The town of Forks always brags that with their 120" of rain a year, they measure it in feet.
Judy Harder of Port Orchard took a different tack -- she accidentally left her measuring bowl outside just before we went through our real rainy stretch earlier this month. After a week or so of rain, the bowl measured just under four cups of rain -- about a quart.
She decided to leave it out there for the rest of the month and see what her unique rain gauge would come up with. Lo and behold, by Monday morning her accumulated rainfall was up to 8 cups, or two quarts (or, a half gallon).
Now, since her measuring cup isn’t a perfect cylinder, she can't just stick a ruler in there and declare there was, say, 4.75" of rain this period (the height of a Coke can -- what she has in the photograph there for height reference). That's because the top part of the cup is larger than the base, so it's not just a 1-to-1 measurement.
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.
That said, Seattle has had 5.45" of rain this month (10th wettest March at Sea-Tac so far) so her measurement is pretty close.
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.