Paper Size Converter

Sending a US-letter PDF to a printer in Europe or trying to fit an A4 layout onto US letter without anything getting cut off is a recurring small annoyance.

This converter handles ISO sizes (A0–A10, B0–B10, C series), North American sizes (Letter, Legal, Tabloid, Junior Legal), and Japanese JIS B series. Pick a source and a target, and it tells you the exact dimensions in millimeters and inches and the scale factor to apply if you want the content to fit.

The scale-to-fit math is the part most converters skip: A4 to Letter requires roughly 94% scaling to fit width, while Letter to A4 needs about 97%. If you scale uniformly to fit one dimension, the other will have margins, which is shown as the residual whitespace at the calculated scale.

The ISO sizes have a lovely property: each size has the same aspect ratio (1:√2) and folding any sheet in half along the long edge gives you the next size down. A0 is one square meter. A1 is half a square meter. A4 is 1/16 of a square meter. This is why office printers handle the whole series consistently — scaling A3 down to A4 is exactly 71% (1/√2 rounded), no aspect-ratio drift. North American sizes don't share this property because Letter and Legal evolved from US government practice, not from a unified mathematical scheme.

The scale-to-fit math is just the ratio of the shorter dimension. A4 to Letter: A4 is 210mm wide, Letter is 215.9mm wide. The longer dimension (A4 297mm vs Letter 279.4mm) is the binding constraint, so 279.4/297 = 94% scaling. Apply that to the entire A4 layout and it fits within Letter with about 6mm of unused width on each side. Reverse direction: 210/215.9 = 97%, with about 13mm of unused height at the top and bottom of the A4 page.

The pain that drives this: a colleague in Berlin sends you an A4 PDF, you print it on US Letter, and the page either gets cropped or scaled in some way the printer driver picked without asking. If you don't know the source size, the result depends on what 'fit to printable area' decided. The converter tells you what the right scale should be, so you can specify it explicitly in your PDF settings rather than letting the driver guess.

What to use instead for digital documents: don't use a paper-size paradigm at all. PDFs render at any size; HTML is reflowable; e-readers reflow text. Paper sizes only matter when paper is the actual deliverable. If you find yourself fighting paper-size conversions for a screen-only artifact, stop and ask whether the document should be a fixed-size PDF in the first place.