How Big a Rainwater Tank Do You Need? (Calculator)
Every inch of rain that falls on your roof is water you can keep. On a typical Central Texas home that adds up to tens of thousands of gallons a year. Enter your roof to see your number.
NOAA 1991–2020 normal at Austin-Camp Mabry: 36.25 inches a year.
Austin Water customers: up to $5,000, based on storage capacity. See every program on the rebate map.
Got your number? Get a quote for that tank size, with the rebates that may apply shown.
The formula
One inch of rain on one square foot of roof equals 0.623 gallons. Real systems lose some to splash, evaporation and the first-flush diverter, so the Texas Water Development Board puts what reaches the tank at 75 to 90 percent. We use 85 percent.
Gallons per year = roof area (sq ft) × annual rainfall (inches) × 0.623 × 0.85
Measure your roof's footprint (the ground area under it), not the sloped surface. A 2,000 sq ft footprint catches about 1,060 gallons from a single one-inch storm.
What your roof catches, by town
Annual yield for 2,000 and 2,500 sq ft roofs, using NOAA 1991–2020 rainfall normals:
| Location | Rainfall (in/yr) | 2,000 sq ft roof | 2,500 sq ft roof |
|---|---|---|---|
| Austin | 36.25 | 38,400 gal | 48,000 gal |
| San Marcos | 36.55 | 38,700 gal | 48,400 gal |
| Bastrop | 36.37 | 38,500 gal | 48,100 gal |
| Lago Vista / Jonestown | 35.55 | 37,700 gal | 47,100 gal |
| San Antonio | 32.38 | 34,300 gal | 42,900 gal |
| Spicewood | 31.27 | 33,100 gal | 41,400 gal |
Sizing the tank: catch vs. carry
Your roof's yearly catch isn't your tank size. The tank only has to carry you through the gaps between rains. In Austin, May is the wettest month (about 5 inches) and July the driest (about 2 inches), so the tank's job is to bank spring rain for summer.
- Garden and trees: 1,000 to 3,000 gallons covers most yards through a normal summer.
- Garden plus livestock or a well backup: 5,000 to 10,000 gallons.
- Whole-house drinking water: 10,000 gallons and up. One to two people typically need 10,000 to 15,000 gallons, and a family of three to four 20,000 to 30,000.
Bigger tanks earn bigger rebates where a program pays by storage: Austin Water, Lost Pines, San Marcos and SAWS all do. For a month-by-month sizing at your address, use the Rain Planner.
Questions
How much rainwater can I collect from my roof?
About 0.623 gallons per square foot per inch of rain, minus 10 to 25 percent in losses. A 2,000 sq ft roof in Austin catches about 38,400 gallons a year.
What size rainwater tank do I need?
Size it for the dry gaps, not the yearly total. Gardens usually need 1,000 to 3,000 gallons; whole-house systems 10,000 gallons or more.
Do I measure the sloped roof or the footprint?
The footprint, the flat area the roof covers.