How to Measure Extension Springs?

Extension springs are powerful tools that consist of a coiled thread of metal that works through a tension load. The spring ends are attached to two items that are supposed to be held apart by the spring. At rest, the spring’s coils are held together by initial tension. It takes a load of specific weight pulling from the side to stretch the springs.

Extension springs store enough energy to provide a steadfast return force that pulls back the load. For all these to work, you need to use the right extension spring for the job, and size is a major determinant here.

There are four main measurements of an extension spring, these are:

Outside Diameter

The outside diameter is the size of the spring from the outer ends of the coils. You are measuring the width of the spring from its outermost margins. It is useful to know this size if the extension spring will fit inside a hole or a tube. The right size will ensure the spring fits the extension appropriately without being too tight or loose.

Inside Diameter

It is similar in all ways to the outside diameter, only that it represents the spring’s width measured from inside the spring’s body, so it’s essential to the diameter of the hollow part of the extension spring. It will help you know if the spring fits a mandrel, shaft, or tube properly.

Notably, the coil’s diameter makes the spring weaker, and decreasing it strengthens the spring.

Wire Diameter

The wire diameter of an extension spring refers to the diameter from which the spring’s coils are made. Extension springs are made from wires of varying thickness, some thin, others thick, and you can only know this by getting the wire diameter which gives you the wire’s width. More importantly, the wire diameter also affects the spring rate, the force a load needs to stretch the spring by an inch.

Body Length

This is the length of the spring but only the coiled part of the spring, not the ends. Usually, extension springs come with hooks and springs at the end for attachment. These are excluded when getting the body length.

Free Length

This refers to the whole spring’s length, including the hooked ends. This measurement covers on top of one end to the other. For both the body and free lengths, you can use the micrometers and calipers to measure the small springs and then a ruler for the larger ones.

Steps to measure an extension spring

You will need a micrometer, dial calipers, and a tape measure or ruler for the large springs.

  1. Start by measuring the wire diameter using the micrometer. For precise readings, take measurements at two spots and find the average.
  2. Get the outside diameter using any of the middle coils since the coils towards the end of the spring tend to be large.
  3. Measure the body length using the appropriate tool depending on the spring’s size.
  4. Get the overall length/ free length.


Accuracy is paramount for the right measurements of the spring. Besides that, you need a trusted extension spring supplier, and that is where GL Metal proves a reliable partner. We provide a wide range of spring options and can even custom-make ones to suit your needs. Contact us today, and let us guide you to the best solution.