A3 paper size in cm, mm, inches and pixels.

A3 dimensions in mm, cm, inches and points

A3 is defined by the ISO 216 standard as 297 mm wide and 420 mm high. Turn the sheet sideways for landscape; the size stays the same and only the order of the numbers changes.

Every A size has the same shape: the long side is √2 (about 1.414) times the short side. That is what makes the series work. Cut an A3 sheet in half across its long side and you get two A4 sheets with exactly the same proportions, so a design can be scaled between sizes without cropping or stretching.

What is A3 size in pixels?

Paper has no pixels, so the pixel size depends on the print resolution you want. Multiply the size in inches by the pixels per inch (PPI), or divide the size in millimeters by 25.4 first.

For a sharp A3 print, aim for 3508 × 4961 pixels (300 PPI). A poster seen from a metre or more away looks fine at 150 to 200 PPI, which needs far fewer pixels: 2339 × 3307 at 200 PPI is under half the pixel count. A 12-megapixel phone photo (4032 × 3024) printed across the full A3 width has about 244 PPI in landscape, which prints well.

To make an image this size, use the Image Resizer with the aspect ratio locked, and crop to the A-series shape first with the Crop Image tool (custom ratio 297 : 420). If a print shop wants the file tagged at 300 DPI, set the tag without resampling in the DPI Converter.

A3 compared with A4, A2 and the rest of the A series

Because the short side of A3 equals the long side of A4 (297 mm), two A4 pages fit side by side on one A3 sheet. That is why A3 is used for printed spreads, folded A4 brochures and booklets, and for enlarging A4 documents such as spreadsheets and drawings. Scaling A4 to A3 is 141%, and A3 to A4 is 71%, which is also the standard “A3 → A4” preset on office copiers.

A3 vs tabloid (11 × 17 inches)

In the US and Canada the nearest size is tabloid, also called ledger when used in landscape: 11 × 17 inches, or 279.4 × 431.8 mm. It plays the same role as A3, being two US Letter pages side by side, but the shapes differ.

A3 is 17.6 mm wider and 11.8 mm shorter than tabloid. If you print an A3 PDF on tabloid paper with “fit to page”, the printer scales it to about 94% and leaves white space at the top and bottom. Printing tabloid on A3 scales it to about 97%. For posters that difference rarely matters; for technical drawings with a printed scale, print at 100% on the paper the file was made for. There is also a larger “A3+” or Super B size, 13 × 19 inches (about 329 × 483 mm), sold for photo printers so that a full A3 image can be printed with a margin.

Printing and setting up an A3 document

1. Check the printer.The printer must list A3 support and have A3 paper loaded. Many office multifunction printers have a separate A3 tray; most home inkjets do not.
2. Set the page size in your app.In Word, choose Layout → Size → A3. In design apps, start a new document at 297 × 420 mm, plus bleed if it will be trimmed.
3. Add bleed for edge-to-edge printing.Print shops usually ask for 3 mm of bleed on every side, making the file 303 × 426 mm, with important text at least 5 mm inside the trim.
4. Choose the right resolution.Photos at 200 to 300 PPI, from the table above. Vector text and shapes in a PDF are sharp at any size.
5. Print at actual size.Turn off “fit to page” or “shrink oversized pages” unless you are printing on a different paper size on purpose.

Sending an A3 PDF by email or to an upload form? Large posters with photos are often several megabytes; the PDF compressor can bring one under 1 MB while keeping its text sharp.

Frequently asked questions

Calculation note: inches = millimeters ÷ 25.4; pixels = inches × PPI, rounded to whole pixels. Points are 1/72 inch, the unit used for PDF page sizes. Tabloid dimensions are exact; A3 inch values are rounded.