What is the meaning of "calculate" in Revelation 13:18?

Click to join the conversation with over 500,000 Pentecostal believers and scholars

Click to get our FREE MOBILE APP and stay connected

This is perhaps the most famous prophesy in popular culture:

This calls for wisdom: let the one who has understanding calculate the number of the beast, for it is the number of a man, and his number is 666.¹
—Revelation 13:18 (ESV)

The ESV footnote is:

Some manuscripts 616

Please note: The question is not about the significance of the number itself or what it might refer to in the past, present or future. Please avoid any speculation, doctrinally-specific interpretations, and wild tangents.

My question is:

1. How are we expected to “calculate the number”? and
2. If this is so important, how did some copyists get the number wrong?

Does reading in the original Greek shed any light on the problem?