A4 vs US Letter: exact sizes in inches, mm and cm, and how to print
A4 measures 210 × 297 mm (8.27 × 11.69 in) and US Letter measures 8.5 × 11 in (215.9 × 279.4 mm). Letter is 5.9 mm wider and 17.6 mm shorter, so neither sheet fits inside the other. This article puts both on one exact scale, shows the scale factor a printer applies when you print one on the other, and explains why a document that looked fine on one paper size breaks its lines and pages on the other.
The two definitions
The two standards start from different places. A4 belongs to the ISO 216 family, where every sheet has a long-to-short side ratio of the square root of 2, so cutting a sheet in half across its long side gives the next size with the same shape. A4 is defined as 210 × 297 mm. US Letter is defined in inches, as 8.5 × 11, and has no such halving rule.
The inch is tied to the metric system exactly: 1 inch = 25.4 mm, fixed by the 1959 international agreement and listed in NIST SP 811 and by the BIPM. That means Letter converts to millimeters without any rounding error, and the figures below are exact apart from the digits we choose to show. For any other size use the inches to cm converter or the cm to inches converter; for millimeters, see the mm to inches converter.
A4, Letter and their neighbors in one table
The last two columns are the shape ratio (long side divided by short side) and the area. A4 has a ratio of 1.414, while Letter is squarer at 1.294.
| Paper | Inches | Millimeters | Centimeters | Ratio | Area |
|---|---|---|---|---|---|
| A4 | 8.27 × 11.69 in | 210 × 297 mm | 21 × 29.7 cm | 1.414 | 623.7 cm² |
| US Letter | 8.5 × 11 in | 215.9 × 279.4 mm | 21.59 × 27.94 cm | 1.294 | 603.2 cm² |
| US Legal | 8.5 × 14 in | 215.9 × 355.6 mm | 21.59 × 35.56 cm | 1.647 | 767.7 cm² |
| Tabloid (11 × 17 in) | 11 × 17 in | 279.4 × 431.8 mm | 27.94 × 43.18 cm | 1.545 | 1,206.4 cm² |
| A3 | 11.69 × 16.54 in | 297 × 420 mm | 29.7 × 42 cm | 1.414 | 1,247.4 cm² |
| A5 | 5.83 × 8.27 in | 148 × 210 mm | 14.8 × 21 cm | 1.419 | 310.8 cm² |
| A6 | 4.13 × 5.83 in | 105 × 148 mm | 10.5 × 14.8 cm | 1.41 | 155.4 cm² |
How much bigger is one than the other?
Letter is 5.9 mm (0.59 cm) wider than A4, and A4 is 17.6 mm (1.76 cm) taller. In area A4 covers 623.7 cm² and Letter 603.2 cm², so A4 is 3.4% larger by area. The practical result is that A4 is a narrower, longer sheet and Letter is a wider, shorter one. A stack of Letter paper does not fit in an A4 tray without the edges overhanging, and the opposite is also true.
Printing one on the other
When a printer shrinks a page to fit, it scales both sides by the same factor, and the tighter side decides how much. A Letter page sent to A4 paper is limited by the width: 210 ÷ 215.9 = 97.3%, so the text shrinks by about 2.7%. An A4 page sent to Letter paper is limited by the height: 279.4 ÷ 297 = 94.1%, a shrink of 5.9%.
A shrink of 3% to 6% is small, but it moves every line, and a printout with a table or a form can break. Without fit-to-page, the printer prints at 100% and clips the page: an A4 document on Letter loses 17.6 mm from the bottom edge, and a Letter document on A4 loses 5.9 mm from the side.
What a 1 inch margin leaves you
Take a standard 1 inch (25.4 mm) margin on all four sides. The writing area on Letter becomes 165.1 × 228.6 mm and on A4 it is 159.2 × 246.2 mm. At 12 pt text with 1.15 line spacing, that is about 46 lines per Letter page and 50 lines per A4 page, so the same document is about 4 lines longer per page on A4 but a little narrower per line.
Word processors show page sizes in points as well: one point is 1/72 inch (0.3528 mm), so Letter is 612 × 792 pt and A4 is 595 × 842 pt after rounding. If you design in a drawing or layout tool, set the page in one of these units instead of eyeballing it.
Letter, Legal and the sizes people confuse
US Legal keeps Letter's width and adds length: 8.5 × 14 in is 35.56 cm tall, 7.62 cm more than Letter. Tabloid, 11 × 17 in, is two Letter sheets side by side and measures 27.94 × 43.18 cm. The closest ISO size to Tabloid is A3 at 29.7 × 42 cm, which is 1.76 cm wider and -1.18 cm taller, so the two are not interchangeable either.
Photo sizes follow a related story. A 4 × 6 in print is 10.16 × 15.24 cm, while the metric 10 × 15 cm print is a little smaller. The photo and paper sizes chart lists the common photo, poster and paper sizes side by side, and the inches to cm chart covers anything else.
A short checklist before you print or order
- Set the paper size in the document first (File, Page Setup) and then write. Changing it afterwards re-flows everything.
- For documents that cross borders, keep content inside a safe area: at least 15 mm from every edge fits both sheets.
- Use PDF export with the paper size embedded, so the receiving side sees the layout you intended.
- When buying folders, binders, frames or envelopes, check the paper standard they were made for. A4 sheets sit loosely in a Letter binder and Letter sheets stick out of an A4 one.
- Choose "actual size" for forms and templates where lengths matter, and "fit to page" only for reading copies.
Related reading
The same exact-conversion idea powers the other series on this site: how to read a tape measure in inches and mm explains the sixteenth ladder behind inch fractions, and phone and tablet screen sizes in cm shows how a screen's diagonal turns into real width and height, the same step you used here. If you work with fractions of an inch, the fractions of an inch guide has the full 1/64 to 1 inch chart. For a feel of how screens compare with paper, laptop screen sizes in cm lists real widths and heights.
Frequently asked questions
What is the size of A4 in inches?
A4 is 210 × 297 mm, which is 8.27 × 11.69 inches, or 21 × 29.7 cm.
What is US Letter in cm?
US Letter is 8.5 × 11 inches, which is 21.59 × 27.94 cm (215.9 × 279.4 mm).
Is A4 bigger than Letter?
A4 is longer (17.6 mm) and Letter is wider (5.9 mm). By area, A4 is slightly larger: 623.7 cm² against 603.2 cm².
What scale do I use to print Letter on A4?
About 97.3%, because the width is the tighter side. To print A4 on Letter, use about 94.1%.
Why is A4 a different shape from Letter?
A4 belongs to the ISO 216 family where the long side is the short side times the square root of 2, so each size halves into the next. Letter is defined in whole and half inches, 8.5 by 11, with no such rule.
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