Using a print resolution of 300 ppi, a photo measuring 4 inches by 6 inches is 6.48 megabytes in size. In terms of pixels, the same photo contains 2.16 megapixels. It should be noted that these values also depend on the compression type.
To calculate the size of a photo in terms of bytes, start by converting the size of the picture in inches to pixels. This can be done by multiplying the picture size in inches with the print resolution in pixels per inch, as in 4 inches x 300 pixels/inch = 1200 pixels and 6 inches x 300 pixels/inch = 1800 pixels. Multiplying the two, 1200 pixels x 1800 pixels = 2,160,000 pixels, or 2.16 megapixels.
Next, the value in megapixels is multiplied by 3. This is because 1 color pixel is made up of red, green and blue elements, with each color given a value of 1 byte each. Therefore, 2,160,000 pixels x 3 bytes = 6,480,000 bytes, or 6.48 megabytes.