I use Webflyer when I have multiple segments. Even though some of the distances on some city pairs vary slightly from what United actually calculates it is pretty close.

To get the exact distance, I use the Flight Status on United's home page. You don't have to put in a flight number (if you don't know one) just put in the city pairs and pick one flight from any that are pulled up.

the only disadvantage to using flight status is that if you are flying multiple segments you need to pull each city pair individually.