John the Revelator number of the Beast was lifted from the book of Daniel.

People have tried to decode the number of the Beast. Interestingly the number of the Beast in Revelation is a rewriting of an earlier source, that is, the book of Daniel. Much of Revelation is taken from Daniel, in fact. Anyhow, without dragging this on, let me get right into it. Here is the verse in question:

Rev 13:18

Here is wisdom. Let him that hath understanding count the number of the beast: for it is the number of a man; and his number is Six hundred threescore and six.


The above is taken from here:

Dan 3:1

Nebuchadnezzar the king made an image of gold, whose height was threescore cubits, and the breadth thereof six cubits: he set it up in the plain of Dura, in the province of Babylon.

The Beast John saw was the statue (image of gold) made in the image of Nebuchadnezzar. And that is why John said it was the number of a man. He was describing the dimensions of the statue of Nebuchadnezzar.

height = 60 (threescore) cubits
breadth = 6 cubits
***length = 600 cubits

600 + 60 + 6 = 666


John added an extra dimension, the length, to represent Nebuchadnezzar himself, which Daniel did not include in his dimension.