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Bitmaps and File Size

As we said at the beginning of this chapter, pixel-based images are rectangles with thousands (or even millions) of pixels. Each of those pixels has to be saved on disk. If each pixel is defined using 8 bits of color information, then the file is eight times bigger than a black-and-white bitmap. Similarly, a 24-bit file is a full three times bigger than the 8-bit image, and a 48-bit file is twice the size of a 24-bit one.

Big files take a long time to open, edit, print, or save. Many people who complain about how slow editing is in Photoshop are simply working with files much bigger than they need. Instead, you can save yourself the complaining and reduce your file size when you can. Here's a quick rundown of how each attribute of a pixel-based image affects file size.

Dimensions and resolution

When you increase the number of pixels, you increase the file's size by the square of the value. That means if you double the resolution, you quadruple the file size (2 x 2); triple the resolution, and your file is nine times as large (3 x 3). There can easily be a multimegabyte difference between a 300-ppi and a 225-ppi image. (And remember that a 225-ppi image is almost always plenty enough resolution for a 150-lpi halftone screen.)

Bit depth

Increasing bit depth increases file size by a simple multiplier. Therefore, a 24-bit image is three times as large as an 8-bit image, and 24 times as large as a 1-bit image.

Image mode

Image mode doesn't necessarily increase file size, but going from RGB (24-bit) to CMYK (32-bit) mode does. It adds an extra 8-bit channel, thereby increasing bit depth (because each pixel is 32 bits deep).

Figuring File Size

Now that you know the factors that affect the size of images, it's a simple matter to calculate file size using the following formula:

File size in kilobytes =

Resolution² x Width x Height x Bits per sample ÷ 8,192

For example, if you have a 4-by-5-inch, 1-bit image at 300 ppi, you know that the file size is 220 K (300² x 4 x 5 x 1 ÷ 8192). A 24-bit image of the same size would be 5,273 K (just about 5 MB). In case you were wondering, this formula works because 8,192 is the number of bits in a kilobyte.

Tip: Faster File Figuring

There's an even easier way to calculate file sizes than doing the math yourselflet the computer do it for you. Photoshop's New Document and Image Size dialog boxes are very handy calculators for figuring dimensions, resolution, and file size. Simply type in the values you want, and Photoshop shows you how big the file would be (see Figure 3-11).