creating a field for calculating

[Guest]

any idea how to create a field where someone accessing my website can enter a
number (say, into a box) without seeing the formulas in order to have a
calculation done for them. for example, entering $150.00 into a box that
would then calculate the total including GST. i want to create this almost
like a drop-down field for people to enter the numerical amount-more
attractive box.

 

[p c]

Need to use script.