xkcd #3138: Dimensional Lumber Tape Measure
Title text:
A person with two watches is never sure what time it is, especially if I got them one of the watches.
Transcript:
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Source: https://xkcd.com/3138/
You don't even need a custom one. Readily available is the universal tape measure where the relationship is marked as 2.54 units, equals 1 inch.
It means that a 2 x 4 is actually 5.08 x 10.16, which leaves you with a margin for surfacing and sanding.