A question I hear quite often is "I have a camera of such-and-so-much megapixels, how large can I print?" The question is in fact fairly easy to answer, but the problem is that many people tend to loose themself in the worship of figures that are only meant to suggest quality.Resolution: Pixels per inch
The resolution of a picture is defined by the number of pixels per inch (ppi), often also referred to as dots per inch (dpi). The latter term is incorrect, but used so often that you can safely assume that ppi is meant. The industry standard for high quality printing (glossy magazines, art books) is 300 ppi. That is allready a high number, but still you can hear people in discussion forums state that 500 or 600 is better. That is plain bullshit. You will need a magnifying glass to spot the difference between 240 and 300 ppi, so it will take a microscope to recognize any difference beyond 300 ppi. For pictures that are closely inspected, 200 to 300 ppi is the range you should be looking for.
Viewing distance
But wait. Most people posing the question from the intro pose the question because they want to make a large print. Will the large print be up for close inspection? No, large prints are looked at from a larger distance. At arms length, 100 to 150 ppi is allready fine, and if the viewer is 5-6 feet away, 70 or 80 ppi might even be okay, provided the picture itself is sharp and you use decent paper and a good printer.
Start calculating
Once you know for what purpose you will be calculating and have choosen the correct ppi, the calculation is easy. Simply divide the length and width of you picture in pixels by the ppi number you've picked, and you find the size in inches. For centimeters, multiply by 2.5. Let's say you have a 3072 x 2048 (=6 MP) picture, and you have decided to print it at 100 dpi. You print size is then 3072/100=30.72 x 2048/100=20.48 inch or 75 by 50 cm.
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