How much is 1 inch of rain on a square meter?
Taking 1 inch to be about 2.5cm, an inch of rain on an area of 1m2 is approximatley 0.025m3, or 25000cm3. That is 25000ml or 25l of water.
A gallon is 231 cubic inches. That's 6.1358*6.1358*6.1358 inches At 1 inch thick, a gallon of water will cover: 15.19868415357066363166874420604 square inches, or 1.26655701279755530263906201717 square feet To answer your question, one would need to know the square foot area to which you are referring. A 2000 sq foot roof would collect 1579 gallons of rain if 1 inch fell, for instance.