Open-source feature-rich calculator. App itself is no longer in development, but the developer does [encourage pull requests](https://github.com/tranleduy2000/ncalc#calculator-n---powerful-calculator-for-android). I've used it frequently and it works great. Not on F-Droid, but a 3rd party play store can probably find it; or the APK's are available as releases on GitHub. You can also compile from source.
- name:Calculator++
url:https://github.com/Bubu/android-calculatorpp
fdroid:org.solovyev.android.calculator
eyes:null
text:|
Open-source calculator packed with features. Thanks @ghost