You can use ASP.Net to create a web application that can be viewed on most any device (ASP.Net renders to plain HTML, Javascript, and CSS). Download the free
Visual Web Developer Express 2010, which allows you to create ASP.Net applications for free.
If you want the calculator to be good looking, you could instead go with a RIA technology, such as Silverlight, but that will reduce the number of devices that can view your calculator. Silverlight has a plugin for popular browsers on the Mac and Windows computers, but that plugin has to be installed for Silverlight to work a user's computer. To get started with Silverlight, go
here.