calc() lets us use calculations to determine CSS values. Its greatest strength is that it allows us to combine different CSS types. For example, you could set the width of an item to be 100% but offset by 5px. With
calc() that would look like
width: calc(100% - 5px). Browser support for
calc() is fantastic with all major browsers supporting it https://caniuse.com/#feat=calc.
Addition, subtraction, multiplication and division can be used in calc():
+ - * /. Below, I have a widget to play around with
calc() including addition and subtraction. It changes the width of the green square.