The abax of ancient Greece, used as a counting board, was found to meet fully the needs also of later Roman mathematicians, who changed its name to abacus, a name that we still employ.
Wooden beads, or the like, strung upon wires, are used on modern boards. But in ancient times the board was divided by partitions into the number of compartments that might be wanted, and small pebbles were used for the counters, such pebbles being moved from compartment to compartment as the reckoning might require.
The Latin name for a pebble was calculus.
One who calculates is, therefore, one who, in ancient times, moved pebbles about upon an abacus for a mathematical reckoning.