Alfred Wallace Rugby is a calculator for the rugby union world rankings based on the rules defined by World Rugby. The official rules are explained here, or on the english wikipedia page. However, the formula is much easier to understand on the french or italian wikipedia pages.
There can be some slight differences (0,01) between the results of my calculator and the official updates on the World Rugby website. This is due to an arbitrary rounding or miscalculation on their part. You can see in the very example they produce that they are making a mistake by stating that Wales (76.92) and Scotland (76.36) exchanged 0.64 points and ending up having Wales at 77.57 = 76.92 + 0.65 . Maybe someone will notice one day ;)