A4 vs US Letter: Paper Sizes Explained

Most of the world prints on A4. North America prints on US Letter. The two are close enough to look interchangeable and different enough to quietly ruin a document when it crosses between them — a header clipped, a footer pushed onto a second page, a grid that comes out 6% too small.

The sizes come from two completely different ideas about what a sheet of paper should be. Once you see them, the conversions and the printing problems both make sense.

The A-series: one clever idea

The international A-series is defined by ISO 216, and the whole family grows from a single rule: every sheet has the same shape, a length-to-width ratio of √2 (about 1.414). That ratio is special because halving a √2 rectangle across its longer side gives two smaller rectangles of exactly the same shape. Fold an A4 sheet in half and you get two A5 sheets with identical proportions; A5 halved gives A6, and so on, up and down the range.

The series is anchored by area, not by any round dimension: A0 has an area of exactly one square metre. Every step down halves the area. That is why the millimetre figures look arbitrary — they are whatever the √2 shape and the 1 m² anchor produce, rounded to whole millimetres.

Size	Dimensions (mm)	Dimensions (in)
A0	841 × 1189	33.1 × 46.8
A1	594 × 841	23.4 × 33.1
A2	420 × 594	16.5 × 23.4
A3	297 × 420	11.7 × 16.5
A4	210 × 297	8.3 × 11.7
A5	148 × 210	5.8 × 8.3
A6	105 × 148	4.1 × 5.8

A4, the everyday sheet, is 210 × 297 mm. Those figures are nominal: ISO 216 allows a tolerance of a millimetre or two, which is why two "identical" reams can differ slightly.

B and C: the relatives

The B-series, also in ISO 216, fills the gaps between the A sizes. Each B size is the geometric mean of the matching A size and the next one up, so B4 sits between A4 and A3. B sizes turn up in books, posters and passports.

Size	Dimensions (mm)
B4	250 × 353
B5	176 × 250
B6	125 × 176

The C-series (ISO 269) is for envelopes, sized so that A-series paper slips inside.

Envelope	Dimensions (mm)	Holds
C4	229 × 324	A4 flat
C5	162 × 229	A4 folded once, or A5 flat
C6	114 × 162	A4 folded twice, or A6 flat
DL	110 × 220	A4 folded into thirds

DL is the everyday business-letter envelope. It is not a true C-series size — its shape is different — but it is the one most window envelopes use.

The US sizes: no system at all

North American sizes come from a different tradition and follow no mathematical rule. The common ones are the ANSI lettered series, with ANSI A = Letter as the base, standardised in ANSI/ASME Y14.1.

Name	ANSI	Dimensions (in)	Dimensions (mm)
Letter	ANSI A	8.5 × 11	215.9 × 279.4
Legal	—	8.5 × 14	215.9 × 355.6
Tabloid / Ledger	ANSI B	11 × 17	279.4 × 431.8
ANSI C	ANSI C	17 × 22	431.8 × 558.8

The millimetre figures are exact, because the inch is exactly 25.4 mm: 8.5 in is precisely 215.9 mm, and 11 in is 279.4 mm.

The ANSI series has one awkward quirk. Doubling Letter gives Tabloid (11 × 17), doubling again gives ANSI C, and so on — but because Letter's ratio is 11 ÷ 8.5 ≈ 1.29, not √2, the shape alternates at each step instead of staying constant. That is the practical advantage the A-series has and the US series lacks: scale an A4 drawing up to A3 and nothing is distorted.

A4 vs Letter, side by side

This is the pair that actually matters, because documents move between the two constantly.

	A4	US Letter
Width	210 mm (8.27 in)	215.9 mm (8.5 in)
Height	297 mm (11.69 in)	279.4 mm (11 in)
Shape	taller, narrower	shorter, wider

Letter is about 6 mm wider and about 18 mm shorter than A4. Neither fits inside the other, which is the root of every cross-printing headache:

• An A4 PDF printed on Letter is too tall, so the driver scales it down to roughly 94% (279.4 ÷ 297) to make it fit. A 5 mm grid then prints at 4.7 mm, and a carefully set margin shrinks with it.
• A Letter PDF printed on A4 is shorter than the sheet, so it usually prints at full size with a wider bottom margin — but anything set close to the right-hand edge can be clipped, because Letter is the wider of the two.

If a printed page comes out the wrong size, the cause is almost always this mismatch combined with a "fit to page" setting. Our guide on printing at the exact size lists the setting to change in each program.

Worked example: will my A4 report fit on Letter?

A colleague in the US needs to print your A4 report.

1. A4 is 297 mm tall; Letter is 279.4 mm tall. The report is 17.6 mm too tall.
2. To fit, the printer scales everything to 279.4 ÷ 297 = 0.94, or 94%.
3. At 94%, 11 pt body text prints at about 10.3 pt — still readable, but your page count and line breaks may shift.

The fix is to design for both from the start. Keep a margin of at least 19 mm (0.75 in) top and bottom and 19 mm left and right, and the content sits safely inside A4 and Letter. Most word processors also offer a "US Letter" page size — switching to it before sending a document abroad avoids the scaling entirely.

Where the sizes came from

The A-series maths was first described by the German physicist Georg Christoph Lichtenberg, in a letter dated 25 October 1786 that noted the convenience of a √2 sheet. It became the German standard DIN 476 in 1922, spread across Europe, and was adopted internationally as ISO 216 in 1975. Today it is the standard everywhere except North America and a handful of other countries.

US Letter has no such tidy origin — the exact reason for 8.5 × 11 in is not documented. It was made the standard size for US federal paperwork in the early 1980s, which cemented it across American offices.

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