What Size Heat Shrink Do I Need? Sizing Guide and Chart

What size heat shrink do I need? It is the question that comes up every time someone stands at the bench with a fresh join and a bag of tube. The answer is not the wire gauge on its own. It is two measurements: the widest part the tube has to slide over, and the narrowest part it has to grip.

Get those two numbers, compare them with the supplied and recovered sizes on the pack, and you will pick the right heat shrink size every time. This guide shows how to measure, gives a sizing chart for every tube Solderstick sells, and works through common 12V jobs on a 4x4, caravan or boat.

Quick answer:

• Measure the widest part of the job (usually the join, lug or plug) and the narrowest part (usually the cable insulation).
• The tube's supplied inside diameter must be bigger than the widest part, so it slides on.
• The tube's recovered inside diameter must be smaller than the narrowest part, so it grips.
• For the best result, aim to shrink the tube at least 20% onto the job, and leave at least 10% of its shrink unused.
• If no single size meets both rules, step up to a higher shrink ratio.

The two-number rule for heat shrink size

Every heat shrink tube has two sizes printed on it. The first is the supplied (expanded) inside diameter, the size straight out of the pack. The second is the recovered inside diameter, the smallest it will go if nothing is inside. Manufacturers give the recovered figure as a maximum, so the tube may finish slightly smaller.

The rule is simple:

• Supplied ID must be larger than the largest part. If it is not, it will not go on.
• Recovered ID must be smaller than the smallest part. If it is not, the tube finishes loose on the cable and will not seal.

TE Connectivity, maker of Raychem tube, adds a sensible margin. It recommends the tube shrinks by at least 20% from its supplied size onto the job, and that at least 10% of its recovery is left unused once installed. Work to that window and the tube will be under tension all the way along, which is what gives a firm grip and pushes the adhesive into the gaps.

How to measure for heat shrink

Wire gauge is not enough to pick a heat shrink size, because insulation thickness varies a lot between cable types. Two cables with the same copper size can have quite different outside diameters. Measure what is actually there.

Step 1: Measure the narrowest part

This is usually the outside of the cable insulation either side of the join. Use a vernier or digital caliper if you have one. If not, wrap a strip of paper around the cable, mark the overlap, measure the length and divide by 3.14.

Step 2: Measure the widest part

This is the finished join, the crimped barrel, the lug or the back of a plug. If you have not made the join yet, measure a sample or allow generously, then slide the tube on first.

Step 3: Measure the length

Measure the length of the join and add enough overlap onto the insulation at each end. As a rule of thumb, aim for at least 10 to 15mm of overlap each side, and cut a little extra because polyolefin tube generally loses around 5 to 10% of its length as it shrinks, depending on the product.

Heat shrink size chart for Solderstick tube

All of these are adhesive-lined dual wall tube in 150mm lengths. The "best grip range" column applies TE's 20% and 10% margins, so it is the range of diameters each tube is happiest gripping. Anything you need to slide over must also be smaller than the supplied ID.

A note on the adhesive: the recovered ID is measured with nothing inside the tube, and the adhesive lining sits inside that figure. The adhesive fills small gaps and steps, but do not rely on it to grip a cable that is smaller than the recovered ID. Pick a size that meets the rule on the tube itself.

Worked examples: choosing a heat shrink size

Example 1: a butt join on an accessory wire

Say your cable measures 3.4mm across the insulation and the crimped join measures 5.5mm. The clear 3:1 7.9mm tube slides over 5.5mm easily, recovers to 2.6mm, and both measurements sit inside its best grip range of about 2.9 to 6.3mm. That is the one.

Example 2: a ring terminal on a heavier cable

Say the cable is 6mm across and the lug barrel is 11mm. The white 3:1 12.7mm tube slides over 11mm, but 11mm is above its best grip range, so it will be tight to fit and barely shrink on the barrel. The 4:1 20mm tube slides over easily and grips anywhere from about 5.5 to 16mm, covering both. Use the 4:1.

Example 3: a trailer plug onto its cable

Say the plug back is 24mm and the cable sheath is 8mm. The 4:1 32mm tube covers 8.8mm and up, so on an 8mm sheath it would have no shrink left and would barely grip. The 6:1 heavy wall slides over 24mm and grips down to about 6.1mm, so it covers both with one piece and adds abrasion protection.

What if one heat shrink size won't cover both?

Sometimes the gap between the widest and narrowest part is too big for any single heat shrink size. You have three options:

• Go up a ratio. Moving from 3:1 to 4:1 or 6:1 widens the window. Our heat shrink ratios guide explains how each ratio behaves.
• Build up the thin side. Shrink a short piece of smaller tube over the cable first, then put the larger tube over everything.
• Use two overlapping pieces. One sized for the join and one for the cable, overlapping by a good margin so the adhesive bonds them together.

Heat shrink size vs Solderstick connector size

If you are joining wire to wire, a solder seal connector may save you choosing tube at all. Solderstick connectors are sized by wire, not by diameter, and each colour covers a set range:

The connector is already a sealed heat shrink tube, so there is nothing extra to size. For more detail see the Solderstick colour chart. Loose heat shrink is still the right choice for lugs, terminals, plugs and sheath repairs, and for anything heavier than 6mm2, where splice connectors are not suitable and crimped or soldered lugs should be used instead.

Shrinking it on properly

The right size only works if it is shrunk properly:

1. Isolate the circuit and disconnect the battery negative.
2. Slide the tube on before you make the join.
3. Centre it over the join with overlap onto the insulation at both ends.
4. Heat with a heat gun from the middle outwards, turning the work so it shrinks evenly.
5. Stop when the tube is tight and a thin ring of adhesive shows at each end.
6. Let it cool before you move it.

A lighter or torch can scorch the tube and is a last resort only.

Which should you use?

• If you're sealing a single wire join on lighting or accessory cable, use the clear 3:1 7.9mm, or a Solderstick connector in the matching colour.
• If you're covering two or three wires or a heavier cable, use the white 3:1 12.7mm.
• If you're repairing a multicore sheath or wrapping a small harness, use the clear 3:1 19.1mm.
• If you're sealing a lug or terminal that is much fatter than the cable, use the 4:1 that slides over the barrel.
• If you're covering a plug or large fitting onto a much thinner cable, use the 6:1 heavy wall.

Size the cable and fuse for the load first, because tube will not fix undersized cable. Our wire gauge guide and auto cable sizes explainer cover that. These products are for low-voltage work only; 230/240V mains wiring must be done by a licensed electrician.

Frequently asked questions

How do I know what size heat shrink to buy?

Measure the widest and narrowest parts of the job. Choose a tube whose supplied size is bigger than the widest part and whose recovered size is smaller than the narrowest part.

Should heat shrink be bigger than the wire?

Yes, before shrinking it must be bigger than everything it slides over. After shrinking, its recovered size must be smaller than the wire insulation so it grips.

What size heat shrink for 6mm2 cable?

It depends on the cable's outside diameter, which varies with insulation type, so measure it. For a lug on 6mm2 cable, a 4:1 tube is usually easier because the barrel is much bigger than the cable.

Can heat shrink be too big?

Yes. If its recovered size is larger than your cable, it will finish loose and will not seal. Pick a tube that still has some shrink left once it touches the cable.

How long should I cut heat shrink?

Cover the full join plus overlap onto the insulation at both ends, and add a little extra. Tube generally gets around 5 to 10% shorter as it shrinks, depending on the product.

Does the adhesive change the size I need?

Not in practice. The recovered size is what counts, and the adhesive fills small gaps inside it. Do not count on the adhesive to make up for a tube that is too big.

What to use

Every size in the chart is in our heat shrink collection, in packs of 9 or 18. If you are not sure which sizes you will need, the Ultimate Heat Shrink Bundle gives you 12 tubes across the full range. For wire-to-wire joins, the 250PCS Solderstick connector kit covers white through yellow. Free express shipping Australia wide, and orders placed by 1pm are dispatched the same day.