The A4 vs Letter paper size difference causes more document formatting problems than any other single issue in international business. A4 is taller and narrower. Letter is shorter and wider. A document formatted for one will not print correctly on the other.
The sizes
A4: 210 x 297 mm (8.27 x 11.69 inches)
Letter: 215.9 x 279.4 mm (8.5 x 11 inches)
Legal: 215.9 x 355.6 mm (8.5 x 14 inches)
A4 is used by every country except the US, Canada, and a few others that use Letter. This means most of the world's documents are formatted for A4.
The A-series design
The A-series paper sizes follow an elegant mathematical principle: each size is the previous size cut in half. A1 is A0 cut in half. A2 is A1 cut in half. And so on.
The aspect ratio is constant: 1:sqrt(2) (approximately 1:1.414). This means cutting an A4 sheet in half produces two A5 sheets with the same proportions. No other aspect ratio has this property.
A0: 841 x 1189 mm (1 square meter area)
A1: 594 x 841 mm
A2: 420 x 594 mm
A3: 297 x 420 mm
A4: 210 x 297 mm
A5: 148 x 210 mm
A6: 105 x 148 mm
Why this breaks documents
A document formatted for Letter with 1-inch margins has a text area of 6.5 x 9 inches. Printed on A4, the width fits (A4 is narrower but the margins absorb it), but the extra height means the page breaks shift. A 10-page Letter document might become 9 pages on A4 or 11, depending on how the printer handles the size mismatch.
CSS addresses this:
@page { size: A4; margin: 20mm; }
@page { size: letter; margin: 1in; }
For a reference of all standard paper sizes with conversion between metric and imperial, I built a guide at zovo.one/free-tools/paper-size-guide. It includes the A, B, and C series plus US sizes.
I'm Michael Lip. I build free developer tools at zovo.one. 500+ tools, all private, all free.
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