Measuring heat shrink tubing comes down to knowing two numbers: the expanded (supplied) inner diameter and the recovered (fully shrunk) inner diameter. The ratio between these two tells you how much the tubing will shrink, and getting the right fit means measuring both your tubing and the object it needs to cover.
Expanded vs. Recovered Diameter
Heat shrink tubing arrives in its expanded form. That’s the larger size you see on the shelf or in the bag. Once you apply heat, it shrinks to its recovered state, which is its final, smaller diameter. Every sizing decision revolves around these two measurements.
Most manufacturers list dimensions for both states, but the convention can vary. Some catalogs list the supplied (expanded) diameter, others list the recovered diameter, and some list both. Always check which number you’re looking at before ordering.
Understanding Shrink Ratios
The shrink ratio tells you how much the tubing will reduce in diameter. A 2:1 ratio means the tubing shrinks to half its supplied diameter. A 3:1 ratio shrinks to one-third. A 4:1 ratio shrinks to one-quarter. So a piece of 2:1 tubing sold at 12 mm diameter will recover down to about 6 mm.
To figure out what size tubing you need, multiply the maximum diameter of the object you’re covering by the shrink ratio. If your wire bundle measures 8 mm across and you’re using 2:1 tubing, you need tubing with a supplied diameter of at least 16 mm. For 3:1 tubing, that same 8 mm bundle would need tubing supplied at 24 mm.
Higher ratios give you more flexibility. A 3:1 or 4:1 tube can fit over bulky connectors or irregular shapes and still shrink tightly around thinner sections, which makes them useful when the object you’re covering isn’t uniform in diameter.
The 80/20 Sizing Rule
Getting the right fit isn’t just about matching diameters on paper. The tubing should recover at least 20% and no more than 80% of its total shrink range. This is sometimes called the 80/20 rule.
If the tubing barely shrinks at all (less than 20% of its potential), it won’t grip the object tightly and could slide or leave gaps. If it has to shrink more than 80% of its range, you’re pushing the material to its limit, which can thin the walls excessively or cause uneven coverage. Staying within that middle zone gives you a snug, reliable seal with consistent wall thickness.
How to Measure the Object You’re Covering
Use calipers to measure the widest point of whatever the tubing needs to slide over. For a single wire, that’s the outer diameter of the insulation. For a wire bundle, gently compress the wires together and measure the overall cross-section. For a connector or splice, measure the fattest part, since the tubing needs to pass over it before it shrinks.
This widest-point measurement determines the minimum expanded diameter your tubing needs. The diameter of the section where the tubing will actually sit (often thinner than the part it passes over) determines whether the recovered diameter is small enough for a tight fit.
Measuring Tubing You Already Have
If you have a piece of heat shrink and need to figure out its diameter, the easiest method is to flatten it and measure the width. Take a pair of calipers and measure the flat width, then use this formula:
Multiply the flat width by 2, then divide by 3.1416 (pi). The result is the inner diameter.
For example, if the flattened tubing measures 3 mm wide: 3 mm × 2 = 6 mm, then 6 mm ÷ 3.1416 = 1.9 mm inner diameter. This works because the flattened tubing forms two layers, and the total width of both layers equals half the circumference.
A ruler works in a pinch, but digital calipers give you the precision you need, especially with smaller tubing where a fraction of a millimeter matters.
Don’t Forget Length Shrinkage
Heat shrink tubing also gets shorter when it recovers. In most cases, the length reduction is less than 10% of the original length, but it’s enough to leave a gap if you cut your pieces right to the edge. Cut your tubing a bit longer than the area you need to cover, particularly on longer runs where that 5-10% adds up. A 100 mm piece could lose up to 10 mm in length after heating.
Wall thickness increases as the tubing shrinks, both radially and longitudinally. This is normal and actually improves the protective qualities of the finished sleeve, but it’s worth knowing if you’re working in a tight space where extra bulk matters.
Putting It All Together
Start by measuring the widest point the tubing must slide over and the diameter of the section where it will sit. Check that the expanded (supplied) diameter is larger than the widest point, and that the recovered diameter is smaller than the narrowest section. Confirm you’re within the 80/20 range so the tubing isn’t working too hard or too little. Then add about 10% to your length to account for longitudinal shrinkage, and you’ll get a clean, tight fit every time.