There doesn't need to be any copyright claim for there to be copyright protection. The protection is given automatically whenever someone authors a work.
If you make money off of your improved version, the owner of the page may sue you for royalties.
The only way to avoid this is to ask him for permission, or to create your page without using any of the source code of the original page.
If you copy and republish the page, it's the same as copying and republishing any copyrighted work. There's no difference for "online copyright".
Automatic copyright isn't a thing everywhere. (although it is in most places.) If you happen to be in such a place, check your local laws, the page may not have copyright protection. (just where you are)
It's unlikely that the original creator of the calculator will attempt to sue you over it. Though, if you're concerned about that happening you could re-implement most/all of the code and layout. Without seeing it myself, I don't know how complicated the implementation is.