DPI is a the term normally associated with scanning and it means Dots Per Inch. DPI is a term that's been carried forward from the early days when scanners could only capture black and white and black was represented with a dot. Today we really scan in Pixels but the term remains and is now used interchangeably with PPI or Pixels Per Inch.

Here is a simple example to show you how you can determine how many pixels you'll get from your photographs.

Suppose you want to scan your 4x6 inch prints at 300 dpi. You'll get an image that is 1200 pixels by 1800 pixels.  We arrived at those numbers by doing some simple arithmetic. Our print is 4 inches high and we are scanning at 300 Dots (pixels) Per Inch. Each linear inch of the print will produce 300 pixels. 4 x 300 = 1200 pixels. We do the same math against the width of the picture and get 6 inches x 300 DPI  = 1800 pixels.  Our print yields 1200 x 1800 pixels. 

Ok, so how many MegaPixels is that?
Everyone is talking MegaPixels these days and we can credit the Digital Camera manufacturers with that. When you buy a digital camera, the price is usually related to how many MegaPixels it can produce. The higher the number, the sharper your images and the more expensive the camera. More pixels generally means more detail. Back to our example of the 4x6 print. We've scanned the photo and we now have a digital image that's 1200 x 1800. That means we have a matrix or grid that is 1200 in one direction and 1800 in the other.

Imagine this is our image. It's only 3 pixels x 3 pixels so its not much of an image, but it can help exemplify how we compute MegaPixels. This image contains 9 pixels, 3 pixels in height by 3 pixels in width. Now, lets just build from there.

If this simple 3x3 pixel image contains 9 pixels, our 1200 x 1800 pixel image must contain 0,000 pixels. 1200 x 1800 = 2,160,000. That's 2.16 million pixels or 2.16 Mega Pixels.

OK! So how big is my file?
The answer to this is, it depends....I know, just when you thought you had a handle on this stuff, somebody had to mix it up. The reason it depends is because of compression, but more on that later. If we take compression out of the equation, it's simple to compute file size. A pixel is made up of the three primary colors of Red, Green, and Blue or simply RGB. The computer stores the representation of those colors in bytes. One byte for each of the three colors. Therefore, 1 pixel = 3 bytes. If our image is 9 pixels, like in the example above, its 9 pixels x 3 bytes big, or 27 bytes. One megabyte is 1 million bytes. Our 4x6 inch print is 2,160,000 pixels so if we do the math, we find that the image is (2160000 x 3 ) 6,480,000 or simply 6.4 Megabytes (MB) in size.

What about compression?
Here's where it become a little complicated. There are two types of compression. 

Lossless - The first type is called “Lossless” because it results in no loss in quality or degradation to the image. It manages to make the file size smaller the same way that zipping works. Lets try to compress our 3x3 image. Lets suppose that all of the pixels in that image are the same color. We could store that image uncompressed and it will take up 27 bytes. But since all the colors are the same, we could just store it in such a way that describes the image as 9 black pixels. Since the pixel takes up 3 bytes, we need three bytes for that then we need one more byte to indicate how many of those black pixels we have, 9 in this case. We've just compressed our 27 byte image down to 4 bytes. Of course, this is over simplified, but that's the basic concept. We can now uncompress the file when we open it and get an exact representation of the image with no loss in quality. Now that is cleaver, huh?

Lossy - The other type of compression is called “lossy” because it results in the loss of data. It works on the assumption that the human eye can't detect certain details in an image so it strips those details out. This type of compression is much more complex and also very configurable. JPEG is considered a lossy compression file format but its degree of loss is configurable. You can vary the quality factor from 0 to 100%. As you lower the quality factor toward 0, your file size gets smaller and your picture loses more detail. The biggest problem with lossy compression comes up when you open and the save the file over and over again. It suffers from 2nd generation degradation as each successive save reduces the detail of an image that already lacked detail. It’s much like a photocopy of a photocopy of a photocopy. Each successive copy gets worse. 

The following charts provided for your convenience list  numbers which are estimates. Actual results may vary

Here is a table of various sizes you can expect from your photograph print images..

Here is a table of the various sizes you can expect from your slide and negative images.
A 35mm slide or negative is approx. 1.3 inches by .9 inches.

The JPEG file sizes are approximate and will vary depending on the image content. Some images compress in JPEG better than others.

Now you're probably wondering how many images fit on a disc. That's easy to figure out, too. A CD can hold 650MB and a DVD can hold 4700 MB (4.7 GB).

Here are the numbers for “reflective” prints (aka Photographs):

Here are the numbers for Slides (positive film) and regular Film (negative film):