“English measurements:
You can collect 600 gallons of water per inch of rain falling on 1,000 square feet of catchment surface.

You can collect 27,000 gallons of water per inch of rain falling on 1 acre of catchment surface.

Metric measurements:
You can collect 1,000 liters of water per each 10 millimeters of rain falling on 100 square meteres of catchment surface.

You can collect 100,000 liters of water per 10 millimeters of rain falling on one hectare of catchment surface.”