It DOES Work on iPhone 6s

This is not a review but rather a report of my experience with the app on iPhones.

1. It DOES switch to and work in landscape mode on my iPhone 6s running iOS 9.0.

2. It IS a fully functional financial calculator. However, the look and functionality of the venerable HP-12C are available only when the phone is turned to landscape mode. In portrait mode, the calculator is pretty much just a "four-banger."

3. The click sound can be disabled in Settings.

4. HP has not updated this app in several years. Consequently, the app uses less than all the screen area available to it on an iPhone 5 or later.

Eilif the Lost about HP 12C Financial Calculator