Use our ready made calculator forms and integrate them via our api. The form will look like a totally integrated part of your webpage. We have calculator APIs for: Finance, Engineering, medicine and a lot of other areas. The service is free for up to 10 calls per minute and 150 calls per day. If you need more calls please look at our licenses. When you need a custom calculator form you can create it by adding a formula at: Add a formula.
To get a test code go to Calculator Form and choose a calculator widget. Then fill in your email and web-address to get the code for your api. There is no need to design your widget, since you will use your own css. Copy and paste the code into your webpage and design the form with your css rules.