Heat Shrink Tubing Splice Calculator
Calculate the overall size of your splice and estimate the best heat-shrink tubing size. For a single-sided splice, use only the Side A column.
Wire lead outer diameters
Enter the OD (mm) of the wire insulator for each lead in your splice. The square of each OD is calculated automatically.
| Lead | Side A | Side B | ||
|---|---|---|---|---|
| Wire OD | OD² | Wire OD | OD² | |
Calculated bundle values
Design splices with cross-sectional areas as balanced as possible on each side of the splice.
| Metric | Side A | Side B |
|---|---|---|
| Sum of OD² | — | — |
| Bundle cross-sectional area (× 0.8) | — | — |
| Total bundle diameter (√ of sum) | — | — |
| Diameter +10% | — | — |
| Diameter −10% | — | — |
HST selection
Use the total bundle diameter values to select the HST size from the Eurotech drawing.
Select an expanded ID larger than the largest side by at least +10%.
Select a recovered ID smaller than the smallest side by at least 10%.
Smaller diameter ÷ larger diameter. 1.00–0.50 = 2:1 · 0.49–0.33 = 3:1 · 0.32–0.24 = 4:1 · below 0.24 = > 4:1
Conductor size reference
| ISO Size | Wire OD | ISO Size | Wire OD | SAE Ga. | GXL | SXL | TXL | GPT |
|---|
Sizing Heat Shrink Tubing for a Wire Harness Splice
Selecting the correct heat shrink tubing size for a spliced wire bundle is a dimensional calculation, not a guess. The tubing has to slide over the largest side of the splice before shrinking, and it has to recover tightly enough on the smallest side to seal. This calculator handles both ends of that requirement.
Each wire in a splice contributes a cross-sectional area proportional to the square of its insulated outer diameter. Summing the squares of every lead OD on a side and taking the square root of that sum gives the theoretical bundle diameter for that side. A bundling factor of 0.8 accounts for the fact that round conductors never pack perfectly, and the ±10% margins give you the working window for tubing selection.
Reading the results
- Total bundle diameter — the effective outside diameter of each side of the finished splice.
- Expanded ID (IDe) — the tubing’s as-supplied inner diameter. It must be larger than the biggest side of your splice by at least 10% so the tubing installs without forcing.
- Recovered ID (IDr) — the tubing’s fully shrunk inner diameter. It must be smaller than the narrowest side by at least 10% so the tubing grips and, in dual-wall constructions, so the adhesive flows and seals.
- Shrink ratio — the smallest bundle diameter divided by the largest. The wider the step between the two sides of a splice, the higher the ratio you need.
Choosing a shrink ratio
Transition splices with lead counts that differ significantly from one side to the other create a step change in diameter, and a 2:1 tubing will not conform across that step. Use the ratio value from the calculator to move up:
1.00–0.50 = 2:1 | 0.49–0.33 = 3:1 | 0.32–0.24 = 4:1 | Below 0.24 = Higher than 4:1Where possible, design splices so the cross-sectional area is balanced on each side. A balanced splice allows a lower shrink ratio, a thinner finished profile, and a more consistent seal.
Tubing Built for Automotive Wire Harnesses
Eurotech supplies OEM-approved heat shrink tubing for automotive and commercial vehicle wire harnesses. Adhesive-lined dual-wall constructions seal splices, terminations, and lug transitions against moisture, road salt, and vibration, while single-wall products handle bundling, strain relief, and abrasion protection. Every product is spec’d the way engineers evaluate it: shrink ratio, wall construction, operating temperature range, and expanded and recovered inner diameters.
4:1 ratio | Semi rigid | Rugged outer jacket. High-expansion adhesive-lined tubing for splices with a large diameter step between sides.
Rated to +125 °C. Adhesive-lined sealing for splices, terminals, and connector backshells in engine-adjacent routing.
Rated to +135 °C. Flexible mechanical protection, identification, and bundling where a moisture seal is not required.
Sealing beyond the tubing
Adhesive-lined tubing seals the outside of a splice, but water can still track longitudinally along the strands inside a conductor. For high-voltage and sealed-circuit applications, Eurotech’s Ultra Water Barrier (UWB) acrylic mastic is applied under the heat shrink as a die-cut or roll to block that path. Eurotech also supplies Coroflex shielded and unshielded high-voltage cable, Coroplast harness tapes for volume and weight reduction, and full harness re-works services for teams re-engineering an existing assembly.
Heat Shrink Sizing FAQ
How do I calculate the diameter of a wire bundle?
Square the insulated outer diameter of each wire in the bundle, add the squares together, then take the square root of the total. This gives the theoretical bundle diameter. Apply a bundling factor of 0.8 to the summed area to account for imperfect packing between round conductors.
What shrink ratio do I need for a splice?
Divide the smaller side’s bundle diameter by the larger side’s. A result of 0.50 or above works with 2:1 tubing. Between 0.49 and 0.33 requires 3:1, between 0.32 and 0.24 requires 4:1, and anything below 0.24 requires a ratio higher than 4:1.
What is the difference between expanded ID and recovered ID?
Expanded ID (IDe) is the tubing’s inner diameter as supplied, before heat is applied. Recovered ID (IDr) is its inner diameter after it has fully shrunk. Select tubing whose expanded ID clears your largest bundle diameter by at least 10% and whose recovered ID is at least 10% smaller than your smallest bundle diameter.
When should I use dual-wall instead of single-wall tubing?
Use adhesive-lined dual-wall tubing wherever a moisture or environmental seal is required, such as splices, terminations, and lug transitions exposed to water, salt, or fluids. Single-wall tubing is appropriate for mechanical protection, abrasion resistance, strain relief, bundling, and circuit identification.
Why should splice cross-sections be balanced?
A balanced splice keeps the diameter step between the two sides small. That allows a lower shrink ratio, produces a thinner and more flexible finished splice, and gives the adhesive a more uniform gap to fill, which improves seal consistency.
Let’s Talk.
Send us your splice dimensions and application requirements. Our engineering team will confirm the tubing size, ratio, and wall construction for your harness.