How do I compensate for resin shrinkage?

Make the mold cavity larger than the finished size you want. Because the part shrinks by a fixed percentage as it cures, the mold dimension equals the finished dimension divided by (1 minus shrinkage over 100). For 1% shrinkage you scale every dimension up by about 1.0101.

How much does resin shrink?

Linear shrinkage is typically 0.1% to 3% depending on the resin type and chemistry. Polyester and some casting resins shrink more, while many epoxy and urethane resins shrink less. Always check the datasheet or measure a test cast for your exact product.

Why is the volume factor larger than the linear factor?

Volume scales with the cube of the linear factor. A 1% linear shrinkage means the part shrinks roughly 3% by volume, so multiply your finished-volume resin estimate by the volume factor to know how much resin the oversized mold actually needs.

Does part thickness affect shrinkage?

Yes. Thicker sections cure hotter and can shrink more, and uneven thickness can cause warping rather than uniform shrinkage. For critical fits, cast a test piece of similar thickness and measure it before committing to a final mold.

Should I scale all axes equally?

Resin shrinkage is generally isotropic, so the same percentage applies to every axis and the tool scales length, width and height by the same factor. If you notice direction-dependent shrinkage in tests, adjust the worst axis individually.

What is the Resin Casting Shrinkage & Mold Size Calculator?

Enter the finished dimensions you want and your resin's linear shrinkage percentage to compute the oversized mold cavity dimensions, the per-axis oversize, the linear scale factor, and the volume factor for extra resin. For resin artists and small-batch makers. It runs free in your browser on Gera Tools, with nothing uploaded.

Resin Casting Shrinkage & Mold Size Calculator

Name: Resin Casting Shrinkage & Mold Size Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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When resin cures it shrinks slightly, so a mold built to the exact finished size produces a part that comes out a little too small. For parts that must fit together or match a spec, you compensate by building the mold cavity oversized. This calculator works out exactly how much oversize each dimension needs.

How it works

If a part shrinks by a linear percentage s as it cures, the finished dimension F relates to the mold cavity dimension M by:

F = M × (1 − s ÷ 100)

Solving for the mold cavity you need to build:

M = F ÷ (1 − s ÷ 100)

The tool applies this to length, width and height and reports the per-axis oversize, the linear scale-up factor, and the volume factor (the linear factor cubed) so you can estimate how much extra resin the larger mold consumes.

Worked example

You want a finished block of 50 × 30 × 20 mm from a resin with 1% linear shrinkage. The scale factor is 1 / (1 − 0.01) ≈ 1.0101, so the mold cavity must be about 50.51 × 30.30 × 20.20 mm. The volume factor is roughly 1.0306, meaning the mold holds about 3% more resin than the finished volume. At 2% shrinkage the cavity grows to about 51.02 × 30.61 × 20.41 mm — already a noticeable difference on a precision-fit part.

Typical shrinkage by resin type

Shrinkage rates vary considerably across resin chemistries. Figures from typical datasheets:

Resin type	Typical linear shrinkage
Polyester casting resin	1.5–3%
Standard epoxy (bisphenol A)	0.5–2%
Urethane casting resin	0.2–1%
UV-cure resin (SLA/MSLA)	0.5–2.5%
Silicone (condensation cure)	0.5–3%

These are approximate typical ranges. Always use your specific resin’s technical datasheet value — and then validate it against a test cast, because mixing ratio, ambient temperature, and cure time all affect the final shrinkage of the same product.

Back-calculating shrinkage from a test cast

If you have already made a test cast and can measure it, you can determine the true shrinkage of your resin:

linear shrinkage % = (1 − finished / mold) × 100

For example, if your 50.00 mm mold cavity produced a part measuring 49.52 mm after cure:

shrinkage = (1 − 49.52 / 50.00) × 100 = 0.96%

Use this measured value instead of the datasheet estimate for your next mold — it captures all the real-world factors including your specific pouring and curing conditions.

Why the volume factor matters

The linear scale factor tells you how to size the mold. The volume factor — the linear factor cubed — tells you how much extra resin to mix. For 1% linear shrinkage, the mold volume is about 3% larger than the finished part volume. For 2% linear shrinkage, it is about 6% larger. If you mix resin for the finished part size, you will be short. Add the volume-factor percentage to your mixing estimate to avoid running out mid-pour.

Notes

Shrinkage varies with resin brand, mixing ratio, temperature and part thickness, so the percentage on the datasheet is a starting point. Always cast a test piece of similar geometry, measure it, and back-calculate the real shrinkage before cutting an expensive master mold. All calculations run locally in your browser.