What Size Heat Shrink Do I Need?

Sizing from the shrink ratio alone produces tubes that either will not slide over the connector or hang loose after recovery. The reliable method works from the six dimensions below, in order, and finishes against the real size table of the SKU you will actually order.

Why ratio alone does not size a tube

A shrink ratio (2:1, 3:1, 4:1) describes only how far a tube can collapse. It says nothing about whether the recovered tube grips anything. A 3:1 tube sized as "ratio × object OD" often recovers far past the cable and sits loose, because the largest object it slides over (usually a connector) and the smallest section it must grip (usually the cable jacket) are two different diameters. Ratio is a property of the tube; sizing is a property of your assembly.

The six checks, in order

1. Supplied ID clears the largest object. Add real clearance, typically 10–20% of that object's OD, so the unshrunk tube slides over the connector without scraping or stretching. The exact clearance figure depends on the assembly: stiffer cable and rigid connectors need more.
2. Recovered ID fits the smallest target section. The recovered ID must be equal to or smaller than the diameter it should grip. The cable jacket, not the connector. If recovered ID is larger than the cable, the tube recovers but never grips.
3. Verify the recovered wall thickness. The wall after recovery is thinner than supplied. Check the datasheet's recovered wall against your abrasion, dielectric or mechanical requirement. The supplied wall value does not survive recovery.
4. Verify longitudinal change. Tubing shrinks along its length as well as in diameter, commonly in the range of roughly −5% to −15% depending on grade. Confirm the exact figure on the datasheet and add it to the cut length, or the finished tube pulls back and exposes the joint.
5. Verify installation clearance. The unshrunk tube needs room: a heat gun must sweep the full circumference, and the expanded tube must slide past everything between its start position and its final position.
6. Confirm against the actual SKU size table. Your four numbers (largest OD, smallest grip OD, wall need, cut length) now select a real row in a real table. The table is the final authority, not the ratio printed on the box.

Worked example — explicit assumptions

Assumed assembly: a cable at 6.0 mm jacket OD, a connector body at 11.0 mm OD that the tube must slide over, and a requirement to cover the joint with no grip lost on the cable.

1. Supplied ID: 11.0 mm + clearance. At roughly 15%: 11.0 × 1.15 ≈ 12.7 mm supplied ID or larger.
2. Recovered ID: must be ≤ 6.0 mm to grip the jacket. A tube recovering to ~4.5–6.0 mm works; at 4.5 mm the grip is firm.
3. Ratio implied: 12.7 ÷ 4.5–6.0 ≈ 2.1–2.8, so a stocked 3:1 size fits, though note the ratio was derived from the assembly, not used to pick it.
4. Recovered wall: read the 3:1 datasheet row for the chosen size; if the recovered wall is 1.0 mm and the spec needs 0.8 mm minimum, it passes.
5. Longitudinal: datasheet says −10% ±; a 50 mm cover length is ordered as ~56 mm cut length.
6. SKU check: the size table confirms a supplied ID of 12.7 mm exists in the 3:1 range with recovered ID in the 4–5 mm band. Both checks pass on one stocked part.

The numbers above demonstrate the method. Always close against the size table of the part you order, see the dual-wall 3:1/4:1 product page or the Shrink Ratio hub for how the stocked sizes are organized.

What to send for a sized recommendation

Largest object OD, smallest section to grip, wall requirement, environment. The quotation returns a stocked size, or a custom size with the tooling implications stated. Send the numbers.

Frequently asked questions