1,048,576 pixels, of course—which is probably useless information.
Megapixels are the number of pixels in a picture. Based on the resolution, it can tell you the size of the picture. Most digital cameras seem to use a 4x3 ratio picture. Looking up the actual megapixel to photo size I can say there is a lot of rounding (mostly up) to define the resolution.
It seems that ~200 dpi is considered "Photo Quality" by the camera companies. This, and possible 150 dpi work for photos moounted on a wall and viewed at a distance. 300dpi is good for up close viewing. 600 dpi is for perfectionists.
It comes down to a balance between size, viewing distance and quality. So, how many megapixels do you need?
Here are two tables that convert a few megapixel sizes to picture size at 150dpi, 200 dpi, 300 dpi, and 600 dpi. Some rounding of size is done.
SLR Cameras have a 3x2 aspect ratio.
| Megapixels | 150 dpi | 200 dpi | 300 dpi | 600 dpi |
| 1 | 8.4” x 5.6” | 6.3” x 4.2” | 4.2” x 2.6” | 2.1” x 1.4” |
| 10 | 26.4” x 17.6” | 19.8” x 13.2” | 13.2” x 8.8” | 6.6” x 4.6” |
| 12 | 29.0” x 19.3” | 21.7” x 14.5” | 14.5” x 9.7” | 7.2” x 4.8” |
| 14 | 31.3” x 20.9” | 23.5” x 15.6” | 15.6” x 10.4” | 7.8” x 5.2” |
| 16 | 33.4” x 22.3” | 25.1” x 16.7” | 16.7” x 11.1” | 8.4” x 5.6” |
| 18 | 35.5” x 23.6” | 26.6” x 17.7” | 17.7” x 11.8” | 8.9” x 5.9” |
| 20 | 37.4” x 23.6” | 28.0” x 18.7” | 18.7” x 12.5” | 9.3” x 6.2” |
| 22 | 39.2” x 26.1” | 29.4” x 19.6” | 19.6” x 13.1” | 9.8” x 6.5” |
| 24 | 41.0” x 27.3” | 30.7” x 20.5” | 30.5” x 13.7” | 10.2” x 6.8” |
| 26 | 42.6” x 28.4” | 32.0” x 21.3” | 21.3” x 14.2” | 10.7” x 7.1” |
Micro Four Thirds Cameras have an aspect ratio of 4x3.
| Megapixels | 150 dpi | 200 dpi | 300 dpi | 600 dpi |
| 1 | 7.9” x 5.9” | 5.9” x 4.4” | 3.9” x 3.0” | 2.0” x 1.5” |
| 10 | 24.9” x 18.7” | 18.7” x 14.0” | 12.5” x 9.3” | 6.2” x 4.7” |
| 12 | 27.3” x 20.5” | 20.5” x 15.4” | 13.7” x 10.2” | 6.8” x 5.1” |
| 14 | 29.5” x 22.1” | 22.1” x 16.6” | 14.7” x 11.1” | 7.4” x 5.5” |
| 16 | 31.5” x 23.6” | 23.6” x 17.7” | 15.8” x 11.8” | 7.9” x 5.9” |
| 18 | 33.4” x 25.1” | 25.1” x 18.8” | 16.7” x 12.5” | 8.4” x 6.3” |
| 20 | 35.3” x 26.4” | 26.4” x 19.8” | 17.6” x 13.2” | 8.8” x 6.6” |
| 22 | 37.0” x 27.7” | 27.7” x 20.8” | 18.5” x 13.9” | 9.2” x 6.9” |
| 24 | 38.6” x 29.0” | 29.0” x 21.7” | 19.3” x 14.5” | 9.7” x 7.2” |
| 26 | 40.2” x 30.1” | 30.1” x 22.6” | 20.1” x 15.1” | 10.0” x 7.2” |