I want to make an app for which i need to include a part of my c++ code for calculation. For GUI i'm using HTML5. So is it possible to include C++ code into HTML5 ?
No. You can't do that. Try porting your C++ to JavaScript. You could take a look at this project that claims it can convert C++ code to JavaScript. I have never tried this myself.
Otherwise, you could write a CGI to use your C++ program and use AJAX to "remote execute it".