If you wonder "how many pixels in A4 paper sheet", it turns out that a clear answer is not there. The fact is that unlike centimeters and millimeters, a pixel has no size. In fact, the pixel represents a minimum of a two-dimensional logical object used in computer graphics. The combination of pixels of certain colors creates a picture on a screen or sheet of paper. Depending on the type of device, the pixel may be square, rectangular, octagonal or circular shape. The word "pixel" or "pixel" is an abbreviation for the English phrase pix element, which means "picture element".
The pixel size is not constant, as it is directly linked with a term such as "resolution". Under the resolution in this case refers to the number of pixels or dots that fit into one or another unit of area or length. Typically, resolution is measured in dpi, which means dots per inch – number of dots per inch. Naturally, the more resolution, the better the image on the screen or paper and Vice versa.
Thus, the number of pixels per A4 sheet depends on the resolution used to create the image and print. Most graphics programs default to creating images with a resolution of 72 dpi, but for printing quality most often used resolution 300 dpi. Knowing the dimensions of an A4 sheet (297x210 mm) or 11,75x8. 25 inches, you can calculate the number of pixels at a particular resolution. So, for the resolution of 72 dpi number of dots on a sheet of A4 is equal to 502524, and at a resolution of 300 dpi – more than 8.7 million pixels.
In practice it looks as follows: for example, you have an image 1000x1000 pixels when printing at 72 dpi it will take a sheet of paper 35 by 35 centimeters, and if you send it to print with a resolution of 300 dpi, the final size will be only 8.5 by 8.5 cm. Of course, the image of a small size can be stretch and on the whole leaf, but it will be a mosaic, grainy. And if you print the image with low resolution, significantly affected color reproduction. Therefore, in the printing industry, usually require a fairly large image, allowing print them with high resolution that provides good quality photos or illustrations.