Hexadecimal numbers go from 0-15 rather than 0-9 because they're base 16, not base 10 like we're used to with our normal numbering system.

The characters A-F are used to represent 10-15 when writing in hexadecimal.

For Windows, you can download an offline hexadecimal color calculator at http://www.lightman.com/software/huey/index.htm -- it's freeware.