How to add a custom and calculating form to my site

Hello excellent support team

Apologies in advance for the amateur-level question BUT:

I am hoping to add a calculator to my site that allows visitors to calculate the BTU output of a radiator/heating system.

An example of what I am trying to achieve is here:
http://www.diy.com/help-advice/btu-radiator-calculator/Dev_npcart_100006.art

Can this be achieved using a standard form plugin? I'm looking for something that will apply calculations to fields and display the result, according to several selected or non-selected variables. I am thinking perhaps formidable pro but is there a wpmudev solution?

Any advice appreciated as always.

Thanks
Tom