Material Shrinkage Allowance Calculator
When designing molds, patterns, or dies, dimensions must be enlarged to compensate for material shrinkage as it cools and solidifies. The mold or pattern dimension is calculated as: Mold dimension = Finished dimension / (1 - Shrinkage rate). Enter the finished part dimension and the material shrinkage percentage to find the required mold or pattern size.
Shrinkage allowance formula
Mold dimension (mm) = Finished dimension / (1 - Shrinkage rate / 100)
Allowance (mm) = Mold dimension - Finished dimension
This formula is the standard foundry and injection molding approach for linear shrinkage compensation. It is documented in the American Foundry Society (AFS) casting design guides and ASTM D955 for plastic injection molding.
Shrinkage rates for common materials
- Gray cast iron: 0.8-1.0% (AFS)
- Carbon steel: 1.5-2.0% (AFS)
- Aluminum alloys: 1.0-1.8% (AFS)
- ABS plastic: 0.4-0.8% (ASTM D955)
- Polypropylene (PP): 1.0-2.5% (ASTM D955)
- Nylon 66: 0.7-1.5% (ASTM D955)
- HDPE: 1.5-3.5% (ASTM D955)
Material shrinkage allowance: frequently asked questions
What is shrinkage allowance in manufacturing?
Shrinkage allowance is the extra size added to a mold cavity or pattern to compensate for the reduction in dimensions that occurs as a material cools and solidifies. It ensures the finished part matches the design dimension after shrinkage.
What is the shrinkage allowance formula?
Mold/pattern dimension = Finished dimension / (1 - Shrinkage rate). For example, if the finished part must be 100 mm and the material shrinks at 2% (0.02), the mold cavity = 100 / (1 - 0.02) = 102.04 mm.
What are typical shrinkage rates for common materials?
Typical volumetric shrinkage rates: Gray cast iron 0.8-1.0%, Steel (carbon) 1.5-2.0%, Aluminum alloys 1.0-1.8%, Polypropylene (PP) 1.0-2.5%, ABS 0.4-0.8%, Nylon 66 0.7-1.5%. Per ASTM D955 for plastics and foundry engineering references for metals.
Is shrinkage the same in all directions?
No. Shrinkage is often anisotropic (different in different directions), especially for fiber-reinforced plastics and directionally solidified metals. This calculator uses a single isotropic shrinkage rate. For critical parts, directional shrinkage rates must be determined experimentally for the specific material, process, and geometry.
What is the difference between linear and volumetric shrinkage?
Linear shrinkage is the reduction in one dimension (length, width, or height). Volumetric shrinkage is the total volume reduction. For isotropic materials, volumetric shrinkage is approximately 3 times the linear shrinkage for small values. This calculator uses linear shrinkage, which is the standard for pattern and mold design.
Official sources
- ASTM International D955: ASTM D955 Measuring Shrinkage from Mold Dimensions of Thermoplastics.
- American Foundry Society: AFS Casting Design and Shrinkage References.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.