Poster Size Guide: A4, A3, Inches and Millimeters
Understand physical poster dimensions and plan the right grid.
Understand the most common paper sizes
ISO A-series paper gets smaller by halving the preceding sheet along its longer side. The familiar A4 sheet measures 210 × 297 mm, while A3 measures 297 × 420 mm and A5 measures 148 × 210 mm. US Letter is 8.5 × 11 inches, which equals 215.9 × 279.4 mm.
Paper orientation changes the width and height but not the sheet itself. Portrait A4 is 210 mm wide and 297 mm tall; landscape A4 is 297 mm wide and 210 mm tall.
Calculate the nominal size of a poster grid
For a tiled poster, multiply the tile width by the number of columns and the tile height by the number of rows. For a grid of three columns and two rows of portrait A4 tiles, the nominal width is 3 × 210 = 630 mm and nominal height is 2 × 297 = 594 mm.
That is 63 × 59.4 cm, or approximately 24.8 × 23.4 inches. If you rotate the tiles to landscape orientation, the same 3 × 2 grid becomes 891 × 420 mm. Always specify orientation when sharing dimensions.
Convert centimeters, millimeters, and inches
There are 10 millimeters in a centimeter and exactly 25.4 millimeters in an inch. To convert 63 cm to inches, calculate 630 ÷ 25.4 ≈ 24.8 inches. To convert 24 inches to millimeters, calculate 24 × 25.4 = 609.6 mm.
If you are working from image pixels instead of paper sizes, a physical conversion additionally needs a target print density, usually expressed in PPI. A file that is 2400 pixels wide does not have a fixed width in centimeters until you decide how densely to print those pixels.
Allow for borders and trimming
Home printers often cannot print to every edge of a physical sheet. Window Pane printing may preserve a white border on each piece, while Seamless printing may require trimming before pieces meet. This can affect the measured size of the visible image after assembly.
A design made of A4-sized logical tiles should not be described as guaranteed A4-sized ink coverage. If the printer reserves a margin on all sides, it may either leave white space, clip the art, or scale it depending on the export and print settings.
Choose a size that fits the room
Measure the available wall area rather than just the overall wall width. Leave space for furniture, windows, lighting, and any frame or mounting board. Use masking tape to mark a rectangle in the planned dimensions before printing.
A wall preview can help compare proportions and placement, but it is still an approximation. Confirm the available width and height with a measuring tape, especially when a frame or multiple panels are involved.
Choose the image resolution for that physical size
A large poster can look sharp from across a room while appearing softer when viewed from a few centimeters away. Check the source image’s effective PPI at the chosen print dimensions, paying attention to the smaller of the horizontal and vertical values.
If the estimated density is too low for your intended viewing distance, either reduce the poster dimensions or find a higher-resolution source rather than relying on export settings to invent detail.
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Use these tips in the poster editor.