How secure are my backend calculations if my website gets hacked?

Since you’re probably starting out, don’t stress too much. If you use standard technologies for your API, like Flask, FastAPI, or Node.js, you’ll be on a good path. Remember to look into authentication libraries to restrict access to your API; typically, users would only see the results of the calculations, not how they're done under the hood.

The security of your calculations really depends on the style of attack. "Hacking" is a broad term; it could range from simple content injections that affect other users to more serious breaches like SQL injections that give access to your entire database. If someone gains full control of your server, they can potentially see everything running on it, including your calculations.

It all boils down to the level of access the hacker gains. Nothing online is 100% safe. If your server gets compromised, they would have access to whatever is on it, including your code and calculations. But there are ways to make it harder for them: using strong passwords, two-factor authentication, and secure hosting can help protect your site. You might also consider storing sensitive formulas in a secure environment, like a Google Sheet with limited access.