I find that calculators remain weirdly unergonomic on most Operating Systems. There are so many times where I want to do some quick math but feel hobbled by the insistence of calculator software writers to ape physical calculator design -- Numi seems like a cool step in the right direction
If you're using OS X, try using Spotlight or Alfred directly (Keypirinha[0] is a reasonable Windows alternative) - just type in `1 + 2` and you get the answer without launching a special app. There are plugins for conversions as well. [0]. https://keypirinha.com/
These days, DuckDuckGo is my go to calculator. I can type math and conversions and get the answer. And if I don’t, I do the !wa bang and let Wolfram Alpha handle it.