Arithmancy

In modern numerological terminology, arithmancy 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. Arithmancy is associated with the Chaldeans, Platonists, Pythagoreans, and the Kabbalah. When arithmancy is applied to a person's name, it is a form of onomancy.
Etymology.
The term "arithmancy" is derived from two Greek words – "arithmos" (meaning number) and "manteia" (meaning divination). "Αριθμομαντεία" Arithmancy is thus the study of divination through numbers.
Methods.
Pythagorean method.
In the so-called 'Pythagorean' method (which uses a kind of place-value for number-letter attributions, as does the ancient Hebrew and Greek systems), the letters of the modern Latin alphabet (with I distinguished from J and U distinguished from V, which was not common before the 18th century) are assigned numerical values 1 through 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 (its digital root).
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 (in other words, all letters) are used and summed.
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.
Chaldean method.
A lesser known method, more popular in the nineteenth and early twentieth century, is the so-called 'Chaldean' method; in this context, "Chaldean" is an old-fashioned name for the Aramaic languages. In the Chaldean method number 9 is not used in the calculations. It is left out because it is thought to be divine and sacred, and therefor unassignable. This method is radically different from the Pythagorean (as well as both the ancient Greek and Hebrew systems) as letters are assigned values based on equating Latin letters with letters of the Hebrew alphabet in accordance with sound equivalence rather than with the ancient and well attested system of place-value (although 'place-value' is almost universally interpreted according to units, tens and hundreds, which nonetheless have the same digital root as place value); in consequence of this several slightly different versions are extant, there being disagreements over some of the letter-sound equivalence (it doesn't help matters that the Hebrew alphabet has only twenty-two letters whilst the modern English alphabet has twenty-six):
Agrippan method.
Heinrich Cornelius Agrippa applied the concept of arithmancy to the classical Latin alphabet in the 16th century in "Three Books of Occult Philosophy". He mapped the letters as follows (in accordance with the Latin alphabet's place-value at that time) :
Note that the letters U, J, and W were not commonly considered part of the Latin alphabet at the time.