If your rain gauge isn't symmetrical, you'll have to do some complex math to figure out how to convert your data. For instance, if your top rain catcher part is twice as wide than the rain holder at the bottom, you have to divide your measurement by 2.
Electronic "official" rain gauges have a little tipping mechanism that knows exactly how much weight 0.01 inches of rain is, and will tip the rain out at that point and add 0.01 to the digital readout and start over.
For More Information -- snow.plymouth.edu
Editor's Note: In the printed P-I article, we erroneously said to multiply, not divide, your measurement by two if your collector is twice as large as your container.