Calculating dots per inch, or dpi, from megapixels, or mgp, may seem complicated. Cameras offer anywhere from two megapixels to more than 10 megapixels, and sometimes refer to "pixels per inch" (ppi) instead. Toss those numbers and terms aside, however, when all that's needed is a simple calculation to convert megapixels to dots per inch.
Things You'll Need
- Camera manual (optional)
- Printer manual (optional)
Determine the maximum image size (horizontal and vertical) in pixels for the camera. Most cameras state the number of megapixels right on the case. This is the product of multiplying the maximum width in pixels by the maximum height of an image in pixels and dividing by 1 million. The numbers are usually in the owner's manual or on the manufacturer's website. For example, a camera that takes 2,048-pixels wide x 1,436-pixels high equals 2,940,928 pixels. Divide by 1 million and you get 2.9 megapixels.
Determine the size at which a photo is to be printed. Divide the width in pixels by the width in inches. The quotient is the number of dots per inch. Repeat the task with the height in pixels by the height in inches. The dpi numbers should be approximately the same. For example, a 2,048-pixel image to be printed 10-inches wide has a printed resolution of 204.8 dpi.
Increase the photo quality by increasing the number of dots per inch. In other words, printing photos at smaller sizes will make the quality better, as the dpi will be higher.
Control the dpi with the output device—a printer or digital-document converter settings (as when making a PDF)When a printer is set to 120 dpi, for example, the digital publisher knows that the maximum image size is 25 inches by 17 inches: The 3,008 pixel width and 2,000 pixel height are both divided by 120 to calculate the size in inches.
Tips & Warnings
- The use of the published image dictates the dpi. When viewed from a great distance, fewer dpi are needed. A poster can be professionally legible at 72 dpi because most viewers stand 3 to 8 feet from the image. A 4 x 6-inch photo to be shared should be published at 300 dpi or higher in order to be viewed at close range. Desktop publishing is 150 dpi, commercial publication at 1,200 dpi.
- When dots per inch are "fixed," such as an image to be viewed onscreen, graphic software should be used to resize the image based on the assigned dpi. Screen resolutions are no better than 96 dpi, so a 3,008 x 2,000-pixel image needing to be completely viewed onscreen would be resampled by specifying the new width in dpi. If the screen image is to be equal to half of a 1,024 x 768-dpi screen resolution, the resampling sets the new width at 1,024 pixels. The conversion results in a smaller file size than the original. This allows for a faster download.
- Calculate the longer of the two measurements, width or height, and use that number to dictate the shorter measurement.
- Putting high-resolution images online for download slows viewers' use of the image and can eat up bandwidth.