes:

Bolt head and nut caps

• Ideal for bolts, nuts, screws and washers
• Excellent for outdoor applications, such as equipment and machinery and construction
• LDPE with UV resistance
• Easy push-fit installation
• Available in black, white and gray

Secure cover caps

• Ideal for bolt heads and nuts 
• Excellent for outdoor applications, such as outdoor recreation equipment
• Cap design snaps securely in place (sold separately)
• Polyethylene
• Available in white, red, yellow, blue, green and black

Learn more in our guide to cover caps.

Banjo bolt caps

• A versatile protection cap & retaining washer in a single unit 
• Holds everything together during assembly, storage and shipping
• Easy to grip
• Protects against damage & ingress of dirt
• Supplied with bridged retaining loops 

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 make sure you&#;ve chosen the right product for what you need. If you&#;re not quite sure which solution will work best for your application, 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?

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