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I need to have an image printed as a poster 32″ by 18″ (about 80x45 cm) at 300 DPI, but I am unsure of how big the file must be.

Also, what file format is best?

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I believe the question is fine now, I don't believe we need to use SI units for the rest of the world to understand...centimeters should be good enough. It would be best (@TFD) to refrain from bickering in the comments...degrades the quality of our forum. Matt has the utmost respect of this community for the quality of his edits. –  jrista Sep 29 '12 at 1:57
    
@jrista I made a simple edit to improve post for ROW. It got edited a few more times, and then muddled. No bickering? Just amazed! What's the point? –  TFD Sep 29 '12 at 2:20

1 Answer 1

up vote 16 down vote accepted

The straightforward answer to your question is very simple arithmetic: 32×300 = 9600 and 18×300 = 5400, so 32 inches by 18 inches at 300 dots per inch is 9600 by 5400.

However, it gets a little more complicated when you consider a more complicated relationship between pixels and colored dots in your output medium. For details on this, take a look at jrista's answer to a similar question: What DPI should I resize my image to for best printing quality?, at What does DPI mean?, and at Is there a general formula for image size vs. print size?

Also consider that 300dpi may be overkill for a poster, unless you expect your viewers to come right up to it and look closely. A much lower resolution will look great from a few feet away.

It sounds like you're doing something specific, though. In that case, the best answer is to ask your print service what they recommend — and the same thing goes for file format (and color space, a whole 'nuther can of worms).

If your print service offers multiple file types as an option, lossless TIFF or PNG will ensure that you don't introduce additional JPEG artifacts, but in practical terms you can get excellent results from a high-quality JPEG. (With high enough JPEG quality and no repeated edits, you probably will not be able to find experts who could tell the difference in a blind test.)

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1  
Great answer. I've never had a print job for which a high-quality JPEG wasn't perfectly adequate. –  Mark Whitaker Sep 26 '12 at 18:49
    
My printing company say that 150DPI is fine for my A0 sized posters (46.8 x 33.1 in) and I can provide them with any size as long it is above that. If your company requies exactly 300 DPI, just resize with bicubic or lanszos resize. Unless you are printing something like an ISO 12233 test chart it will be fine. –  Michael Nielsen Sep 28 '12 at 13:10

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