Inch (source code)
= Inch
{title2=$\mathrm{in}$}
{wiki}
An <inch> is a <unit of length> defined by
$$
1\,\mathrm{in}=0.0254\,\mathrm m=25.4\,\mathrm{mm}
$$
exactly. Thus an <inch> is a fixed fraction of the <metre>, not an independently measured physical constant. The United States announcement of the international conversion made it effective on 1 July 1959.<SI Units – Length by National Institute of Standards and Technology>[FAQ, When did the metric redefinition of the inch occur?; derivation from 0.9144 metre per yard and 36 inches per yard]{ref}
A part specified as $3/8\,\mathrm{in}$ long therefore has nominal <length> $(3/8)\times25.4=9.525\,\mathrm{mm}$. If its manufacturing tolerance is $\pm0.001\,\mathrm{in}$, the converted tolerance is $\pm0.0254\,\mathrm{mm}$. Exact conversion preserves the uncertainty or tolerance already present in the specification; it does not make the manufactured part exact.
For an <area>, square the conversion: $1\,\mathrm{in}^2=(0.0254)^2\,\mathrm m^2=0.00064516\,\mathrm m^2$. Multiplying an area by only $0.0254$ would fail <dimensional analysis>. The same distinction matters when converting <pixel density>, where inches appear in the denominator.
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