Read the image size in pixels, for example 2400 x 1600 pixels (width and height). Decide on the size of the picture you need to print; for example, say you want a 6" x 4" picture. Divide the width in pixels by the picture size in inches to calculate the horizontal DPI resolution. In our example, 2400 pixels / 6 inches = 400 DPI. Divide the height in pixels by the picture size in inches to calculate the vertical DPI resolution. In our example, 1600 pixels / 4 inches = 400 DPI.
Dots per inch (DPI) is a measure of spatial printing or video dot density, in particular the number of individual dots that can be placed in a line within the span of 1 inch (2.54 cm).
Monitors do not have dots, but do have pixels; the closely related concept for monitors and images is pixels per inch or PPI. Many resources, including the Android developer guide, use the terms DPI and PPI interchangeably.
Old CRT type video displays were almost universally rated in dot pitch, which refers to the spacing between the sub-pixel red, green and blue dots which made up the pixels themselves.
A dot matrix printer, for example, applies ink via tiny rods striking an ink ribbon, and has a relatively low resolution, typically in the range of 60 to 90 DPI (420 to 280 µm). An inkjet printer sprays ink through tiny nozzles, and is typically capable of 300–720 DPI. A laser printer applies toner through a controlled electrostatic charge, and may be in the range of 600 to 2,400 DPI.
The DP measurement of a printer often needs to be considerably higher than the pixels per inch (PPI) measurement of a video display in order to produce similar-quality output. This is due to the limited range of colors for each dot typically available on a printer. At each dot position, the simplest type of color printer can either print no dot, or print a dot consisting of a fixed volume of ink in each of four color channels (typically CMYK with cyan, magenta, yellow and black ink) or 24 = 16 colors.
Contrast this to a standard sRGB monitor where each pixel produces 256 intensities of light in each of three channels (RGB).
Most printers must therefore produce additional colors through a halftone or dithering process, and rely on their base resolution being high enough to "fool" the human observer's eye into perceiving a patch of a single smooth color.
This dithered printing process could require a region of four to six dots (measured across each side) in order to faithfully reproduce the color in a single pixel. An image that is 100 pixels wide may need to be 400 to 600 dots in width in the printed output; if a 100×100-pixel image is to be printed in a one-inch square, the printer must be capable of 400 to 600 dots per inch to reproduce the image. Fittingly, 600 dpi (sometimes 720) is now the typical output resolution of entry-level laser printers and some utility inkjet printers, with 1200/1440 and 2400/2880 being common "high" resolutions.
Some digital file formats record a DPI value, or more commonly a PPI (pixels per inch) value, which is to be used when printing the image.
For example, a bitmap image may measure 1,000 × 1,000 pixels, a resolution of 1 megapixel. If it is labeled as 250 PPI, that is an instruction to the printer to print it at a size of 4 × 4 inches. Changing the PPI to 100 in an image editing program would tell the printer to print it at a size of 10×10 inches.
Since the 1980s, the Microsoft Windows operating system has set the default display "DPI" to 96 PPI, while Apple/Macintosh computers have used a default of 72 PPI.