Input boxes and maths

What would be the most effective way of performing mathematical functions on data that is entered into input text fields.

I have a client who wanted a method of predicting how many floor tiles would be needed for a floor depending on the square metres of the floor.

A simple width by height calculation would suffice but I seem to have encountered a little trouble.

If I had two input boxes in which to enter a width and a height and a further dynamic box to display the result of the calculation width x height, what sort of Actionscript would I be looking at??

Whenever I try to trace the result to an Output box I get NaN. This is not the answer I wanted. Back to books.