The "Download Now" link directs you to the iTunes App Store, where you must continue the download process. You must have iTunes installed in order to open the link, and you must have an active iTunes account to download the application. This download may not be available in some countries.
From RealSimple Software:
Have you ever wondered how much rain falls on your roof during a typical rain storm? Use this simple rain harvesting calculator to find out. You will be amazed at how much water can be collected from your roof (or any other surface) during even the shortest of rain storms. Why not collect and store this wonderful natural resource and put it to good use later to water your organic garden? A simple calculator interface allows for the input of: Catchment surface area in square feet or square meters Rainfall in inches or millimeters Surface efficiency as a percentage Number of collection points The final result is then presented in total gallons or liters of collected runoff. Gallons or liters per collection point is also calculated when more than one collection point is entered into the calculator. Additional enhancements are planned for future releases and I welcome and will consider any ideas you might have. Happy Rain Harvesting!