CLK-Calculator is a skinable calculator for Windows. The handling is especially simple and clear. You can choose your preferred input mode algebraic or RPN. Unit conversions, matrix calculations, complex numbers, vectors handling and function plotting are done especially simple. If you have used a HP 11/28/32/41/42/48, or any other RPN calculator, you will feel right at home. But even if you used a algebraic calculator before you will find it comfortable to see the last operations and results are right at hand on the stack. The program can handle basic operations but can handle complex numbers and vectors as well. Often used constants can be picked out of the library. Additional the program can convert 261 units of measure, work with matrices, offers a formula collection and can work with logic operations. And this is not everything at all.