myrpncalcdemo

basically you push enter to place something on the "stack", and then when you want to complete an operation, you don't have to push =, just the operator which will pull the top thing from the stack and, for example, add it to the last thing you typed.
try putting in 12, enter, 3, +.
It shows you the answer as well as placing the answer on the stack.

How Reverse Polish Notation works:
if you wanted to add 2 and 5:
on a normal calculator 2 + 5 : you would push the 2 button, then the plus button,
then the 5 button, then the equals button
in RPN 2 5 + : you push the 2 button, enter it onto the stack, then push the 5 button, and then the plus button.

the nice thing about reverse polish notation is that when you do calculations with lots of numbers, there are no parenthesis. If we wanted to computer 3 * (5 + 2) , on a regular calculator we would have to add 5 and 2 first, then multiply it by 3. In RPN we can type 3 5 2 + * . Just that simple.