Touch 11i Free scientific calculator

Touch 11i is a full-featured RPN calculator that implements most functions of the beloved HP-11C calculator. It is feature-complete (including programming and subroutines).

The free version is ad-supported. If you like this app, check out the premium version (Touch 11i) that has many more features, like vertical layout, saving multiple memory instances in the cloud (and share them among your devices), etc.