A4 versus Letter: 6 mm that ruins a print run

The numbers

A4 is 210 × 297 mm, or 8.27 × 11.69 inches. US Letter is 8.5 × 11 inches, or 216 × 279 mm. Letter is 6 mm wider and 18 mm shorter.

What that does to a document

Send an A4 layout to a Letter printer without scaling and the bottom 18 mm never arrives — usually a footer, a page number or the last line of a table. Going the other way leaves an unexpectedly deep bottom margin and a layout that looks unbalanced.

Why A4 halves so neatly

The A series is built on a ratio of 1 to the square root of two. Cut any sheet across its long side and you get the next size down with identical proportions. A3 is exactly two A4s; A5 is exactly half of one. Letter has no such relationship with Legal or Tabloid, which is why enlarging between American sizes always distorts.

Working across both

1. Set the page size before you write, not at print time.
2. Use the standard of the country the document is going to.
3. Keep at least 15 mm of margin so a mismatch clips white space rather than text.
4. If you must cross standards, enable scale-to-fit rather than accepting the crop.

One detail for commercial print

Add 3 mm of bleed on every edge for anything that prints to the paper's edge. Trimming tolerance is real, and a background that stops exactly at the trim line will show a white sliver on some copies.

Where each standard came from

The A series was proposed in Germany in 1922 and standardised internationally as ISO 216. Its defining property — the 1:√2 ratio — was chosen precisely so that halving a sheet preserves the proportions, which makes scaling between sizes lossless and enlargement percentages predictable.

US Letter has no such derivation. The 8.5 × 11 inch sheet became standard through manufacturing convention, and was formalised for US federal use in the 1980s. Because it has no consistent ratio with Legal (8.5 × 14) or Tabloid (11 × 17), scaling between American sizes always distorts or crops.

The full A series

Each step halves the area. That is why enlarging A4 to A3 is exactly 141 per cent, and reducing A3 to A4 is exactly 71 per cent — figures printed on every office photocopier.

What scale-to-fit actually does

Printing A4 content on Letter with scaling enabled reduces everything to about 94 per cent. Body text survives that comfortably. Two things do not:

• Fine rules and hairlines can drop below the printer's minimum stroke width and disappear in patches.
• Pre-printed stationery no longer aligns, because the letterhead was printed at 100 per cent and your content is not.

For anything that will be signed, filed or bound, set the correct page size instead of scaling.

Margins that survive both standards

If a document genuinely has to print on either standard, design for the intersection: the narrower width of A4 (210 mm) and the shorter height of Letter (279 mm). Keep at least 15 mm of margin on every edge and avoid full-bleed elements. The result prints acceptably on both without scaling.

Envelopes follow the same logic

The C series is sized to hold the A series: a C4 envelope takes an unfolded A4 sheet, C5 takes A4 folded once, and C6 takes it folded twice. American envelope sizes — No. 10, for instance — relate to Letter by convention rather than construction, which is another place the two systems refuse to interoperate cleanly.

The takeaway

Six millimetres of width and eighteen of height sound trivial and are not. Set the page size at the start, match the destination country, and reserve scale-to-fit for drafts.