Poster sizes: A4, A3, 18 x 24 and pixels at 300 dpi

The standard poster sizes are A4 (210 × 297 mm), A3 (297 × 420 mm) and A2 (420 × 594 mm) in most countries, and 11 × 17, 18 × 24 and 24 × 36 inches in North America. To design one for print, work at 300 dpi: an A4 poster is 2480 × 3508 pixels, A3 is 3508 × 4961 and an 18 × 24 inch poster is 5400 × 7200. The table below lists every common size, and the sections after it explain which to choose and how to handle the very large ones.

Standard poster sizes in pixels at 300 dpi

How to work out a poster's size in pixels

Pixels equal inches multiplied by dots per inch. For a size given in millimetres, divide by 25.4 first. So an A3 sheet is 297 mm wide: 297 ÷ 25.4 = 11.69 inches, and 11.69 × 300 = 3508 pixels. The same sum gives the height of 4961. Our explainer on DPI vs PPI covers why print shops talk about dpi when they mean pixels per inch.

pixels = (millimetres / 25.4) x dpi

A3 width:   297 / 25.4 x 300 = 3508 px
A3 height:  420 / 25.4 x 300 = 4961 px

18 x 24 in at 300 dpi = 5400 x 7200 px
18 x 24 in at 150 dpi = 2700 x 3600 px

Which poster size to choose

• A4 or US Letter for noticeboards, staff rooms, lifts and anywhere people stand close. It is also what most office printers can handle.
• A3 or 11 × 17 for shop windows, café walls and gig listings. It is the default size for most event posters.
• A2 or 18 × 24 when the poster has to compete across a room or a street. Headlines can be large enough to read from several metres.
• A1 or 24 × 36 for film posters, exhibitions, trade stands and framed prints. These are printed by a print shop, not at home.

Check the frame or the space before you start. Ready made frames in the UK and Europe are sold in A sizes; in the US they come in inch sizes such as 18 × 24 and 24 × 36. An A2 print will not sit neatly in an 18 × 24 frame, and the shapes differ enough that you can't simply scale one to the other.

Large posters and the 150 dpi rule

At 300 dpi, a 24 × 36 inch poster is 7200 × 10800 pixels, a heavy file for any design tool. Large posters are viewed from further away, and many print shops happily accept artwork at 150 dpi for them, which halves each dimension. Ask your printer what they want before you start, and design everything else at 300. Image resolution for printing shows how viewing distance changes the numbers.

Setting up a poster file

1. 1

   Pick the size with your printer

   Confirm the trim size, the bleed they want and whether they print at 300 or 150 dpi.

2. 2

   Create the page at the trim size

   In Gradiently, A4, A3 and 18 × 24 are in the size list; A2 and anything else can be set as a custom size up to 8,000 pixels on each side, so a 24 × 36 inch poster is designed at 150 dpi (3600 × 5400).

3. 3

   Keep words inside a safe margin

   Leave at least 5 mm inside the trim on small posters and 10 mm on large ones.

4. 4

   Export a print PDF

   A PDF with bleed and crop marks tells the printer exactly where to cut. Read print ready PDF for the details.

If one poster becomes three sizes, design once and resize rather than starting again. Gradiently's Pro plan resizes a design to every size in one step and exports a print PDF with bleed and crop marks, and a Mark renders fresh at each size instead of being stretched. For layout ideas, see poster design and gradient posters.