Let's say you have a 12 MP digital camera, and you are considering an upgrade to a camera with 18 MP. At first glance one might think that this is a 50% increase in image size: (18 - 12) / 12 = 0.50, or 50%, but that is incorrect. Let me show you why!
Now consider the ratio of 3:2, or 3 units of length by 2 units of height, which is the same aspect ratio of 35mm film. Here are some specifications taken from current models of Nikon and Canon digital SLRs. All of these share the same 3:2 aspect ratio.
12 MP 4256 x 2832 pixels
18 MP 5184 x 3456 pixels
24 MP 6016 x 4016 pixels
36 MP 7360 x 4912 pixels
Now let's compare the area increase among combinations of these sensor sizes:
12 MP -> 18 MP 22%
12 MP -> 24 MP 41%
12 MP -> 36 MP 73%
18 MP -> 24 MP 16%
18 MP -> 36 MP 42%
24 MP -> 36 MP 22%
So you see, the increase is based on the linear dimensions, not the total MP count. That is why a 24 MP image cannot be printed twice as large as a 12 MP image while maintaining the same pixels per inch (ppi) resolution. What does this mean in terms of print size? Here are a few examples using 300 ppi:
12 MP 14.2" x 9.4"
18 MP 17.3" x 11.5"
24 MP 20.1" x 13.4"
36 MP 24.5" x 16.4"
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