The amount of rain water that your roof can collect depends on the size of your roof and the amount of rain that comes down in a given time.
However as a general rule of thumb approximately 550 gallons of rainwater can be collected for every 1000 square feet of your roof size or collection surface per inch of rain.
To estimate amount collected in one year, take the square footage of your roof or collection surface, divide by 1000, multiply by 550 and then multiply by the average annual rainfall for your area.
In every gallon of water there are 231 inches of water.
Or more precisely there are 231 cubic inches of water per every gallon of water.
It takes a lot of rain water to equal 1 gallon.
However it takes more inches of snow to make 1 gallon of water.
An inch of snow falling evenly on 1 acre of ground is equivalent to about 2,715 gallons of water.
This figure, however, based upon the "rule-of-thumb" that 10 inches of snow is equal to 1 inch of water, can vary considerably, depending on whether the snow is heavy and wet, or powdery and dry.
You can get more water in rain falling down for the same amount of snow.
Snow is also good to add moisture and water to the earth and give us the water we need but rain water falling is much better.