A3 Paper Size — 297 × 420 mm and Inches

A3 paper size is 297 × 420 millimeters. In inches, rounded to two decimals, that is 11.69 × 16.54. Those millimeters come from ISO 216. The inches are 297 and 420 divided by 25.4.

A3 is the drawing and poster size one step above A4. It is not a wider A4. The short side of A3 is the long side of A4. Two A4 sheets placed side by side along their long edges make one A3 sheet, with no trim left over. That is the point of the series, described in Markus Kuhn’s notes on ISO 216 (International standard paper sizes).

A3 measurements

Unit	Width	Height
Millimeters	297	420
Centimeters	29.7	42.0
Inches (rounded)	11.69	16.54

A3 paper dimensions and a3 paper in inches are this same pair. Portrait is 297 mm wide. Landscape is 420 mm wide. Swapping them does not create a new format name.

In PostScript points the sheet is about 842 × 1191 pt. Use millimeters on a printer ticket so the point rounding never becomes the spec.

A3 and A4

	Millimeters	Inches (rounded)
A4	210 × 297	8.27 × 11.69
A3	297 × 420	11.69 × 16.54

The 297 mm edge is shared. An A3 page can hold two A4 pages at full size, side by side, for a spread or a workbook sheet. It cannot hold two A4 pages plus a margin between them unless you scale the pages down.

A3 vs A4 in practice:

• A report or a letter stays on A4. Moving it to A3 makes the type look stranded unless you redesign the grid.
• A plan, a map, a poster, or a newspaper-style spread that must be read at arm’s length is a reason to go to A3.
• Imposing two A4 pages onto one A3 sheet is a production choice. The reader still thinks of the page as A4. The printer thinks of the sheet as A3.

A4’s own numbers, and the A5 cut, are in A4 Paper Size.

A3 and US tabloid

US tabloid is 11 × 17 inches, which is 279.4 × 431.8 mm. A3 is 297 × 420 mm.

	Width	Height
A3	297 mm / 11.69 in	420 mm / 16.54 in
Tabloid	279.4 mm / 11 in	431.8 mm / 17 in

A3 is wider and shorter than tabloid. The difference is about 17.6 mm in each direction, the same kind of mismatch as A4 versus Letter. “Fit to page” will scale a drawing and move title-block text. If the shop is in North America and stocks 11 × 17, design for tabloid. If the shop stocks ISO, design for A3. The tabloid and ledger names are sorted out in Tabloid Paper Size.

When A3 is the right sheet

A3 is common for:

• Diagrams and simple architectural sketches that are not yet on A2 or A1
• Posters that must ship in a standard ISO sleeve
• Workbooks and exam papers that show two A4 pages on one leaf
• Desktop inkjet printers that list A3 as the largest tray

It is a poor default for a contract or a slide handout. Those want A4 or Letter so the reader can file them. A3 that gets folded into an A4 folder needs a fold that does not cut through a title. Fold once, parallel to the short side, and the result is A4-shaped. The fold is a production mark, not a change of format.

The next sizes up, A2 and A1, are for posters and drawings that A3 cannot hold at a readable scale. Those dimensions are in A2 Paper Size.

Bleed sits outside 297 × 420. Ask the printer for the bleed they cut. A frequent digital-print request is 3 mm. Do not add that 3 mm to the page size in the design file’s “trim” field. Trim stays 297 × 420. Bleed is an extra.

A two-up imposition is arithmetic, not a style. Each A4 page is 210 mm wide. Two of them are 420 mm, which is exactly the long side of A3. The gutter between the two pages has to come from inside those 210 mm margins, or the pair no longer equals 420 mm and the crop marks move. If you need 10 mm between the pages, each A4 live area shrinks. Do not “add 10 mm” to the A3 width. The sheet will not grow to match the file.

Office printers that claim A3 sometimes mean the tray accepts it and the margin still eats 5 mm a side. A line drawn at the trim will not print. Keep rules at least a few millimeters inside 297 × 420 unless the shop has confirmed borderless A3.

FAQ

What are A3 paper measurements?

297 mm wide by 420 mm tall in portrait, or 11.69 × 16.54 inches when rounded to two decimals.

How many A4 sheets fit on A3?

Two, at 100 percent, placed on the long side of each A4 against the short side of the A3. There is no leftover strip.

Is A3 the same as tabloid?

No. Tabloid is 11 × 17 inches (279.4 × 431.8 mm). A3 is wider and shorter. See Tabloid Paper Size.

Is A3 bigger than A4?

Yes. A3 has twice the area of A4. The long side of A4 becomes the short side of A3.

What to open next

Set the artboard to 297 × 420 mm and only then place the drawing. If the printer quoted 11 × 17, switch to the tabloid size instead of scaling this file at the end. The series index is the Paper Sizes chart.