What century is 1666 AD?

Answer 1

1666 AD resides in the 17th century. Unless the year ends in -00, you can add 1 to the first number if it is a three digit year and 1 to the first two numbers if it is a four digit year.

For example, 411 AD is in the 5th century. This is a 3 digit year so take the first number, (4) and add (1), 4+1=5th Century.