More than 50 years ago Toni Nett published his book Der Lauf. In the book, he presented thumb rules to calculate possible personal running records for common distances for middle and long distances. The possible personal bests are calculated based on known performances on the next shorter distance. Manfred Steffny extended Toni Nett's rules to cover the marathon and the 100 km. He found these rules surprisingly accurate when comparing personal bests of hundreds of runners of different levels from beginner to world champion. I have been writing about these thumb rules previously here. Now I made a small php script that does the calculation for you.