My calculator icon is gone. How do I get it back?

If it got deleted entirely, you can download it again from the App Store App. https://apps.apple.com/us/app/calculator/id1069511488

If you get an "Open" button instead of the download cloud icon, then look for it on other Home Screens, or in the App Library. Click here ➜ Organize the Home Screen and App Library on your iPhone - Apple Support

Did you move it to another screen? Swipe left to the App Library and see if it is there. If not, check the App Store to download it.