Surface Area Calculator for Web Page

We are looking for a surface area calculator to be designed for one of our webpages, we intend to offer pond fibreglassing kits, the calculator will enable the customer to calculate the total surface area of their pond.

I have an example below of the type of calculator we require :

https://www.calculator.net/surface-area-calculator.html
The calculator " Rectangular Tank Surface Area" on this page is what we require.

We would like the unit options to be in feet & meters , with the result of the feet calculation also showing in meters.

The calculator would need to be styled to suit the design of our website mbfg.co.uk