On a 1 foot square flat surface area 1 inch of rain will produce .62 gallons of water. Using this value, you can calculate the amount of water that will come down your downspout on a given rain.
Using this drawing of a house as an example the house is 30’x 50’ that equals 1500 sq. ft. of total roof area and it will produce 930 gallons of water for every 1 inch of rain fall.
The average rain fall in Marquette for the seven month period of April through October is 25 inches, that equals 23,250 gallons of rain water coming off your roof. If you were paying for that water from a municipality to water your lawn, garden, wash cars etc. it could cost you approximately $ 1,828.00.
Based on a water & sewer charge of 12.72 pr. 1000 gallons.