how to measure bolts

How To Measure A Bolt

There are a few different ways to measure bolts and screws. 

To replace like-for-like, you will need to know exactly what size bolt you are looking for, so here is a simple guide to measuring bolt sizes, with a few of the most common tools to help you measure bolts and screws:

  • A ruler
  • A tape measure
  • Callipers
  • A bolt gauge

At Fixabolt, you will find the sizes on all our bolts and nuts, but occasionally you will need to replace like-for-like and in this instance you will need to know your exact measurements. To measure the diameter of a bolt or screw, you will need to measure the distance across the widest part of the thread. To do this, you can use a ruler or tape measure, although it is not as precise as a set of callipers or a gauge.

To measure the length of a bolt or screw, you will need to measure from the tip of the bolt to the bottom of the bolt head. 

To measure the head diameter of a bolt or screw, for example a M16 hex head bolt, you will need to measure from one side of the bolt head to the other, generally called the flats, which is also the size of the spanner you would need to tighten the bolt.

When measuring bolts and screws, it is important to make sure that you are using the right tools and taking accurate measurements. This will ensure that your bolts and screws fit properly and function as intended.

What is a thread pitch?

Thread pitch is the distance between threads on a bolt or screw. With metric screw threads, the pitch is measured in millimetres by measuring the distance between two adjacent threads.

This is an important measurement when selecting bolts and screws, as the wrong thread pitch can cause problems.

Because this measurement is very small it is advisable to measure the distance between 10 or 20 threads then divide that number by 10 or 20 to give you one thread count.

This will give you the measurement of one complete thread.

What is a bolt gauge?

A bolt gauge is a device that is used to measure the diameter of a bolt or screw. Bolt gauges come in a variety of sizes, and are typically made of steel, plastic or brass. Here's an example of a nut, bolt and thread gauge:

measure a bolt with this thread, nut, and bolt gauge

How do you use a bolt gauge?

To use a bolt gauge, simply place the bolt or screw into the appropriate size hole on the gauge. If the bolt or screw fits snugly into the hole, then it is the correct size. If the bolt or screw does not fit into the hole, or if it is too loose, then it is the wrong size.

How to use a bolt calliper to measure bolts

A bolt calliper is a tool that is used to measure the diameter of a bolt. To use a bolt calliper, follow these steps:

  1. Place the bolt in the jaws of the bolt calliper.
  1. Adjust the jaws of the bolt calliper until they are snug against the bolt.
  1. Read the measurement on the bolt calliper. This is the diameter of the bolt.
  1. Repeat steps 1-3 for the length of the bolt.
  1. To find the thread count, count the number of threads per inch on the bolt.

What are some tips for measuring bolts and screws?

  • Pay attention to the thread pitch when measuring bolts and screws. The thread pitch is the distance between threads, and it can vary from one bolt or screw to another.
  • Be sure to measure the entire length of the bolt or screw, not just the part that protrudes from the object it is attached to.
  • Use a bolt gauge that is made of durable material, such as steel or stainless steel. This will ensure that the bolt gauge is accurate and will last for a long time.
  • When in doubt, always err on the side of caution and choose a bolt or screw that is slightly larger than the hole you are measuring for. It is much easier to make a hole bigger than it is to make a bolt or screw smaller.

Once you have these measurements, you can then select the appropriate bolt or screw for your needs.

Bolt Sizes UK

There are a few different bolt-size systems used in the UK. The most common is the metric system, which uses metric units such as millimetres (mm) and centimetres (cm). Another common system is the British Imperial System, which uses imperial units such as inches (in) and feet (ft).

Metric bolt sizes are specified by the diameter of the bolt, followed by the length of the bolt and finally the pitch (distance between threads). For example, a bolt with a diameter of 6 mm and a length of 60mm with a pitch of 1 mm is called a "6 x 60mm x 1 mm bolt".

The British Imperial System uses a different naming convention, in which the bolt size is specified by the diameter of the bolt, followed by the thread count (number of threads per inch). For example, a bolt with a diameter of 1/4 inch and a thread count of 20 is called a "1/4-20 bolt".

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