Just Google it..tons of info..State of Florida has a big page with the math to find out big you need to make it per square foot of catchment..having said that we are a Watershead Friendly Household.at least thats what the sign in our rain garden says..the city of Minneapolis will help pay for the plants to go into a rain garden.those are plants that can be flooded and sit in water untill the rain soaks in.water from the downspots runs in and pools and gets back into the groundwater supply rather that running off the streets and into the river..our rain barrel collects water that we save for watering in door plants during our long winters.