How do you calculate material shrinkage for molded rubber?

Subtract the cured part dimension from the mold dimension, divide by the reference dimension, and multiply by 100. A part measuring 49.25 mm from a 50 mm cavity gives (50 − 49.25) / 50 × 100 = 1.5% shrinkage.

What is a typical shrinkage rate for rubber?

Most molded elastomers shrink between about 1% and 4% depending on polymer and filler loading. Silicones often run 2 to 4%, while many filled organic rubbers land near 1 to 2% — the 1.5% in this example is typical for a well-filled compound.

How do I size a mold cavity for shrinkage?

Cut the cavity larger than the target part so that after shrinkage it lands on nominal. For 1.5% shrinkage, a 50 mm target feature needs a cavity around 50.76 mm so the cured part finishes near 50 mm.

Why use a reference dimension separate from the mold dimension?

The reference fixes the basis the percentage is taken against. Using a consistent reference — usually the nominal or mold dimension — keeps shrinkage figures comparable across features and from one tool to the next.

Does post-cure affect shrinkage?

Yes. Post-curing drives off volatiles and completes crosslinking, which can add measurable shrinkage. Always measure parts in the same cure and temperature state you intend to ship, or your compensation will be off.

Why do my parts shrink differently in different directions?

Flow direction, fiber or filler orientation, and feature geometry make shrinkage anisotropic. Calculate it per critical feature and direction rather than applying one number to the whole part.

Gaskets, Seals, O-Rings & Elastomer Components calculator

Material Shrinkage Calculator

Rubber and elastomers shrink as they cool and cure, so a mold cavity must be cut larger than the target part by the material's shrinkage rate. This calculator takes a cured part dimension and the mold dimension it came from and returns the shrinkage percentage you need to compensate the tool. Tooling engineers and compounders use it to dial in cavity dimensions, validate a new compound against its datasheet shrinkage, and diagnose why parts are coming out under or over size. Get shrinkage right and first-article parts land on print; get it wrong and you scrap a steel cavity or chase tolerances forever.

What this calculator does

Estimate elastomer material shrinkage by comparing the mold or tooling dimension with the cured part dimension.
Use it when tooling, quality, or process engineering needs a quick shrinkage percentage for rubber sheets, molded seals, O-rings, grommets, diaphragms, or custom elastomer components.
It computes the percentage a molded part shrinks relative to the mold by comparing cured and tool dimensions against a reference dimension.

Formula used

Material shrinkage = (mold or tool dimension - cured part dimension) ÷ shrinkage reference dimension × 100
Use the same feature, temperature condition, and measurement method for all three inputs.

Inputs explained

Cured part dimension:
Mold or tool dimension:
Shrinkage reference dimension:

How to use the result

Use it when cutting or correcting a mold cavity, qualifying a new compound, or root-causing parts that consistently measure off nominal.
Shrinkage varies with compound, durometer, cure state, part geometry, and post-cure; a single linear figure will not perfectly predict shrinkage across all features or directions.

Current U.S. benchmarks

The U.S. has 11,391 plastics and rubber products establishments employing about 815,988 workers (Census County Business Patterns, 2023).

Common questions

How do you calculate material shrinkage for molded rubber? Subtract the cured part dimension from the mold dimension, divide by the reference dimension, and multiply by 100. A part measuring 49.25 mm from a 50 mm cavity gives (50 − 49.25) / 50 × 100 = 1.5% shrinkage.
What is a typical shrinkage rate for rubber? Most molded elastomers shrink between about 1% and 4% depending on polymer and filler loading. Silicones often run 2 to 4%, while many filled organic rubbers land near 1 to 2% — the 1.5% in this example is typical for a well-filled compound.
How do I size a mold cavity for shrinkage? Cut the cavity larger than the target part so that after shrinkage it lands on nominal. For 1.5% shrinkage, a 50 mm target feature needs a cavity around 50.76 mm so the cured part finishes near 50 mm.
Why use a reference dimension separate from the mold dimension? The reference fixes the basis the percentage is taken against. Using a consistent reference — usually the nominal or mold dimension — keeps shrinkage figures comparable across features and from one tool to the next.
Does post-cure affect shrinkage? Yes. Post-curing drives off volatiles and completes crosslinking, which can add measurable shrinkage. Always measure parts in the same cure and temperature state you intend to ship, or your compensation will be off.

Last reviewed 2026-05-12.