There are many sources that state that the Washington Monument is in total 666 feet tall. This is hard to verify from information that is provided on the internet. It does seem to be a excepted fact that the monument stands at 555 feet tall above ground. It is also stated that the foundation is 20 percent of the monuments height. That would put the total height of the monument at 666 feet, although this does not seem to be a verified fact.
The thing that is clear, is that if the monument stands at 555 feet tall (above ground) then the height would be 6660 inches, this could be where we get the reference to its height being the same as the “number of the beast”.