Digital Reduction

Definition: the procedure of reducing a multi-digit number to a single digit (1–9) by repeatedly summing its digits; mathematically, this is the digital root of the number. The process continues until a single digit from 1 to 9 is obtained; mathematically, the procedure is equivalent to taking the original number modulo 9 (except when the root equals 9).

[source: Numerology — Wikipedia (ru); Digital root — Wikipedia (en)] Related: kornevoe-chislo, master-chisla, numerologicheskiy-kalkulyator.