Calculate casting shrinkage to find the pattern size needed from finished casting dimensions and metal shrinkage rate, or the shrinkage percent.

Nominal metal presets use shrinkage as a percentage of pattern size. Verify a process-specific rate before making a pattern.

Related Calculators

Casting Shrinkage Formula

Choose pattern size needed, measured shrinkage percentage, or final casting size. The following formula uses linear shrinkage as a percentage of the original pattern dimension.

FS = OS - (OS × (SR / 100))

Variables:

  • FS is the finished casting dimension (inches, or any consistent length unit)
  • OS is the original pattern dimension (same length unit as FS)
  • SR is linear shrinkage as a percentage of the pattern dimension (%)

For pattern-based shrinkage, FS = OS × (1 − SR / 100), so OS = FS / (1 − SR / 100) and SR = (OS − FS) / OS × 100. A casting-based pattern allowance A instead uses OS = FS × (1 + A / 100); these percentage bases are not interchangeable.

What is Casting Shrinkage?

Casting shrinkage is the change in dimensions as a casting cools. This calculator estimates linear dimensional change, not liquid-volume shrinkage or feeding requirements. Nominal metal presets are estimates retained from the supplied calculator, using its pattern-dimension basis. Actual changes depend on alloy, shape, section thickness, mold restraint and measurement temperature; verify your foundry’s rate and its reference dimension. Machining, draft and distortion allowances require separate treatment. NPTEL’s pattern-allowance lesson explains these separate allowances and dimension-dependent contraction values.

How to Calculate Casting Shrinkage?

The following steps outline how to calculate the final size of a casting after shrinkage.

  1. First, determine the original pattern dimension (OS) and its length unit.
  2. Next, determine the shrinkage rate (SR) as a percentage.
  3. Use the formula FS = OS – (OS * (SR / 100)) to calculate the final size (FS).
  4. Finally, calculate the final size of the casting after accounting for shrinkage.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem :

Use the following variables as an example problem to test your knowledge.

original pattern dimension (OS) = 12 inches

pattern-based shrinkage rate (SR) = 2%. Final casting size = 12 × (1 − 0.02) = 11.76 inches. The equivalent casting-based allowance is approximately 2.0408%.