Not sure if this is what you are talking about, but something like blockchain.info has? but for all coins?
https://blockchain.info/apiI dont know where the calculator sites get their info, but lots of times, it seems outdated. I would guess running own node would be only way to get realtime realtime without going through service like blockchain.info
Yeah its a major pain in the ass, I'm needing it for all SHA-256 coins that are currently being mined.
I resorted to writing a wrapper to pull statistics from bitinfocharts using Beautiful Soup and then parsing the html for the data. It works but I really did not want to do that as it adds to my coding load lol.