Posters · Glossary
18x24 poster size: cm, pixels and the 3:4 ratio
An 18x24 poster is 18 by 24 inches, or 45.72 by 60.96 centimetres, with a 3 to 4 aspect ratio. It is the US medium poster size, matching Arch C. At 150 DPI the artwork is 2700 by 3600 pixels, and at 300 DPI it is 5400 by 7200 pixels.
The 18x24 is the poster that fits a door, a lift lobby or a lawn sign frame. It is close to A2 in size, but its 3 to 4 shape is squarer than any A size, so the two are not interchangeable without a crop.
Nuwan Madhusanka · Co-founder
4 min read · Published
| Measure | 18 x 24 in | A2 | 24 x 36 in |
|---|---|---|---|
| Inches | 18 x 24 | 16.54 x 23.39 | 24 x 36 |
| Centimetres | 45.72 x 60.96 | 42 x 59.4 | 60.96 x 91.44 |
| Pixels at 100 DPI | 1800 x 2400 | 1654 x 2339 | 2400 x 3600 |
| Pixels at 150 DPI | 2700 x 3600 | 2480 x 3508 | 3600 x 5400 |
| Pixels at 300 DPI | 5400 x 7200 | 4961 x 7016 | 7200 x 10800 |
| Aspect ratio | 3 : 4 (1 : 1.333) | 1 : 1.414 | 2 : 3 |
| Also called | Arch C, US medium poster | ISO 216 | Arch D, US large poster |
Medium poster, Arch C and a squarer shape
Papersizes.org lists 18 by 24 inches as the US medium poster, printed on Arch C sheets. The architectural series alternates between 4 to 3 and 3 to 2, and Arch C is the 4 to 3 one, the same proportion as a classic computer monitor or a 4 to 3 photo. That squarer shape suits posters with a single image and a short message, and it is common for yard signs, event posters and retail displays. A2, the nearest ISO size, is 420 by 594 millimetres, a little smaller and noticeably taller and thinner.
How many pixels are enough
An 18x24 poster is usually read from about a metre or two. At 150 DPI the canvas is 2700 by 3600 pixels, about 9.7 megapixels, which keeps text crisp at that distance. At 100 DPI, 1800 by 2400, photographs still look fine from a few steps away, but small type and thin lines begin to soften. At 300 DPI the file is 5400 by 7200 pixels; it is only worth the size if people will read fine print up close, for instance on a menu board or a timetable.
Fitting A sized artwork onto 18x24
A design made at A2 or A0 is 1 to 1.414. On an 18 by 24 inch sheet, scaled to the height, it is about 17 inches wide, leaving half an inch either side; scaled to the width, it is about 25.5 inches tall and loses three quarters of an inch top and bottom. Neither is a disaster if the design has a margin. A full bleed photograph is where the crop hurts, so either compose with space around the subject or create a 3 to 4 version with the aspect ratio calculator before sending it to print.
Common mistakes
The first is ordering 18 by 24 for a conference that specified A1 or 36 by 48 inches, which leaves the poster lost on the board. The second is using a frame bought in centimetres; a 45 by 60 centimetre frame is a little small for an 18x24 print. The third is a phone photo cropped in a hurry so the subject's head touches the top edge. The fourth is thin text in a light colour, which disappears under a window's glare in a shop display.
Making an 18x24 from the A0 board
The poster tab makes A0 portrait and A0 landscape boards only, so there is no 18 by 24 inch canvas. An A0 export is 3312 by 4680 pixels. Printed 24 inches tall it lands at about 195 DPI and about 17 inches wide, so the print shop leaves a half inch band on each side or fills it with the background colour. If that band is not acceptable, the image resizer or print scale calculator can produce the exact 3 to 4 pixel size from the export, and the blank PNG above is 5400 by 7200 pixels, 18 by 24 inches at 300 DPI, for building at size in another tool.
Questions people ask
Is 18x24 the same as A2?
No. A2 is 420 by 594 millimetres, about 16.5 by 23.4 inches, and 18x24 is 457 by 610 millimetres. The 18x24 is larger and squarer, with a 3 to 4 ratio against A2's 1 to 1.414. A file made for one needs cropping or a margin to fit the other.
What frame fits an 18x24 poster?
An 18 by 24 inch frame, which is a stock size in the United States. For a mat around the print, use a 24 by 30 or 24 by 36 inch frame with a mat opening slightly smaller than 18 by 24 so the edges of the print are covered.
Should an 18x24 poster be portrait or landscape?
Portrait suits a single photo with a headline above and details below, the usual layout for events and shop windows. Landscape suits a wide image, a timeline or a set of three side by side panels. Decide before choosing images, because cropping a portrait photo into a 4 by 3 landscape frame loses a lot of the picture.
How far away can people read an 18x24 poster?
It depends on letter height, contrast and light rather than on the sheet. Large headline letters carry across a room, while body text needs the reader within a step or two. The poster text size calculator gives a minimum letter height for any viewing distance you choose, which is a better guide than a fixed point size.
Can a home printer print 18x24?
Not in one piece. It needs a large format printer or a print shop. Some home software tiles a poster across several letter or A4 sheets, but the joins and white margins make that suitable only for a rough proof, not a finished poster.
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