Form calculation before submit

Hi,
I am writing a script that will allow a user to update the details of a recipe. I have got as far as getting all of the ingredients for the recipe and printing them out into editible form fields. I want the user to be abled to amend the "grammes" field and by doing so automatically updating the ounces field, which is disabled. I want this calculation to be done prior to the user submitting the form.
If a user entered '500' into the grammes field for one of the ingredients the ounces…
Form calculation before submit

See original post by karl_murphy