The VDOT running calculator uses a formula developed by legendary American coach Jack Daniels. Using a recent race finish time, the calculator provides you with equivalent finish times for other distances, along with scientifically devised paces with which to devise your training runs. Scientific in the sense that the paces provided offer the best chance for improvement. I have not tried the training aspect for long enough to offer opinion, but I have found that the predictor aspect of the calculator tends to be highly accurate. Of course this works less well if you are predicting a marathon based on a 5k race time than if you were predicting a half marathon finish based on a 10k time.
Enter your most recent race time below and the calculator will provide you with everything that you need. Be sure to use your most recent time though, even if it is not your PB, as this is how you are currently running so it will provide the most accurate results.