Arithmancy is a Hogwarts subject taught by Professor Vector. The classroom is on the seventh floor. It involves complicated number charts, but nothing else is known about the subject. It is apparently a more accurate (or at least more accepted) way of predicting the future, similar to an obscure but existing hobby of numerology - the belief that number patterns existing in real life can be calculated to give information regarding the future. This would make it a more sensible version of Divination, in that it does the same thing through more appropriate or exacting means. The accuracy of this information is unknown, however. It is presumably closely related to the subject of Numerology, as one known textbook for the subject is Numerology and Grammatica.