How the rainwater calculator works

Rainfall sources and geographic coverage

The calculator offers PRISM for Texas and NASA POWER for locations beyond Texas where data are available. Coverage in this tool is different from a provider's full archive. NASA POWER integration is being tested; compare it with local observations when evaluating a project.

PRISM: Texas coverage in this calculator

The Texas option uses approximately 4 km gridded precipitation data. A grid represents an area rather than a rain gauge at your property. See the PRISM Climate Group for dataset documentation. Expansion of PRISM coverage in this tool depends on additional funding and data integration.

NASA POWER: U.S. and international locations

For selected years before 2001, this tool requests MERRA-2 corrected precipitation through NASA POWER. For 2001 onward, it requests daily IMERG precipitation and sums complete calendar months. A period spanning 2001 uses both sources. The datasets have different methods and grid sizes, so a change in the historical series may reflect a dataset difference as well as weather.

MERRA-2 monthly mean rates in millimeters per day are multiplied by the number of days in each month. IMERG daily values are summed by month. Millimeters are divided by 25.4 to obtain inches. Missing values are excluded; IMERG months with missing days are flagged and left out rather than treated as zero rainfall. Check available months and years before comparing scenarios.

Consult NASA POWER precipitation methodology for source comparisons and limitations. The historical record is not a forecast, and recent data may be incomplete. Use the latest complete period available in the tool; this documentation date does not imply that every dataset is current to that date.

Roof collection equation

Capture (gallons) = roof area (sq ft) × rainfall (inches) × 0.623 × collection efficiency

The factor 0.623 converts one inch of rain over one square foot to U.S. gallons. Use the horizontal roof area connected to your collection system. The main calculator estimates collection efficiency from roof material and condition; these are planning assumptions, rather than measurements of your gutters or filters.

At 80% efficiency, one inch on 1,000 square feet produces approximately 498 gallons. Across 30 inches of rainfall, the same assumptions give about 14,952 gallons of potential collection. These totals describe capture before overflow and withdrawals; they are not a guarantee of usable supply.

Tank sizing, demand, and reliability

Storage connects rainfall to the months when you need water. The main calculator works with monthly rainfall and demand. Compare storage capacities and inspect the resulting supply, shortages, and overflow across the selected historical period instead of sizing a tank from annual rainfall alone.

Monthly modeling cannot show the exact timing of storms and withdrawals within a month. A month with enough rain in total can still contain a dry spell. Starting storage, the selected years, demand, and collection efficiency all affect the result; keep these assumptions consistent when comparing tanks. A high historical reliability is not a guarantee of future supply.

If you have measured rainfall records or want other scenarios, use the custom rainfall and firm-yield tools. Their time steps and input assumptions may differ from the main calculator. Use the site planner to explore roof catchment and tank footprints.

What to check before using an estimate

• Compare gridded rainfall with a nearby rain gauge when possible, especially in areas with strong local variation.
• Check that the chosen period contains complete years, including dry years relevant to your project.
• Try lower collection efficiency and higher demand to see how sensitive the result is to your inputs.
• Have a qualified professional verify the catchment, storage, installation, and intended water use before buying equipment.

This page documents the calculation approach; it does not establish field validation of a proposed system. If an output looks wrong, report the location, dataset, years, and inputs so it can be investigated.

Explore rainwater system components and planning resources and standards, or return to the rainwater harvesting calculator to compare scenarios.