It's for macOS instead of iOS, but numi is a great scratchpad calcualtor. https://numi.io/

Never tried it, but Soulver looks like a good solution for iphone: https://acqualia.com/soulver/

and CalcNote on Android - just installed this and its' just like numi. https://play.google.com/store/apps/details?id=com.burton999....