How do you calculate die swell allowance?

Subtract the target finished size from the expected extrudate size, then divide by the reference dimension. With a 1.08 in extrudate, 1.00 in target, and 1.00 in reference, the allowance is (1.08 - 1.00) / 1.00 = 8%.

What is a typical die swell percentage?

For common polyolefins it often lands between 5% and 40% depending on shear rate and draw, with rigid PVC on the low end and high-molecular-weight PE or rubber-modified compounds much higher. The 8% in our example is realistic for a moderately drawn pipe or profile.

Why does extrudate swell after the die?

Inside the die the polymer chains are stretched and sheared; once free of the wall they recover elastically, so the cross-section expands. Higher molecular weight, lower melt temperature, and shorter die land all increase swell.

Does draw-down cancel out die swell?

Partly. Pulling the extrudate faster than the die output stretches and thins it, offsetting swell. The allowance you design in should account for your normal draw ratio so the finished size lands on print.

Die swell vs draw ratio — what's the difference?

Die swell is elastic expansion happening at the die exit; draw ratio is the mechanical thinning from take-off pulling faster than extrusion. Net finished size is the swell up-size minus the draw-down.

How do I reduce excessive die swell?

Raise melt temperature, lengthen the die land to give chains time to relax, lower output rate to drop shear, or switch to a lower-molecular-weight or broader-distribution resin.

Plastics Extrusion - Pipe, Film & Profile calculator

Die Swell Allowance Calculator

Die swell (or extrudate swell, the Barus effect) is the elastic expansion a molten polymer undergoes as it exits the die and relaxes from shear. The die swell allowance expresses that expansion as a percent of the finished dimension, telling the tool designer how much larger — or in some drawn products smaller — the die land must be relative to the target part. Die designers, extrusion process engineers, and profile toolmakers use it when cutting a new die or dialing in a first article. Getting it wrong means scrapping tooling or fighting the line with draw-down and take-off tweaks that never quite hit print.

What this calculator does

Calculate the percent allowance between target finished size and expected extrudate swell at the die.
Use it when tooling, process engineering, or quality needs a quick die swell allowance for pipe, profile, sheet, or film dimensions.
It computes the percentage difference between the size the extrudate reaches at the die exit and the target finished dimension, referenced to a chosen baseline dimension.

Formula used

Allowance = expected extrudate size - target finished size
Percent allowance = allowance ÷ reference finished size

Inputs explained

Expected extrudate size at die exit:
Target finished part size:
Reference dimension for percent:

How to use the result

Use it when designing or correcting an extrusion die, or when documenting the swell behavior of a specific resin and melt condition for future tool builds.
Swell is highly sensitive to resin, melt temperature, shear rate, and draw ratio — a percent measured on one condition will not transfer cleanly to a different material or line speed.

Current U.S. benchmarks

The producer price index for steel mill products stands at 348.53 (BLS, May 2026), up 6.7% from a year earlier. Quotes priced off last quarter's material cost miss this move.
The producer price index for aluminum mill shapes stands at 404.859 (BLS, May 2026), up 36.8% from a year earlier. Quotes priced off last quarter's material cost miss this move.
The producer price index for plastic resins and materials stands at 319.371 (BLS, May 2026), up 19.5% from a year earlier. Quotes priced off last quarter's material cost miss this move.
The U.S. has 3,569 primary metal manufacturing establishments employing about 354,911 workers (Census County Business Patterns, 2023).

Common questions

How do you calculate die swell allowance? Subtract the target finished size from the expected extrudate size, then divide by the reference dimension. With a 1.08 in extrudate, 1.00 in target, and 1.00 in reference, the allowance is (1.08 - 1.00) / 1.00 = 8%.
What is a typical die swell percentage? For common polyolefins it often lands between 5% and 40% depending on shear rate and draw, with rigid PVC on the low end and high-molecular-weight PE or rubber-modified compounds much higher. The 8% in our example is realistic for a moderately drawn pipe or profile.
Why does extrudate swell after the die? Inside the die the polymer chains are stretched and sheared; once free of the wall they recover elastically, so the cross-section expands. Higher molecular weight, lower melt temperature, and shorter die land all increase swell.
Does draw-down cancel out die swell? Partly. Pulling the extrudate faster than the die output stretches and thins it, offsetting swell. The allowance you design in should account for your normal draw ratio so the finished size lands on print.
Die swell vs draw ratio — what's the difference? Die swell is elastic expansion happening at the die exit; draw ratio is the mechanical thinning from take-off pulling faster than extrusion. Net finished size is the swell up-size minus the draw-down.

Last reviewed 2026-05-12.