Enter your roof size and local rainfall — get gallons captured per year, per storm, and the cistern size to hold it. Free, instant, tuned for San Antonio & the Hill Country.
A roof captures about 0.6 gallons per square foot for every inch of rain. So in San Antonio's ~32 inches a year, a 2,000 ft² roof yields roughly 34,000 gallons a year — about 1,000 gallons from a single 1-inch storm — at a realistic 85% capture efficiency.
That's water you're currently sending to the street. Caught and stored, it carries a garden through a Hill Country summer, establishes trees, and softens the next flash flood.
§ 1 · How it works
Four inputs, one number.
Step 1
Roof footprint
The plan area that sheds water, in square feet — not the sloped surface.
Step 2
Annual rainfall
Your town's inches/year: San Antonio ~32, Boerne ~34, Bulverde ~33.
Step 3
Capture efficiency
Leave it near 85% — accounts for splash, first-flush, and evaporation.
Step 4
Read your yield
Gallons/year, per-storm capture, and a recommended cistern + first-flush size.
§ 2 · The Hill Country numbers
Why catch it here.
The Hill Country gets its rain in bursts — long dry spells broken by hard storms that run off caliche and limestone faster than the ground can take them. Stored rain bridges the dry months, and a first-flush diverter plus an overflow to a bioswale keeps the cistern clean and the next storm out of the street.
The money also works: SAWS rebates rainwater cisterns at $1 per gallon (up to $2,000), and Texas exempts rainwater-harvesting equipment from sales tax. We never quote a rebate figure we can't source.
How much rainwater can I collect from my roof in San Antonio?
A roof captures about 0.6 gallons per square foot per inch of rain. In San Antonio (~32 in/yr), a 2,000 ft² roof yields roughly 34,000 gallons a year — about 1,000 gallons from a single 1-inch storm — at 85% capture efficiency.
What size rainwater tank do I need?
It depends on your use and dry-spell length. For garden irrigation, target ~30 days of storage. A common Hill Country setup is one 2,500-gallon poly cistern plus a couple of IBC totes, with a first-flush diverter and an overflow path to a bioswale.
Is rainwater harvesting worth it in the Hill Country?
Yes — long dry spells and watering restrictions make stored rain valuable, and the incentives help: SAWS rebates cisterns at $1 per gallon (up to $2,000), and Texas exempts rainwater-harvesting equipment from sales tax.
Can I drink harvested rainwater?
With proper filtration and treatment, yes — the calculator includes a potable mode. Most systems here start with irrigation and tree establishment; potable conversion adds filtration and disinfection.
Start here
Run the numbers.
Free, instant, no signup. Then book a Site Read and we'll size the real system on your slope.