This app was mentioned in 6 comments, with an average of 4.00 upvotes
It depends on if you are looking to use it for basic calculations or if you want an advanced calculator. I personally use Floating Calculator by Pnapp. I love it because it draws over apps. It will float above any app and allows you to perform calculations while working in other apps. You can also adjust the size.
https://play.google.com/store/apps/details?id=pnapp.pncalc.free
I use this free and simple floating calculator app which can float on top of the rest of your apps. The paid version is just $2 and the developer seems to listen to feedback.
For those who need a simple calculator while writing notes, there is this floating calculator, it floates over the notes (or any other app)
Well. Google should provide this option by default. Anyways I tried some apps and here are some solutions. (They are not PIP but It should work for you.)
Floating Calculator (free version) from PNApp
Other good option is Floating calculator & Calculator air. Looks good but doesn't have many options.
Someone else posted this:
https://play.google.com/store/apps/details?id=pnapp.pncalc.free&hl=en&gl=US
Not a widget, but I use and like Floating Calculator Free