Sizing a pattern for shrinkage
A casting keeps contracting after it solidifies, so the pattern has to be built oversize. The working relation is pattern size = (finished size + machining stock) × (1 + shrinkage). This calculator adds the machining allowance to both sides of each dimension before applying shrinkage, which matches a part machined on opposing faces — enter half the value if only one face is machined.
US patternmakers handle the same correction with a shrink rule, a scale whose inch is stretched by the shrink allowance. The common rules are 1/8 in/ft for gray iron, 3/32 in/ft for ductile iron, 1/4 in/ft for carbon steel, 5/32 in/ft for aluminum and magnesium, and 3/16 in/ft for brass. Each one is shown alongside the equivalent percentage so you can check either way.
Real shrinkage varies with restraint. Cores and heavy ribs hold a section back so it contracts less than the table value, while thin sections that are free to move contract more. For tight-tolerance features and first articles, work from the foundry own shrink data and correct the pattern after measuring sample castings.
Frequently asked questions
It depends on process and size. Small to medium sand castings commonly carry 1/16 to 3/16 in per surface, large castings more, and investment casting or die casting much less. Confirm against the print and the foundry standard.
It is fine for first design, but final verification has to be physical. Because restraint and pouring conditions change the result, patterns are normally corrected after measuring the first articles.