Article
A4 vs. Letter Printing: The Difference in Millimetres
A4 is 210×297mm, US Letter is 216×279mm. We work out exactly what that 6mm/18mm gap does to a printed page, in millimetres and percent, for nine real handling-and-content combinations.
If you have ever downloaded a PDF template made for the “wrong” paper size for your country, you have probably already noticed that it mostly still works. The page prints, the text is readable, nothing looks obviously broken. That is exactly why this is worth explaining carefully: the mismatch between A4 and US Letter is small enough to be invisible on a coloring page and large enough to break a ruler. Below is the arithmetic, not a claim about what any one printer will do, since driver behavior varies (more on that at the end).
The two numbers that matter: 6 mm and 18 mm
A4 is 210 × 297 mm. US Letter is 215.9 × 279.4 mm, usually rounded to 216 × 279 mm. Subtract the two and you get the entire problem:
- Width: Letter is 5.9 mm wider than A4 (216 vs 210 mm).
- Height: A4 is 17.6 mm taller than Letter (297 vs 279.4 mm).
Neither difference is large on its own. What makes it awkward is that they go in opposite directions — A4 is narrower but taller, Letter is wider but shorter — so there is no single “shrink by X%” answer. The correct scale factor depends on which dimension a given handling option decides to respect, which is what the next three sections work out.
As a side note, A4 also holds slightly more paper: 210 × 297 mm is 623.7 cm², against 215.9 × 279.4 mm at 603.2 cm² — about 3.4% more area on an A4 sheet. That has no bearing on printing, but it explains why an A4 template rarely feels cramped when it lands on the smaller Letter sheet.
What gets clipped at 100 percent, and which edge
“Actual size” or “100% scale” applies no scaling at all, so the millimetre differences above turn directly into clipped content, not shrunk content:
- A4 file printed on Letter paper: the width fits with room to spare (5.9 mm of extra blank margin, split or not depending on the viewer), but the extra 17.6 mm of height does not fit. That strip — commonly the bottom of the page, though some viewers center the page and split the loss 8.8 mm off the top and 8.8 mm off the bottom — is not printed.
- Letter file printed on A4 paper: the reverse. Height has 17.6 mm of slack, but 5.9 mm of width does not fit and is lost off one side (or split between both).
This is the failure mode people usually notice first, because something visibly disappears: a footer, a page number, the last line of a table, the edge of a border. It is also the failure mode that is easiest to diagnose, precisely because it is visible.
What “Fit to page” changes: 94.1% one way, 97.3% the other
“Fit to page” scales the whole page down (or up) until both dimensions fit inside the target sheet, keeping the aspect ratio intact. Because A4 and Letter have different proportions, the scale factor is set by whichever dimension is tightest:
- A4 → Letter: the limiting dimension is height. Scale = 279.4 ÷ 297 = 94.1%. The page shrinks by 5.9% in both directions and lands with extra blank margin on the sides.
- Letter → A4: the limiting dimension is width. Scale = 210 ÷ 215.9 = 97.3%. The page shrinks by only 2.7%, with extra blank margin top and bottom.
Nothing is clipped either way — every element on the page survives, just smaller. That is the trade-off against the 100% option above: Fit to page loses no content but corrupts every measurement on the page uniformly; Actual size preserves every measurement but loses whatever falls in the overflow strip.
Why “Shrink oversized pages” rarely helps between A4 and Letter
Most PDF viewers offer a third option that sounds safer than “Fit”: “Shrink oversized pages” (Adobe Acrobat’s wording; other apps call it something similar). The idea is that it behaves like Actual size for pages that already fit, and only scales down pages that are too big — so a smaller page is never enlarged and a matching page is never touched.
The catch is specific to A4 and Letter: each one is oversized relative to the other in exactly one dimension. A4 is too tall for Letter’s height; Letter is too wide for A4’s width. Since there is always an overflowing dimension in this particular pairing, “Shrink oversized pages” ends up triggering every time and computing the same proportional scale as “Fit to page” — 94.1% for A4 on Letter, 97.3% for Letter on A4. It behaves differently from Fit only when the source page already fits inside the target in both dimensions (for example, printing an A5 file onto A4), which is not the situation here. If your goal was to avoid the shrink, this setting will not deliver it for an A4/Letter mismatch.
Nine combinations: handling choice against content type
Putting the three handling options against three kinds of printable content shows why the same 5.9%/2.7% gap matters enormously for one page and not at all for another. Figures below are for A4 content printed on Letter paper; see the next section for the reverse direction.
| Handling choice | Decorative (coloring page) | Ruled (lined paper) | Measured (graph paper, ring sizer) |
|---|---|---|---|
| Fit to page (94.1%) | Shrinks whole image by 5.9%; nothing lost, wider white border | Line spacing shrinks (8 mm → 7.53 mm); every line still there | 5 mm grid becomes 4.71 mm; the printed ruler is now wrong by 5.9% |
| Actual size (100%) | Bottom ~17.6 mm of artwork is not printed | Bottom 2–3 lines (of typical 8–9 mm ruling) are cut off; remaining lines stay true to size | Grid stays true to 5.00 mm, but ~3–4 rows near the bottom edge are missing |
| Shrink oversized pages | Same result as Fit to page — see note above | Same result as Fit to page | Same result as Fit to page |
Read the columns, not just the rows. A decorative page tolerates either failure mode without real harm. A ruled page tolerates the scaling failure fine (nobody measures handwriting-guide spacing with a ruler) but loses actual lines under Actual size. A measured page is the mirror image: Actual size keeps every printed number honest at the cost of losing a few rows, while Fit to page keeps every row but makes every number quietly wrong. For a graph paper grid or a ring sizer, that second failure is the dangerous one because nothing on the page tells you it happened.
A 5 mm square at 97.3%: 4.86 mm, and why that is worse than it looks
Take the gentler of the two Fit-to-page scale factors, 97.3% (Letter content resized to fit A4 width). A 5 mm grid square becomes 5 × 0.973 = 4.86 mm. Over a 20-square span meant to be 100 mm, you get 97.3 mm — off by 2.7 mm, which is smaller than the width of a pencil line and will not be visible to the eye on the page.
It is not invisible to a caliper, a seam allowance, or a ring. Published US ring-size charts put a size 6 at roughly 51.8 mm inside circumference and a size 7 at roughly 54.4 mm — a gap of about 2.6 mm per whole size. Scale a size-7 mark by 97.3% and it becomes 54.4 × 0.973 ≈ 52.9 mm: almost exactly halfway to a size 6. Scale it by the harsher 94.1% (the A4-on-Letter direction) and it becomes 54.4 × 0.941 ≈ 51.2 mm, reading as a size 6 outright. This is arithmetic on published ring-size dimensions, not a claim about any one printed strip we measured — but it shows precisely why a scaling error too small to see with the naked eye is large enough to hand someone the wrong ring size, or throw off a sewing pattern, a stencil, or a name-tracing letter height by a comparable margin.
A4 on Letter versus Letter on A4: not mirror images
Because the two dimensions differ by different amounts, printing in each direction fails differently rather than being symmetric:
| Direction | Actual size (100%) clips | Fit to page scales content to |
|---|---|---|
| A4 file → Letter paper | ~17.6 mm off the bottom (or split top/bottom) | 94.1% — limited by height |
| Letter file → A4 paper | ~5.9 mm off one side (or split both sides) | 97.3% — limited by width |
Practically: an A4 template printed on a US printer at Actual size loses more content (17.6 mm of height) than a Letter template printed on an A4 printer at Actual size (5.9 mm of width). But flip to Fit to page and it reverses — the A4-on-Letter shrink is the more aggressive one (94.1%, a 5.9% reduction) while Letter-on-A4 is gentler (97.3%, a 2.7% reduction). There is no version of this that is “safe by default” in one direction only; each direction just breaks differently.
When none of this changes anything
For most printable content, none of the above matters in practice. A coloring page, a chore chart, a certificate, a to-do list, a crossword — anything read or filled in rather than measured — survives a 3–6% scale change or a centimeter of edge clipping without losing its function. If you print our chore chart or any coloring page on the “wrong” paper size at Fit to page, you will not notice, because nothing on the page depends on a specific millimetre value.
The line is content that gets compared to something outside the page after printing: a ruler, another sheet, a real object, a hole punch, a pre-drilled binder. Lined paper sits in between — the ruling itself tolerates scaling, but a three-hole-punch template or a page meant to face a pre-printed insert does not.
What we can’t tell you from here
This article is arithmetic on the two paper standards, cross-checked against how mainstream viewers describe their own scaling options. It cannot tell you what your specific printer driver will do. Some drivers add their own non-printable hardware margin (commonly 3–5 mm on inkjets, sometimes more) on top of whatever the application already scaled, which shifts the exact millimetre figures slightly. Some center an oversized page and split the clipped strip between both edges; others anchor it to a corner and take the whole loss from one side — we described the common case, not a universal one. And “borderless” or “auto-fit” modes on some consumer printers apply an additional silent enlargement specifically to avoid white edges, which is a different effect again and not something we can quantify without your exact printer model. If a measurement matters, the only way to be certain is the calibration test described in our printing guide: print a known length at 100% and check it with a ruler before trusting the sheet.
The short version: if you can’t verify it with a ruler, don’t build something in reality from it. Downloading a template made for the size your printer actually holds — or generating it fresh at that size from printable paper — removes the question entirely instead of requiring you to correct for it.