Go to this link-
http://sheldonbrown.com/cyclecomp_a.html

Multiply by 10 should get you close. It appears the computer default is a 26X 2.125 tire.

When you have a chance to actually ride a precisely mesured distance, you can "tweek" the numbers then.

PS- Tires tend to run a bit smaller than their "listed" size, so you might want to use a slightly smaller number, like about 3% smaller.

Edit- I forgot which forum this was. Are you old enough