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Rainwater Harvest Calculator

Estimate how many gallons your roof can collect each year — plus tank size, days of supply, full system cost, water-bill savings and payback — with U.S. rainfall data, a roof material comparison, and a downloadable PDF.

Uses the standard 0.623 gallons per sq ft per inch of rain with 2024-2025 U.S. pricing. Rainwater harvesting rules vary by state and county — check local codes before installing. For planning purposes only.

Roof, Rain & System
1 Catchment Area
ft
×
ft
Use the flat footprint — roof pitch does not change collection area
sq ft
sq ft
2 Roof Surface & Rainfall
Smooth metal sheds the most water; porous or planted roofs absorb a large share.
0-1
in / yr
Check your local NOAA 30-year normal for the most accurate number
10%
First-flush diversion, gutter splash-out, evaporation & tank overflow. Typical 10-20%.
3 Storage, System & Cost
$total
$total
Gravity-fed barrels can run $0 — set to 0 for a passive system
$total
$total
Toilets & laundry need purple-pipe separation from potable lines
$total
$total
Potable use is regulated — many jurisdictions require inspection
gal
A common rule is 1 month of average harvest
$/ gal
Poly tanks ~$0.60-$1.20/gal; buried concrete cisterns run higher
gal / day
Garden ~60-150 gal/day; toilets + laundry ~50-80 gal/day per household
$/ 1,000 gal
U.S. combined rates commonly run $3 - $12 per 1,000 gal
$total
Pro install of a full system ~$1,500 - $6,000. Set 0 for DIY.
ft
$/ ft
Harvest Estimate
Annual Rainwater Harvest
—
Enter your roof to begin
Gallons / year —
Monthly average —
Days of supply —
Full System Cost Breakdown
Catchment area —
Storage tank —
TOTAL SYSTEM COST—
Cost / gal capacity —
Water savings / year —
Simple payback —
System Cost Chart
Estimated cost by category ($)
Roof Material Comparison — Your Roof
Roof MaterialRunoffGallons / YearMonthly AvgWater Value / Yr

For your 1,200 sq ft roof at 30 in/yr rainfall, after 10% collection losses. Water value at $4.50 / 1,000 gal.