Paper Sizes Explained: A4, A3, A5, US Letter and Legal in mm and Inches

I spend half my print-prep time confirming a number I should already know by heart: how big is A4, really? Every few weeks a client sends artwork sized for the wrong sheet, the press kicks it back, and I end up measuring margins with a ruler against a PDF. So this guide is the cheat sheet I wish I had pinned to my wall. It covers the ISO A series, the US sizes, the exact dimensions in both millimetres and inches, and the one geometric trick that makes the A series so much nicer to work with than anything else.

The A series, and why A4 is 210×297 mm

The ISO 216 A series is the world's office standard everywhere except the United States and Canada. The headline number to memorise is A4: 210 by 297 mm, which is 8.27 by 11.69 inches. From that single sheet the whole family follows.

Here are the sizes you actually touch:

A3 — 297 by 420 mm (11.69 by 16.54 in)
A4 — 210 by 297 mm (8.27 by 11.69 in)
A5 — 148 by 210 mm (5.83 by 8.27 in)
A6 — 105 by 148 mm (4.13 by 5.83 in)

Look closely and you'll spot the pattern: the long edge of A5 (210 mm) is the short edge of A4. The long edge of A4 (297 mm) is the short edge of A3. Each size is the next one rotated and doubled. That is not a coincidence, and it is the reason the A series beats every other paper standard for day-to-day work.

Why the A series halves cleanly

Cut a sheet of A4 in half across its long edge and you get two sheets of A5. Stack two sheets of A4 side by side and you get one A3. The areas line up perfectly: A5 is exactly half of A4, A3 is exactly twice A4, and so on up the chain to A0, which is precisely one square metre.

The trick is the aspect ratio. Every A size has the same proportion: 1 to the square root of 2 (about 1.414). That ratio is the only one that survives being folded in half — fold it and you get the same shape at half the size, every single time. So when I scale a layout from A4 up to A3 for a poster proof, nothing gets cropped and nothing gets letterboxed. The artwork just grows. Try doing that with US Letter and you'll fight white bars on two edges.

This is also why the C-series envelopes work so neatly: a C5 envelope (162 by 229 mm) swallows a flat A5 or an A4 folded once, because the folding math is built into the standard. If you ever need to scale a design across sizes without distortion, the aspect ratio calculator does the same proportion check for any width and height, not just paper.

US Letter, Legal and Tabloid

North America never adopted ISO 216. Instead it uses sizes defined in inches, which then convert to slightly awkward millimetre figures:

US Letter — 8.5 by 11 in (215.9 by 279.4 mm)
US Legal — 8.5 by 14 in (215.9 by 355.6 mm)
Tabloid / Ledger — 11 by 17 in (279.4 by 431.8 mm)

US Letter is the default in the States the way A4 is everywhere else. Legal is the same width but three inches taller, made for contracts and forms. Tabloid is two Letters side by side, the rough equivalent of A3.

The catch with US sizes is that they don't halve cleanly. Folding Letter in half gives you a 5.5 by 8.5 inch sheet (called Statement), which has a different aspect ratio than its parent, so scaled artwork shifts. The US system trades the elegant geometry of the A series for round inch numbers.

A concrete example: A4 versus US Letter

This is the swap that bites people most often, so it's worth seeing the two sheets together:

A4 — 210 by 297 mm
US Letter — 215.9 by 279.4 mm

A4 is about 6 mm narrower and 18 mm taller than Letter. That sounds tiny, but it is enough to ruin a layout. Send an A4-sized PDF to a US printer loaded with Letter and the page either gets scaled down (shrinking your margins and type) or clips along the bottom edge. Go the other way and you get a strip of empty margin running down the long side.

The fix is boring but reliable: set your document size to the actual target paper before you start, not whichever your software defaulted to. When a client in Europe needs your Letter document on A4, pull the side margins in by a few millimetres and let the page run taller, then re-export. Don't trust "fit to page" to do it invisibly.

Choosing the right size for the job

A few rules of thumb from the press floor:

Body documents, letters, reports — A4 (or Letter in North America). It's the tray default, so it just prints.
Posters, plans, large infographics — A3 or Tabloid. Big enough to read across a room, still feeds through most office machines.
Flyers, postcards, booklets — A5 and A6. Two-up or four-up on an A4 press sheet, which keeps the cost down.
Contracts and legal forms — US Legal where the extra length matters.

Paper size is only half of a print spec, though. The other half is weight — the difference between flimsy 80 gsm copy paper and a stiff 300 gsm business card. If you're picking stock, the paper weight gsm calculator converts between gsm, pounds and caliper so you order the right thickness.

And when you need the full table at a glance — every A, B and C size plus the US sheets, in mm, cm, inches and pixels at 72/150/300 DPI — keep the paper size reference open in a tab. It spells out the folding rule and copies a whole size block in one click, which has saved me more ruler-against-the-screen moments than I'd like to admit.

Memorise one number — A4 is 210 by 297 mm — and the rest of the A series falls out of simple halving. Everything else is just knowing which sheet the job is going to land on before you start designing.

Made by Toolora · Updated 2026-06-13