For estimating an irregular shape, you'll have to average the shape into something that you can calculate or look up, such as the example below. If your irregular shape has somewhat of a square or rectangle shape, try to visualize where the "average" is for the four sides. An irregular circle shape can be averaged into a regular circle as well. It ain't perfect, but better than nothing.
A more precise but longer method would be to take numerous X and Y measurements across the irregular area to get your average sides. Make sense?