Printed length in Layouts package inaccurate?

Question

From the layouts package manual (page 3 and 4):

The textwidth is \printinunitsof{pt}\prntlen{\textwidth} which is also \printinunitsof{in}\prntlen{\textwidth} or \printinunitsof{mm}\prntlen{\textwidth}.
prints out:
The textwidth is 433.62pt which is also 6.00117in or 152.39821mm.

But 1 inch = 25.4 mm, so 6.00117in should equal 152.429718mm. I noticed this type of inaccuracy also in a document I am currently preparing.
The number of decimals provided in the manual example seems to imply a high accuracy, but practice seems to show otherwise.
Am I seeing/doing something wrong?
If not, how accurate are the printed lengths?

Answer 1

Floating point calculations in layouts are based on dimension arithmetic of TeX, which is not really accurate.

You get better results with the floating point module of expl3. Still not accurate to five decimals, but much more than the coarser result by layouts.

\documentclass{article}
\usepackage{layouts}

\usepackage{xparse}
\ExplSyntaxOn
\RenewDocumentCommand{\prntlen}{m}
 {
  \dim_to_decimal_in_unit:nn { #1 } { 1\use:c {l@yunits} } \, \use:c{l@yunits}
 }
\ExplSyntaxOff

\setlength{\textwidth}{6in}

\begin{document}

The textwidth is \printinunitsof{pt}\prntlen{\textwidth}
which is also \printinunitsof{in}\prntlen{\textwidth} or
\printinunitsof{mm}\prntlen{\textwidth}.

\end{document}