To convert to US gallons:
V = (pi * r2 * h) / 231
Where pi = 3.1416, r is the radius of your kettle (diameter divided by 2), and h is the height of your liquid in inches. This will give you a cubic inches figure which is why you divide the answer by 231 to convert to US gallons.
Great idea. I'm going to add this to my brew workbook. Brewday will be even better/easier.