How Much Water Is One Inch Of Rain?

It's a real broiler out here in San Diego today. Dry as a bone, with temperatures in the 90s where I live. Our potted plants on the patio are going to take a real beating. As I watered them this morning in preparation for the day's desiccation, I wondered how much I should be giving them. What if I was trying to simulate an inch of rain?

Well, the calculation is actually pretty simple. The volume of a 1" cylinder is Pi times the pot diamter squared, divided by 4. (Remember Pi R Squared is the area of a circle and the diameter is twice the radius, R.) This equation gives the result in cubic inches and each gallon is 231 of them. Here's a table of the results.

Pot Diameter in Inches	Volume in Cubic Inches	Quarts, Rounded to 1/2
6	28.27	0.5
9	63.62	1.0
12	113.10	2.0
15	176.71	3.0

The surprising thing to me is that other than our really big pots, most would do just fine with a half gallon of water.