Doing calculations in microcontroller based project is not an uncommon thing. That’s what CPU is for, isn’t it? However, all these calculations done by MCU are pre-written as a part of code – which means these formulae are part of the program written by programmer and cannot be changed or altered for each new calculation, if the situation demands so. What if your system has to solve equations on the fly? What if the user wants to define an equation to be evaluated and MCU has to perform the task based on that? Well, here comes handy a Postfix Notation to solve any type of equations in run-time that too without changing any source code. This method not only helps in solving trivial expressions like 3 + 4 but also can handle anything that involves parenthesis or algebraic operator precedence. We will also build a somewhat basic calculator based on this technique using Atmel’s AVR microcontroller.