Texas A&M AgriLife Extension publishes two calculators to aid in designing rainwater collection systems:

• Rainwater Harvesting Supply Calculator

  • Allows the user to determine stored water volume over the course of three years and whether supplemental water is needed.
  • Output is shown in both graph and table form.

• Hazen Williams Pressure Loss Calculator

  • When working with piping in a rainwater harvesting system, friction can cause water to back up, especially in a wet-line conveyance system. To determine how much water will back up, the Hazen Williams pressure loss formula can be used to determine pressure loss, both in feet and psi.
  • Output is shown as pressure loss in feet and psi.

Links in the Uses of Rainwater area, and further down the page, take you to other pages with many resources and dowloadable PDFs..