Paper Sizes in cm, mm and Inches

A4, Letter, and why the difference matters

Most of the world prints on A4, which is 210 × 297 mm, or 21.0 × 29.7 cm. North America prints on US Letter, 215.9 × 279.4 mm. Those two look close enough to be interchangeable and are different enough to ruin a print job. Letter is roughly 6 mm wider and 18 mm shorter. Send an A4 layout to a Letter printer without scaling and the bottom of the page is clipped. Go the other way and the margins come out uneven. If a document has ever emerged from a printer mysteriously cropped, this is usually the reason.

The A series comes from ISO 216 and rests on one elegant rule: every sheet has proportions of 1:√2, so halving one across its long side gives the next size down with exactly the same shape. A0 is defined as one square metre of area and everything follows. A1 is half of A0, A4 is a sixteenth, and scaling artwork between any two A sizes never leaves an awkward margin. US sizes have no such relationship, which is why they scale badly against one another.

One detail worth knowing: the official A-series figures are rounded down to whole millimetres. A4 is quoted as 210 × 297 mm rather than the 210.22 × 297.30 the raw geometry would give. The rounded values are the standard, and they are what printers and software use.

The full history of the 1:√2 ratio, from an eighteenth-century letter to an international standard, is in the ISO 216 paper standard explained. For exactly what changes between the two formats and how to avoid a cropped print, see A4 vs Letter.

Pixel dimensions for printing

Paper is measured in millimetres, design software works in pixels, and resolution is the bridge between them. Taking A4 as the example:

• 300 DPI, the normal target for print, gives 2480 × 3508 pixels.
• 150 DPI, fine for a draft or an office document, gives 1240 × 1754.
• 600 DPI, used for fine art and photographic printing, gives 4961 × 7016.

The table above gives these for whichever size and orientation you select, so you can set up a canvas at the right dimensions first time.

Working at 72 DPI suits screen previews and nothing else. A file prepared at 72 DPI and sent to print looks soft, because the printer has roughly a quarter of the detail per inch that it needs.

Seeing paper at actual size

Numbers only take you so far when the real question is whether something will fit. The viewer above draws the selected sheet at its true physical size, using the same calibration as the online ruler. Hold a real sheet against the display and it should match edge to edge.

Screen size is the limit, and the tool says so rather than quietly misleading you. A4 in portrait needs about 8.3 inches of screen width, so it fits most laptops and desktop monitors but not a phone. Anything too large, meaning A0 through A2 or Tabloid, is scaled down with the exact percentage stated underneath, so you always know whether you are looking at real size or a reduction. Fullscreen gives the viewer more room to work with. To check what your display can actually accommodate, the screen size checker will tell you.

Envelopes: which C size takes which sheet

ISO 216 defines three series, and the one nobody mentions is the one that answers the envelope question. A is paper. C is envelopes, sized so that a C envelope swallows the A sheet of the same number. B sits between the two, which is why B4 and B5 turn up as folder and book sizes.

• C4, 229 × 324 mm, holds an A4 sheet flat and unfolded.
• C5, 162 × 229 mm, holds A4 folded once, or A5 flat.
• C6, 114 × 162 mm, holds A4 folded twice, or A6 flat.
• DL, 110 × 220 mm, is the business envelope: A4 folded into thirds. It is not part of the C series, which is why it is the odd shape in the drawer.

The pattern is worth remembering because it removes guesswork at the post office. Each C size is a little larger than its A counterpart so the sheet actually goes in, and each fold moves you one number down the list.

Sending a document to the other side of the Atlantic

A PDF laid out for A4 and printed on Letter loses about 18 mm off the bottom, because Letter is the shorter sheet. Going the other way, Letter artwork on A4 leaves an uneven strip down one side, because A4 is the narrower one. Neither is a printer fault and neither shows up until someone prints it.

Three habits avoid it. Set the paper size in the document before you write it rather than at the print dialog. Keep at least 15 mm of margin all round, which is wider than the difference between the two formats and so absorbs it. And when you are sending a file you cannot control the printing of, say which size it was laid out for, either in the covering note or in the filename. The scaling table above gives the exact percentage in either direction if the sheet is already fixed.

Bleed, margins and why designers ask for a larger canvas

Anything meant to print colour right to the edge has to be drawn larger than the finished sheet. Printers cut through a stack, the stack shifts a fraction, and a design that stopped exactly at the trim line would show a white sliver on some copies. The convention is 3 mm of bleed on every side, so an A4 flyer is drawn at 216 × 303 mm and trimmed back to 210 × 297.

The same reasoning applies inwards. Keep text at least 5 mm clear of the trim line, and further on anything bound, because the fold steals a few more millimetres. Home and office printers add their own limit: most cannot print to the edge at all and reserve a margin of around 5 mm, which is why a full-bleed design run off on a desktop printer comes back with a white border whatever the file said.