To work out a value by fixed steps. Reasoning done so it can be repeated by someone else and reach the same answer.
Latin calculare, from calculus (a small stone, a pebble), diminutive of calx (limestone); Romans reckoned sums by moving pebbles on a counting board, and the word for the pebble became the word for the reckoning.
To work a value out by fixed steps — reasoning done so that anyone repeating the same steps reaches the same answer.