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666 is a natural number after 665 and before 667. It is a number with many properties. One of these properties is that it is the sum of the first 36 natural numbers. That makes 666 a triangular number. 666 is the sum of the first 144 decimals (digits after the decimal point) of pi.[1] The number is also used in the 666 dating rule or rule of sixes which judges the attractiveness of men.

665 666 667
Cardinal six hundred and sixty-six
Ordinal 666th
(six hundred and sixty-sixth)
Factorization 2 · 32· 37
Roman numeral DCLXVI
Binary 10100110102
Ternary 2202003
Quaternary 221224
Quinary 101315
Senary 30306
Octal 12328
Duodecimal 47612
Hexadecimal 29A16
Vigesimal 1D620
Base 36 II36

The Bible says in the Book of Revelation that the number 666 is associated with the Antichrist, the counter-messiah of the devil. People will be forced to accept this number as a mark on the penalty of death if they refuse. However, people who do accept this mark will be rejected by God.


  1. Alfred S. Posamentier and Ingmar Lehmann 2009. Mathematical Amazements and Surprises: Fascinating Figures and Noteworthy Numbers. ISBN 978-1-5910-2723-2.