Mole Calc is a free tool that seems much more than a classic calculator, and it is. It has the particularity of recognizing atomic compounds from the periodic table and calculates their mass without any error margin.
Its interface is minimal and very simple to use. It has a box to insert the formula, a Calculate button that does the operation, and another box that shows the result.
Mole Calc accepts large and complex chemical formulas, is easy to install/uninstall, and hardly consumes system resources. It is a great program for chemists, investigators, engineers, students, etc. And, it is also free.