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Imagine if buying a heat shrink tubing was as simple as buying a coat. You measure the object you want to cover and then buy a heat shrink to fit.
Unfortunately, it’s not.
Buying a right size heat shrink is much more complex than buying a right size coat. Because the tubing shrinks, when heated, does not retain the same dimensions it originally has pre-heating.
As such, it’s common for buyers (especially, first time buyers) to make sizing mistakes when buying a heat shrink tubing, ending up with a purchase that just won’t fit into their application.
This piece will guide you on how to avoid sizing mistakes and buy a right sized heat shrink tubing for your application.

First things first…

Before we get into the nitty gritty of how to measure a heat shrink when purchasing one, it’s worth noting that heat shrink tubing is first made to the diameter it will be when it’s shrunk. After that, it is expanded and packaged for sale.
So when you buy a heat shrink tubing, its shrunk dimensions are already known; they’re stated on the specification sheet.
Having this explained, we can now move on to the actual measuring step.

Dimensions and Dimension Facts—You Should Know About!

When measuring a heat shrink tubing, there’re basically two types of dimensions you’ll be working with: i) Recovered dimensions and, ii) Expanded dimensions.
Recovered dimensions are the shrunken dimensions and expanded dimensions are the dimensions of the tubing in the stretched form.
On a typical heat shrink datasheet, you’ll find the following dimensions listed:

  • Recovered internal diameter
  • Recovered wall thickness
  • Expanded internal diameter

Let’s say you want to buy a heat shrink to cover a cable which has an outer diameter of 7 mm.
The heat shrink that you buy must have:

  • An expanded internal diameter greater than 7 mm
  • A recovered internal diameter of just under 7 mm

Please note that you won’t find the exact dimensions listed on a heat shrink tubing’s datasheet. They will always have some tolerance range which will be mentioned on the sheet. So make sure you keep that in mind.
What about the wall thickness?
It depends on the application in which you want to use the heat shrink. If wall thickness is really important for your application (for e.g. If you’re looking to insulate a piece of wire), we would recommend that you purchase a heat shrink tubing with the maximum recovered wall thickness.
Let’s take a look at another example.
Let’s say you want to buy a heat shrink tubing to insulate a cable wiring. The cable has a diameter of 7 mm. It has bare wires running through it which are 4 mm in diameter. Plus, the cable has a connector at each of its end, which has a diameter of 15 mm.
The heat shrink that you buy must have:

  • An expanded internal diameter greater than 15 mm
  • A recovered internal diameter of just under 4 mm

For wall thickness the rule is same as discussed above.

One more thing you should keep in mind…

A heat shrink tubing not only shrinks in diameter when heated, but it also shrinks in length. Although the change in length is minimal, usually 10% of the original expanded length. So if you need a heat shrink tubing to cover an object of length 2 meters, you should buy a heat shrink which is at least 2.2 meters long. You can later cut off the tailing end if you wish to.
And that is how you can buy the right size shrink tubing for your application that will fit like a glove.
A BONUS read: The Chemistry Behind the Working of Heat Shrink