You can use this calculator on your own website for free. Download the pocket calculator source files or copy the following HTML code and paste it into your website or blog. The zip file contains all related files to run the calculator locally on your computer or on your web server.
If this HTML code doesn't work on your website, you can download and modify it or try the IFrame version.
All three sources were published under the MIT license and can therefore be downloaded and used for free.