US Letter vs A4

The sizes

US Letter is wider and shorter: about 6 mm wider and 18 mm shorter than A4. A4 is 97.3 % of Letter's width and 106.3 % of its height.

The reasons are historical. Letter descends from an imperial measurement and has no relationship to the metric series. A4 belongs to the ISO 216 series, where each size is produced by halving the previous one and the aspect ratio is 1 : √2.

Margins

The two traditions arrive at similar margins from different directions:

For a plain letter the two are nearly interchangeable. The difference appears as soon as a pre-printed letterhead is involved: a letterhead designed for the DIN 5008 header zone assumes 45 mm of space at the top, and placing it on Letter changes the proportions.

Folding and envelopes

This is where the formats genuinely diverge.

A4: a sheet is folded twice into thirds for a DIN lang envelope (220 × 110 mm). DIN 5008 places fold marks at 105 mm and 210 mm from the top edge for exactly this fold, so the recipient address lands behind the envelope window.

US Letter: the sheet is folded into thirds for a #10 envelope (4.125 × 9.5 in, or 104.8 × 241.3 mm). There is no comparable system of printed fold marks in North American business correspondence — the fold follows the eye, not a marked line.

Which format for which country

The recipient decides, not the sender:

• Germany, the rest of Europe, the United Kingdom, most of Asia: A4. Note that the United Kingdom uses A4, not US Letter.
• United States, Canada: US Letter. An A4 sheet fits a Letter envelope, but it does not fold to the expected proportions and looks unfamiliar.
• Mixed correspondence: many companies keep both letterheads. Printing an A4 PDF on Letter paper usually means scaling it down, which slightly reduces the type size.

For electronic delivery the paper size does not affect what is read, but it does affect what is printed: a recipient in the US printing an A4 PDF will normally use "fit to page" and get a slightly reduced layout.

Converting an existing layout

Changing a letterhead from A4 to Letter — or the reverse — is not a matter of scaling. Stretching the artwork moves every measurement: a 25 mm margin becomes 25.7 mm, and the address field shifts out of position relative to the envelope window.

The correct approach is to rebuild the layout: set the new page size, apply the margins for that format, and reposition the address field and any fold marks. In the TwiceDocs studio, switching the paper format does this automatically and deactivates the DIN-specific fold marks for US Letter.