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I have a document typeset in letter paper size. I obtained it with the fullpage package, which sets all 4 margins to be 1 inch.

Now I would like to use the a4paper option, but keeping exactly the same text width (I spent time to adjust some equations).

What size should I pass to the geometry package together with the a4paper option? Seems that 1 inch is not the right solution.

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Please see that an exact solution depends on how you define 1 Inch in cm or mm. I simply use here the fixed version for

1 Inch = 25.4 mm.

Be aware of rounding errors for the conversion of Inch to mm ...

Because you gave no mwe (please next time add one!) I can only guess what you are doing. Please see the following MWE containing the "formula" to get the current \textwidth with paper letter and the calculation for the needed length of margins for the useage of paper A4 with the same \textwidth. Simply move the % from one call of geometry to the other to get first paper letter, second a4:

\documentclass{article}

\usepackage[showframe, letterpaper, margin=1in]{geometry} % <====================
%\usepackage[showframe, a4paper, margin=22.4mm]{geometry} % <====================
\usepackage{blindtext}

\begin{document}

Format \texttt{letter} is defined to be: 216mm x 279mm.

1 inch is defined as 25.4mm, 
so your text width here is 216mm-25.4mm-25.4mm = 165.2mm % <=====================

Format \texttt{A4} is defined to be: 210mm x 297mm. 

The difference in paper width is 6 mm (rounded!). 
Therefore to get the same text width you need 
margins with 22.4mm: 210mm-22.4mm-22.4mm = 165.2mm. % <=====================

\blindtext

\end{document}

and the resulting letter pdf:

resulting letter page

The marked text width (with both red arrows pointing on the typing area at the same height) is 165.2 mm for letter paper and for a4 paper after moving the % as already mentioned ...

So at last the answer to your question is:

use

\usepackage[a4paper, margin=22.4mm]{geometry}

to get the same text width, but be aware of changing page numbers in longer documents, because the text height is changed too between the two paper sizes letter and a4 ...

  • Thanks for your answer. So it was only basic computations... By the way I checked the geometry package documentation and it is possible to set the textwidth instead of the margin. Which option would you choose ? – J Coulet Jul 12 at 8:08
  • @JCoulet depends on what you need ... Do you need same margin right and left? Use margin. Need only textwidth fixed, margin not fixed: use textwidth= ... – Mensch Jul 12 at 8:14

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