What kind of information do you want to know about it?
Would you like to know how stable the site is, who could use it, how accurate it is, accessible, what problems users have with it, its costs, if it’s compliant to standards, if it’s connected to some other trackers/subsystems, what data it collects from you, how it compares with other calculators, how it’s integrated, if it’s compatible and what it is compatible with, identify what functions it has, see if you can stress it or the site, compare the online one with pocket calculator, verify it’s functions, do we want to make a list of online calculators, do you care about the site or the functions of the site, or a single element - which one specifically? do you want to know if it’s indexed and SEO score, do you want to find/trigger errors - what type? etc…etc…
To start testing you need a mission, otherwise, you have no start or end.
Have you decided yet what your mission is for us as tester(consider you’re a stakeholder)?