ψηφίζωpsēphízō

to use pebbles in enumeration, i.e. (generally) to compute

Greek (New Testament) · G5585

Meanings

to count (properly, with pebbles), reckon, calculate: τ. δαπάνην, Luke 14:28; τ. ἀριθμόν (i.e. calculate the number's meaning), Revelation 13:18 (in classical Greek chiefly middle, to vote by casting a pebble; cf. συγ-κατα-, συμ-φηφίζω).

Same root

ψῆφοςpsēphos · G5586a pebble (as worn smooth by handling), i.e. (by implication, of use as a counter or ballot) a verdict (of acquittal) or ticket (of admission); a vote
συγκαταψηφίζωsynkatapsēphízō · G4785to count down in company with, i.e. enroll among
συμψηφίζωsympsēphízō · G4860to compute jointly

Where it appears

Appears 2 times in the Bible.

Luke 14:28 “But don’t begin until you count the cost. For who would begin construction of a building without first calculating the cost to see if there is enough money to finish it?
Revelation 13:18 Wisdom is needed here. Let the one with understanding solve the meaning of the number of the beast, for it is the number of a man. His number is 666.

Source: G. Abbott-Smith, A Manual Greek Lexicon of the New Testament (1922), public domain.