Thou
Glossary · Tolerance & Measurement
At a glance
- Category /
- Tolerance & Measurement
- Also called /
- thousandth, thousandths, mil
- Related /
- 2 terms
- Source /
- Written in the shop
A thou is one thousandth of an inch, written 0.001", and it is the everyday unit of precision on a machine shop floor.
Also called: thousandth · thousandths · mil
In practice
Machinists rarely say "one thousandth of an inch"; they say "a thou". Five thou is 0.005". For scale, a sheet of printer paper is roughly four thou thick and a human hair is around three.
Why it matters
Almost every conversation about fit, clearance and wear happens in thou. Misreading "five thou" as five hundredths is a factor-of-ten error that will not fit anything.
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