Molten metal will shrink as it cools. To allow for this, patterns are made slightly over sized. A pattern that is 12 1/8" long will produce a 12" long part when cast in iron. Shrink rules are elongated rulers that take shrinkage into account. (A one foot shrink rule with 1/8" shrink per foot looks like an ordinary ruler. It is actually 12 1/8" long.) By using a shrink rule to measure parts for a pattern, the pattern maker does not have to scale up the dimensions of the desired casting.
Each metal has its own shrinkage rate:
Metal Shrinkage
Britannia* 1/32" per foot
Tin 1/12" per foot
Iron 1/8" per foot
Bismuth 5/32" per foot
Brass 3/16" per foot
Aluminum 3/16" per foot
Copper3/16" per foot
Steel 1/4" per foot
Lead 5/16" per foot
Zinc 5/16" per foot
* Lead free form of pewter

From Stanley Folding Rules A History and Descriptive Inventory   by Alvin Sellen's

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