When calculating a value, you can usually just run the calculation directly. But when both values involved have uncertainties, a direct calculation no longer works—that's where the propagation of uncertainty comes in.
Doing this by hand every time gets tedious. Unfortunately, there doesn't seem to be an existing calculator for propagation of uncertainty, so I ended up finding an open-source project by nicoco007 and tried deploying it on this website.
The tool is very powerful and easy to use. It still has some issues that need to be worked out, but I believe it's good enough for simple calculations.
You can try it here. nicoco007 also provides a page to use it.
Thanks to nicoco007 for the excellent work.