What is dpi on a scanner?

Another misuse results from an incomplete understanding of what the DPI measurement means. In order for any "per inch" measurement to be meaningful, some number of inches must be specified. A digital image captured by a scanner or digital camera may have an intended resolution (in pixels per inch), but this need not correspond to the DPI at which it is printed. For example, a 1000×1000-pixel image could be printed at 4×4 inches and 250 pixels per inch, or at 10×10 inches and 100 pixels per inch. Digital images contain some number of pixels; the size at which they are printed is relatively arbitrary. When someone asks for a "300 DPI image", they may be expecting an image with 300 pixels per inch of printed output; unless the size of the printed output is known beforehand, the measurement is meaningless. A more complete specification would include the desired print size in addition to the number of desired pixels per inch. A yet more complete specification would also include the DPI capability of the printer that will be used to print the image; if the printer is only capable of faithfully reproducing 100 pixels per inch, there is no reason to use a higher-resolution image.