Heat Shrink Ratios Explained: 2:1, 3:1, 4:1 and 6:1

Heat shrink ratios are the numbers printed on every pack of heat shrink tube: 2:1, 3:1, 4:1 or 6:1. They tell you how far the tube will shrink when you heat it, and that one number decides whether a piece of tube will slide over your join and still grip the wire tightly once it is shrunk.

Pick the wrong ratio and you get one of two problems. Either the tube will not fit over the bulky part of the job, or it slides on easily and then shrinks down loose around the thinner cable, leaving a gap for water and dust. This guide explains what heat shrink ratios mean, works through the maths with real sizes, and shows which ratio suits which job on a 4x4, caravan, boat or workbench.

We also cover where 2:1 tube fits in, even though Solderstick stocks 3:1, 4:1 and 6:1 adhesive-lined tube, so you can compare like with like when you shop.

Quick answer:

• The ratio is the supplied inside diameter divided by the fully shrunk (recovered) inside diameter. A 3:1 tube shrinks to about a third of its supplied size.
• 2:1 suits jobs where everything under the tube is close to the same diameter.
• 3:1 is the everyday choice for wire joins and small bundles where the join is a bit fatter than the cable.
• 4:1 handles bigger steps in size, such as a connector or lug onto a thinner cable.
• 6:1 heavy wall covers large, awkward shapes and adds extra abrasion protection.

What heat shrink ratios mean

Heat shrink tube is made from a cross-linked plastic, usually polyolefin, that is expanded at the factory and then cooled so it holds that larger size. When you heat it again, it tries to return to its original, smaller size. That smaller size is called the recovered diameter.

The shrink ratio compares the two:

• Supplied (expanded) inside diameter: the size of the tube as it comes out of the pack. This decides what the tube will slide over.
• Recovered inside diameter: the smallest size the tube will reach if nothing is inside it. This decides what the tube will grip.

Divide the first by the second and you have the ratio. A tube supplied at 12mm that recovers to 3mm is 12 divided by 3, which is 4:1. Manufacturers list the recovered size as a maximum, so the real figure may be slightly smaller, never larger.

The heat shrink ratio says nothing on its own about wall thickness, voltage rating or whether the tube has an adhesive lining. Those are separate specs, and you should check them on the product listing.

How to work out a heat shrink ratio from the sizes

Most packs list two numbers rather than the ratio itself, so it helps to be able to do the sum. Here is how the sizes in Solderstick's range work out:

• 7.9mm to 2.6mm: 7.9 divided by 2.6 is about 3.0, so 3:1.
• 12.7mm to 4.2mm: about 3.0, so 3:1.
• 19.1mm to 6.36mm: about 3.0, so 3:1.
• 12mm to 3mm, 20mm to 5mm, 32mm to 8mm: each is exactly 4:1.
• 33mm to 5.5mm: exactly 6:1.

The useful part is the range between those two numbers. Anything inside the tube must be smaller than the supplied size (or it will not go on) and larger than the recovered size (or the tube will finish loose). The bigger the heat shrink ratio, the wider that window, and the more mismatch between thick and thin parts the tube can cope with.

Leave room at both ends of the range

TE Connectivity, the company behind the Raychem heat shrink brand, gives a practical window for choosing a size. Its guidance is that the tube should shrink by at least 20% from its supplied size onto the job, and that it is best to have at least 10% of recovery left unused once it is installed. In plain terms: do not pick a tube that is only just big enough to slide on, and do not pick one that has to shrink all the way down to its smallest size to touch the cable.

Using that rule, a 3:1 tube supplied at 7.9mm is happiest on parts between roughly 2.9mm and 6.3mm across. A 4:1 tube supplied at 20mm is happiest between roughly 5.5mm and 16mm. Our heat shrink sizing guide has the full chart for every size we sell.

2:1 heat shrink: the common baseline

Photo: Ag PIC on Unsplash

2:1 is the most widely sold of the heat shrink ratios and the one most people picture when they think of heat shrink. A typical example is general purpose thin wall polyolefin tube, which is flexible, comes in lots of colours, and is usually single wall with no adhesive inside. TE's RNF-100 range is a well known 2:1 product of this type.

2:1 tube does a good job when the thing you are covering is close to one diameter all the way along: a bundle of wires, a soldered joint on matching wires, a handle or a component lead. It is also handy for colour coding and labelling.

The limit is the narrow window. A 2:1 tube supplied at 6mm recovers to about 3mm, and with TE's margins it is best on parts from roughly 3.3mm to 4.8mm. If your join is 6mm across but the cable either side is 3mm, a 2:1 tube that fits over the join will barely touch the cable. That is why automotive and marine joins usually call for a higher ratio, and why Solderstick does not stock 2:1. If you already have a drawer full of 2:1 tube, use it for bundling and labelling, and use 3:1 or higher adhesive-lined tube anywhere you need a seal.

3:1 heat shrink: the everyday all-rounder

3:1 is the heat shrink ratio most DIYers should reach for first. It gives enough range to slide over a crimped or soldered join and still close down firmly on the cable insulation either side, which is exactly the job on most 12V and 24V repairs.

Solderstick's 3:1 tube is all adhesive-lined dual wall, in 150mm lengths:

• Clear 7.9mm to 2.6mm: single wire joins on light and medium cable, sensor and lighting circuits.
• White 12.7mm to 4.2mm: heavier single wires, two or three wires together, small breakouts.
• Clear 19.1mm to 6.36mm: bundles, multicore cable sheaths and larger joins.

Clear tube has one extra advantage: you can see the join underneath. That makes it easy to check the wires are fully inside before you heat, and to spot corrosion later without cutting anything open.

4:1 heat shrink for big steps in size

4:1 heat shrink earns its place when there is a large difference between the biggest and smallest part under the tube. Think of a ring terminal barrel onto its cable, a plug body onto a thin lead, or a join where several wires come together into one.

Solderstick's 4:1 dual wall tube comes in three sizes, each in black, white and red:

• 12mm to 3mm: joins and terminals on light to medium cable where the join is much fatter than the wire.
• 20mm to 5mm: heavier cable, connector backs and multi-wire splices.
• 32mm to 8mm: large terminals, plug bodies and thick cable bundles.

Red and black are useful for marking positive and negative runs, for example at the battery end of an accessory feed. If you are working on battery cables, see our dual battery wiring guide: anything heavier than 6mm2 should be terminated with crimped or soldered lugs, and heat shrink then goes over the lug barrel and cable to seal it.

6:1 heavy wall heat shrink for awkward shapes

6:1 is a specialist heat shrink ratio. It covers very large changes in diameter in one piece, and the heavy wall version is built thick for extra mechanical protection where cable rubs on chassis rails, rocks or hull fittings.

Solderstick's 6:1 is an extra heavy duty, adhesive-lined heavy wall tube in black, supplied at 33mm and recovering to 5.5mm. With TE's margins that is a working range of roughly 6mm up to about 26mm, so a single piece can go over a large connector body and still close down on a modest cable. That makes it a good fit for trailer plugs, junction points under a ute tray and anywhere a join is likely to be knocked about.

The trade-offs: heavy wall tube takes longer to heat through, uses more heat, and finishes stiffer than thin tube. Do not use it where the cable needs to flex constantly right at the join.

Heat shrink ratios compared

Does a higher heat shrink ratio mean better tube?

No. A higher heat shrink ratio means more range, not more quality. Inside its working window, a 3:1 tube seals a simple join just as well as a 4:1 tube. Where a higher ratio helps is when the parts are uneven, or when you want fewer sizes in the kit to cover more jobs.

A few points to keep in mind:

• The adhesive matters more than the ratio for sealing. A 4:1 tube without adhesive will not keep water out as well as a 3:1 tube with it. Our dual wall vs single wall guide explains why.
• Thicker tube is tougher but stiffer. Heavy wall tube resists abrasion but does not like constant bending.
• Tube gets shorter as it shrinks. As a general rule of thumb, polyolefin tube loses around 5 to 10% of its length, depending on the product. Cut it a little longer than you need. As a rule of thumb, aim for at least 10 to 15mm of overlap onto the insulation each side once shrunk.

How to shrink adhesive-lined tube properly

Whatever the heat shrink ratio, the method is the same. Polyolefin tube typically starts shrinking somewhere between 80 and 125°C depending on the product (TE lists a minimum shrink temperature of 80°C and full recovery at 125°C for its ATUM adhesive-lined range), so a heat gun is the right tool.

1. Isolate the circuit first. On a vehicle, boat or caravan, disconnect the battery negative.
2. Slide the tube onto the cable before you make the join.
3. Make the join, then centre the tube over it with enough overlap onto the insulation at each end.
4. Heat from the middle outwards, turning the work or moving the gun so the tube shrinks evenly all round. This pushes air out of the ends instead of trapping it.
5. Keep going until the tube has closed down firmly and you can see a thin ring of adhesive at each end. That ring is the usual sign the lining has melted and bonded.
6. Let the join cool fully before you bend it, clip it or load it.

A lighter or torch can scorch the tube and leave cold spots where the adhesive has not flowed. Treat a flame as a last resort for a roadside fix, and redo the join properly when you get home.

Heat shrink and Solderstick connectors

A Solderstick connector is itself a short piece of clear polyolefin heat shrink, with a low-temperature solder ring in the centre and coloured sealing rings at each end. When you heat it, the solder flows into the join and the tube shrinks and seals, so you do not need a separate sleeve on top. Each colour covers a set wire range, from white (0.25 to 0.34mm2) to yellow (4.0 to 6.0mm2). See our guide to how Solderstick connectors work for the details.

Loose heat shrink tube then covers the jobs a connector does not: sealing over crimped lugs and terminals, protecting several joins in one sleeve, repairing cut sheathing on multicore cable, and adding an extra abrasion layer where a harness runs past a sharp edge.

Which heat shrink ratio should you use?

• If you're joining a single wire on a lighting, sensor or accessory circuit, use 3:1 7.9mm to 2.6mm, or a Solderstick connector sized to the wire.
• If you're sealing two or three wires together, or a heavier single cable, use 3:1 12.7mm to 4.2mm.
• If you're covering a multicore cable sheath repair or a small harness, use 3:1 19.1mm to 6.36mm.
• If you're sealing a ring terminal, lug or plug onto a much thinner cable, use 4:1 in the size that slides over the biggest part.
• If you're wiring trailer plugs, under-tray junctions or anything that will be knocked about, use the 6:1 heavy wall.
• If you just need to bundle or colour code wires indoors, 2:1 tube is fine, but it is not what we would use for a sealed outdoor join.

Remember that heat shrink and connectors do not fix undersized cable. Size the cable and fuse for the load first: our wire gauge guide for 12V and 4x4 wiring covers how. Solderstick products are for low-voltage work such as 12V and 24V automotive, marine, caravan, solar DC and garden lighting. The 600V rating on the tube is an insulation rating, not a reason to use it on 230/240V mains wiring, which in Australia must be done by a licensed electrician.

Frequently asked questions

What does 3:1 mean on heat shrink?

It means the tube can shrink to about one third of its supplied inside diameter. A 3:1 tube supplied at 12.7mm will recover to about 4.2mm if nothing is inside it.

Is 3:1 or 4:1 heat shrink better?

Neither is better across the board. 3:1 suits most wire joins where the join is only a little bigger than the cable, while 4:1 suits jobs with a bigger size difference, such as a terminal or plug onto a thin lead.

Is 2:1 heat shrink good enough for car wiring?

For bundling and labelling inside the cabin it is usually fine. For joins under the bonnet, under the vehicle or outside, a 3:1 or higher adhesive-lined tube gives a better grip on the cable and keeps water out.

How much does heat shrink shrink in length?

As a rule of thumb, around 5 to 10% depending on the product. Cut each piece slightly long so it still overlaps the insulation at both ends after shrinking.

Can I use a lighter to shrink heat shrink?

You can in an emergency, but a lighter or torch can scorch the tube and leave the adhesive unmelted in places. A heat gun gives an even shrink and a proper seal.

Can I put heat shrink over a solder seal connector?

You do not need to, because the connector is already sealed heat shrink. If a join will rub on something, a larger adhesive-lined sleeve over the top adds abrasion protection.

What heat shrink ratio do I need for battery lugs?

Usually 4:1, because the lug barrel is much bigger than the cable behind it. Choose a size whose supplied diameter slides over the barrel and whose recovered diameter is smaller than the cable insulation.

What to use

Browse every size in our adhesive-lined heat shrink range, all dual wall in 150mm lengths, in packs of 9 or 18. If you want one of everything for the shed or the 4x4, the Ultimate Heat Shrink Bundle gives you 12 tubes across the 3:1, 4:1 and 6:1 range.

For wire-to-wire joins, pair the tube with a 100PCS Solderstick connector kit, which includes all four colours from white to yellow. All orders ship free by Express Australia Post anywhere in Australia, and orders placed by 1pm are dispatched the same day.