Usually people that inquire about my digicams has a wrong conception about the size of images a digicam can shoot in terms of megapixels. Megapixels isnt everything, BUT it is something. Personally, I look at a digicam model's optical zoom, LCD screen size, and battery type first before considering a digicam's max MP, because nowadays it is very rare to see a digicam being sold Mhaving less than 4MP and 4MP is already more than enough. Megapixels matter when you intend to print the image and hang it on a wall or something. Also, it is recommended to print the image @ 300 PPI (pixels per inch), though 200 (or even 150) PPI is still acceptable.

Formula: (W x DPI) x (H x DPI) = X
round of X to the nearest million value then divide it by 1,000,000 and you'll get the megapixel needed.

So, to be able to print a high quality image on a 4" x 6" paper,
(4 x 300) x (6 x 300) = 2,160,000 pixels~ 2MP

4 x 6 = 2.16 megapixels~ 2 MP
5 x 7 = 3.15 megapixels~ 3 MP
8 x 10 = 7.20 megapixels~ 7 MP
9 x 12 = 9.72 megapixels~ 10 MP
11 x 14 = 13.86 megapixels~ 14 MP
15 x 20 = 27.00 megapixels~ 27 MP
planet earth = ask-Einstein megapixels- EMC2 MP