Calculateis from the Latin calculi (pebbles), used by the Romans for counters. In the abacus, the round balls were called calculi, and it was by this instrument the Roman boys were taught to count and calculate. The Greeks voted by pebbles dropped into an urn a method adopted both in ancient Egypt and Syria; counting these pebbles was “calculating” the number of voters. (See page 2, col. 1, Abacus.)
“Your aunt sets two tables, I calculate; don't she?” —Susan Warner: Queechy (vol. i. chap. xix.) Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894 More on Calculate from Fact Monster:

