A4 and Letter differ by 6mm in width and 18mm in height. That is small enough to look interchangeable and large enough to ruin a print run.
The sizes
| Name | mm | inches | Used for |
|---|---|---|---|
| A4 | 210 × 297 | 8.27 × 11.69 | Standard almost worldwide |
| US Letter | 216 × 279 | 8.5 × 11 | US, Canada, Mexico, Philippines |
| US Legal | 216 × 356 | 8.5 × 14 | US contracts, some court filings |
| A3 | 297 × 420 | 11.69 × 16.54 | Posters, spreads, large tables |
| A5 | 148 × 210 | 5.83 × 8.27 | Booklets, notebooks |
| Tabloid / Ledger | 279 × 432 | 11 × 17 | US large format |
The A series has a property Letter does not: every size is exactly half the one above, because the ratio is the square root of two. A3 folded in half is A4. That is why scaling between A sizes is clean — 141% or 71% — while there is no tidy factor between A4 and Letter.
What happens when they meet
A4 is 6 mm narrower and 18 mm taller than Letter. Printing one on the other has three possible outcomes and no good one.
- Scale to fit. A4 onto Letter shrinks to about 94%, so everything is slightly smaller and the margins grow. Usually acceptable, occasionally not — a form with fixed field positions no longer aligns.
- Print at actual size. A4 onto Letter loses 18 mm from the bottom. If the page has a footer, page number or signature line there, it is gone.
- Letter onto A4 gains 18 mm of vertical margin and loses 6 mm of width, so the right edge can clip.
For most documents the 94% scale is invisible and fine. For anything printed to measure — a technical drawing, a form, a document with a signature block positioned exactly — it is not, and the fix is the right paper rather than a print setting.
Legal size
US Legal is Letter width and 77 mm taller. It exists for documents that benefit from more lines per page, and its use is narrower than the name suggests — plenty of legal work in the US is on Letter.
Its practical problem is that many printers do not have a Legal tray, so a Legal document arrives at a Letter-only printer and either scales to 78% or loses the bottom quarter.
If you receive a Legal PDF and need it on Letter, scaling to fit is usually right — the content stays complete and shrinks a fifth. If it must stay full size, splitting across two pages preserves legibility better than shrinking, and Split PDF is not the tool for that. It needs re-laying out in the source.
Portrait or landscape
A simpler decision, made badly surprisingly often.
Portrait for anything read as prose. Line length is the reason — a landscape A4 line of body text is around 110 characters, well past the 45 to 75 that reads comfortably, and the eye loses its place returning to the next line.
Landscape for wide tables, timelines, charts and anything presented on a screen, which is landscape itself.
A document mixing both is fine and often correct — a report with one wide table. Note that mixed orientation prints unpredictably on some drivers, so check every page rather than trusting a preview of the first.
Deciding before you lay out
Nearly all of this is avoidable by picking the size before the design rather than after.
- Who prints it? North America means Letter. Anywhere else means A4. Both means designing for the smaller intersection.
- The safe intersection is Letter width with A4 height — 216 mm wide content area within 297 mm of height. Keep content inside that and it prints acceptably on either.
- Keep 10 mm clear of every edge regardless. No consumer printer reaches the sheet edge, which is separate from the size question and covered in why your PDF will not print properly.
- If it is screen-only, stop worrying. Page size only matters at the moment something is printed.
Frequently asked questions
What is the difference between A4 and Letter?
A4 is 210 × 297mm and Letter is 216 × 279mm. Letter is 6mm wider and 18mm shorter, so neither dimension matches. A4 printed on Letter either scales to about 94% or loses 18mm from the bottom.
Can I print an A4 document on Letter paper?
Yes, scaled to about 94%, which is invisible for most documents. It matters for anything printed to measure — a form with fixed field positions, a technical drawing, or a page with a signature line near the bottom edge.
When should I use Legal size?
When a document genuinely benefits from more lines per page, which is narrower than the name suggests — much US legal work is on Letter. Bear in mind many printers have no Legal tray, so it may arrive somewhere that can only scale it to 78%.
Should my document be portrait or landscape?
Portrait for prose — a landscape A4 line runs to about 110 characters, well past the 45 to 75 that reads comfortably. Landscape for wide tables, charts and anything shown on a screen, which is landscape itself.
How do I make a document that prints well on both A4 and Letter?
Design for the intersection: Letter width, A4 height. Keep the content area within 216mm wide and 279mm tall, with 10mm clear of every edge, and it prints acceptably on either without scaling surprises.
Why is scaling between A sizes always a round number?
Because the A series uses a ratio of the square root of two, so each size is exactly half the one above. A3 to A4 is 71%, A4 to A3 is 141%. There is no equivalent tidy factor between A4 and Letter — the standards were designed independently.