Some thoughts on Escape Velocity...

If you follow the link above it will take you to a fairly straight forward description of how an escape velocity is calculated. It is important to note that it depends on the mass and radius of the planet you are attempting to leave (Wikipedia refers to the value of 'r' as the height from which you are trying to escape-which for our purposes will usually be the surface of the planet).

If you don't like the Wikipedia article I did a quick set of notes HERE