No, there are no sites that accept a formula and provide you code that you can embed within your own web site to perform the calculation.
There is though some simple code that can allow such calculations to occur.
var result = a * b * b / 800;
Or, if you want to get fancy:
var result = a * Math.pow(b, 2) / 800;
That's not where your problem lie. The real problems are in connecting together an HTML form so that scripting code can process the values, and then pass them back to an appropriate location on the page.