Pixels are not units of measurement. They are fluid and variable in size. This is the toughest concept to grasp when dealing with resolution. As a result, a digital file has NO SIZE. It is variable. The resolution of the output device determines the size of the file. The majority of monitors have a resolution of 72 pixels per inch, while printers generally have a resolution of 300 dots per inch. The only true measurement of file size in digital terms is its storage size (ie: Bytes or Megabytes).

So how do you determine what size you need your images to be? To determine what the file size is going to be (storage size in MB), here is the equation:

1,000 x 1,000 pixel file in RGB mode would equal 2.87MB

1,000 x 1,000 x 3 / 1,048576 = 2.87MB

The same image in grayscale would be .95MB. Why? Because it only has one Channel. The same image in CMYK mode would be 3.82MB. It has 4 Channels

To figure out the width and height in inches, here is your formula:

If you have a file that is 1,000 pixels wide x 2,000 pixels high and your output resolution is 300 DPI, then your image size will be 3.33" wide x 6.66" high.

So, it then follows that to determine the amount of pixels needed for a Printable image that is 4" x 5" (a half sheet of regular paper), you have to multiply the inches by the output resolution of the printer.

4 x 300 = 1,200 pixels for the width

5 x 300 = 1,500 pixels for the height

If you really hate math, you can use Photoshop as a calculator. Do the following: Go into File -> New and enter in the width and height in inches or centimeters, along with your output resolution. Photoshop will provide you with the Storage Size (MB). Now switch the inches drop down menu to Pixels. Photoshop automatically recalculates how many pixels are required for the resolution you entered. Now you can set your digital camera up to take an image at or above those pixel requirements. When you are done calculating the size, Cancel out of the "New" dialog box in Photoshop. You no longer need it.

If you are scanning, there is no need to determine the size in inches or centimeters. You already have this information. What you do need to do is set your resolution. This will determine the quality of your image.

If your pixels remain the same, then increasing resolution decreases image size. You are putting more pixel data into an inch. Decreasing resolution will increase the image size. You are spreading out the pixel data over the inch. In this way, they are directly related.

If you want your size (in inches) to remain static, you will have to resample the image (either upward or downward). Doing so will force Photoshop to add or subtract pixel data using a mathematical algorithm. Going downward, this is not such a problem because the image becomes smaller and the changes are not as noticeable (though softening does occur). However, resampling upward will always result in a loss of detail, softening of the image, and noticeable changes such as artifacts and jagged edges. It is a good idea to avoid this at all costs. For this reason, always take the highest quality image you can on your camera or set the scanner to the highest resolution setting possible. This way, you have the most Pixel data to work with. If you have to resample, always save the image first, so that you can always go back to the original image before the data changed. And resample in increments, rather than all at once.

Resampling Data is a destructive change. By leaving the Resample Image checkbox checked (which it is by default), you are destroying image data that you cannot recover, unless you undo the operation.

If you uncheck the Resample Image checkbox in the Image Size dialog, Photoshop is merely changing the numbers around, not changing the data in your image. This is a non-destructive change.