- For the use of the word in the Harry Potter books, see Hogwarts subjects - Arithmancy.
In modern numerological terminology, arithmancy (a shortened form of Greek ἀριθμομαντεία divination by numbers) is a form of divination based on assigning numerical value to a word or phrase, by means of a simplified version of ancient Greek Isopsephy or Hebrew/Aramaic Gematria, as adapted to the Latin alphabet.
Etymology of the name 
The Agrippan method 
In this method, the letters of a recent version of the Latin alphabet (with "U" and "V" considered to be separate letters, and "I" and "J" also considered distinct, which was not common until the 18th century), are assigned numerical values 1-9 as follows:
Based on these values, the value for a person's name is calculated. If the result is greater than 9, the values of the digits in the number are added up until it is reduced to a single-digit number.
This is a system used to predict the strengths and weaknesses in a person, by using the heart number, the social/life number, and the character/personality number. The heart number is determined by adding together only the vowels in a person's name. The social number is calculated by using only consonants. The character number is determined when both vowels and consonants are used.
A similar approach is to use the numbers from a person's birthday to derive their character number. Each of these numbers is considered to have a suitable predictive meaning. Heinrich Cornelius Agrippa first applied the concept to the current Latin alphabet in the 16th century and it has been widely used. It is often called “Pythagorean,” but is not connected to Pythagoras.
The Chaldean method 
A lesser known method is the Chaldean method (in this context, "Chaldean" is an old-fashioned name for the Aramaic languages and their speakers). The most significant difference between the Agrippan method and the Chaldean method is that the number 9 is not used in the calculations. This method is otherwise similar to the Agrippan method, but the letters were assigned values as follows (partially based on equating Latin letters with letters of the Hebrew alphabet):
References and sources 
- "Arithmancy". The Element Encyclopedia of the Psychic World. Harper Element. 2006. p. 31.