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Fits & size limits

Thou and mil: convert the length, then read the tolerance

One thou, or one mil in a length/thickness context, is 0.001 inch = 0.0254 mm = 25.4 µm exactly. A microinch is one thousandth of a mil.

Sources and scope ↓

Half a thou

Half a thou is a length of 0.0005 in = 0.0127 mm. The phrase alone does not say whether this is a one-sided allowance or the entire permitted width.

Plus/minus changes total width

A callout of ±0.0005 in has a total width of 0.001 in. A total tolerance width of 0.0005 in is half as wide. Read the explicit limits or ± sign before converting.

Continue your task

Tolerance width versus allowance between mating parts →Which roughness conversions are valid? →Calculate clearance from hole and shaft limits →

All tolerance and surface references →

Sources and review scope

Reviewed 23 October 2026. Arithmetic examples are labelled separately from source-table facts and standard scope.

NIST SI Units — Length

International inch definition: exactly 25.4 mm. Other unit values on these pages are derived arithmetic.

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