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The A3 format gives the dimension in centimeters, but if I draw an image programmatically I may know just the pixels, for example I have an image of 800x1200 and so on ...

So the question is: I must provide a JPEG file that fits into an A3 format, how many pixels will it be?

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It depends on pixel density, there's no fixed relationship between centimetres and pixels. –  Indrek Jan 9 '13 at 18:13
    
So let's say that I have to give that JPEG image to a person who prints it: do I have to know the pixel density of his printer? –  Ramy Al Zuhouri Jan 9 '13 at 18:20
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@RamyAlZuhouri: You either know it or you don't. I suggest you use a different file format, like PDF, which handles this problem. –  martineau Jan 9 '13 at 18:45
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3 Answers 3

up vote 2 down vote accepted

Paper size of A3 11.69 x 16.54 in inches and 29.7 x 42 in cm according to paper sizes.

Image dimension in pixels per page can be calculated using the following formula:

width in pixels = paper width in inches * dpi horizontal/scaling horizontal 
height in pixels = paper height in inches * dpi vertical/scaling vertical 

if you set A3 as current paper size, and print at 200 x 100 dpi at that page, the generated image dimension per page will be (approximately):

11.69 X 200dpi = 2338 pixels in width.
16.54 X 100dpi = 1654 pixels in height.

Hope it helps.

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A3 format is 420mm x 297mm. A3 aspect ratio is 1:1.414 (As are all A formats)

What you're really asking is not the number of pixels to fit in an A3. What you need to know is the aspect ratio in equivalent pixels.

TO FIT IN A3 FORMAT
So, if you have a starting image of 1200x800, you have two options:
1) If you want to keep the short side the same, then crop it to 800 x 1131
2) If you want to keep the longer side the same, then expand the image to 1200 x 848. If you are printing this image, you can expand the image without distortion by adding empty border space of 24 pixels on the top and bottom.

TO PRINT IN A3 FORMAT
Printing quality is another thing to consider, since you asked about pixels:
Your screen likely displays at 72 pixels per inch. Your printer likely prints at 300 pixels per inch.

Pixel dimensions for printing in A3 with a device at 300 dpi results in an image that must be 4960 x 3508

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This is determined by the pixel density. The pixel density, most of the times expressed in DPI or PPI (dots/pixels per inch), is a property of the image.

If you have an image of 900 x 600 pixels, the size at which it will be printed depends on the DPI of the image. For example, if it is set to be 300, then the image will be printed at 3 x 2 inches.

So to know how large your image will be printed in centimeters, you have to divide both width and height by the DPI value and then multiply both values with 2.54 to convert the inches to centimeters.

The other way around also works. An A3 sheet of paper is 29.7 x 42.0 centimeters. Divide by 2.54 to get the size in inches: 11.69 x 16.54. Then multiply with the DPI setting you want to get the size in pixels. With a DPI of 300 this gives you 3,507 x 4962 pixels.

Update about printers

As far as I know it wont influence the size (as long as the printer support that size of paper), but it can influence the quality somewhat. An inktjet printer has a maximum DPI. For example, an HP Deskjet 2050A has a maximum DPI of 4800 x 1200. When your image has a lower DPI (very likely :-)), the printer has to somehow "resample" the DPI. In such a case it is best that both the horizontal and the vertical maximum DPI of your printer are divisible by the DPI of your image. If you set the DPI of your image to 800, you might get some quality loss because 1,200 / 800 is not a round number. Setting it to 300, 150, 100 will probably yield better results because both 4800 and 1200 are divisible by those values.

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So the DPI is determined by the type of the image, not by my computer, right? –  Ramy Al Zuhouri Jan 9 '13 at 18:33
    
Not really by the type of image. You can actually set the DPI yourself per image. –  Kristof Claes Jan 9 '13 at 18:35
    
Thank you, last doubt alive: let's say that I create an image with a certain DPI, if then I print this image with a printer, the type of the printer will not affect the size (in cm) of the image right? –  Ramy Al Zuhouri Jan 9 '13 at 18:41
    
I have updated my answer to explain some more about the printing aspect. It was too much to fit in a comment :-) –  Kristof Claes Jan 9 '13 at 18:58
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