Megapixels and DPI, or dots per inch, are two ways to measure resolution. Megapixels are used to describe the resolution of a digital image, with each megapixel representing one million little sensors in the camera. DPI is more often used with printers and printed images to describe how many dots can be used per inch. When you print a digital picture, the two terms overlap. When you know the number of dots per inch that a printer is capable of printing, you can determine the minimum number of megapixels that an image would need to be to be printed at the maximum resolution.
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Things you need
Measure the number of inches of the length and width of the photo size that you want to print. For example, you may want to print a four inch by six inch photo.
Look up the maximum resolution the printer is capable of printing in the owner's manual. For most printers, 300 DPI is the highest capacity.
Multiply the resolution times the length and the width of your photo to determine how many dots are needed for the length and the width. For example, if you wanted to print a four inch by six inch photo at 300 DPI, you would multiply 300 by 4 and 6 to find you need 1,200 pixels on the width and 1,800 pixels on the length.
Multiply the length in pixels times the width in pixels to calculate the total number of pixels needed. In this example, you would multiply 1,200 by 1,800 to get 2,160,000.
Divide the result from by 1 million to convert from pixels to megapixels. Finishing the example, you would divide 2,160,000 by 1,000,000 to find that the image would need to be at least 2.16 megapixels to print at maximum resolution.
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