Yes? And are we supposed to guess at the contents of the dropdown lists?
I say again, create an HTML page *FIRST* and show that to us.
As for not knowing how to import HTML and CSS into a VS project and use it to create an ASP.NET page: Sorry, but that's just your inexperience showing. It really isn't that hard, honest.
But that's probably irrelevant. This actually looks trivial to do all in JS once you have the HTML created.
You'll pardon me, I hope, if I say that just from inspection of your C# code I don't think your code is very user-friendly. You don't even tell people why you aren't giving them an answer when they ask for one (e.g., "You must select both the number of inches and number of squares", or something to that effect).
An optimist sees the glass as half full.
A pessimist sees the glass as half empty.
A realist drinks it no matter how much there is.