*Calculate* derives from the Latin word for count––it means either to figure out an equation, i.e., to count it up, or to count on something happening.

You might calculate on having your friends taking your side in a fight with a bully, or you might calculate on reaching home before nightfall, when you're on a road trip. You might calculate the answer to a simple equation in your head but need a *calculator* to figure out something more complex.