Rainwater Catchment.
Rainwater Catchment Calculator
How much rain hits your roof, how much you can capture, what tank size you need, and how much irrigation it covers.
The catchment formula
Gallons captured = Roof area (sq ft) × Rainfall (in)
× 0.623 × Roof efficiency
→ 1" of rain on 1,000 sq ft of metal roof
= 1000 × 1 × 0.623 × 0.95
= ~590 gallons
The 0.623 conversion factor comes from: 1 inch of rain over 1 sq ft = 144 cubic inches × 1 gal/231 cu-in = 0.623 gal.
Reference: catchment by US city
| City | Annual in | From 1000 sq ft (gal) | From 2000 sq ft (gal) |
|---|---|---|---|
| Phoenix, AZ | 8 | ~5,000 | ~10,000 |
| Albuquerque, NM | 10 | ~6,000 | ~12,000 |
| Denver, CO | 14 | ~8,500 | ~17,000 |
| San Francisco, CA | 23 | ~14,000 | ~28,000 |
| Minneapolis, MN | 30 | ~18,000 | ~36,000 |
| Seattle, WA | 38 | ~23,000 | ~46,000 |
| Chicago, IL | 38 | ~23,000 | ~46,000 |
| New York, NY | 47 | ~28,000 | ~56,000 |
| Atlanta, GA | 50 | ~30,000 | ~60,000 |
| Boston, MA | 44 | ~26,000 | ~52,000 |
| Houston, TX | 50 | ~30,000 | ~60,000 |
| Miami, FL | 62 | ~37,000 | ~74,000 |
| New Orleans, LA | 64 | ~38,000 | ~76,000 |
Tank sizing approach
Two extreme strategies; most installations are in between:
| Approach | Tank size | Result |
|---|---|---|
| Capture-everything | = annual catchment | Never overflows; massive cost |
| Carry-through-dry-season | = peak monthly demand × dry months | Most economical for irrigation use |
| Storm-event | = 2-3" rainfall capture | Stormwater management; small irrigation use |
| Code minimum (some jurisdictions) | ~1 gallon per sq ft roof | Stormwater detention requirement |
Legal status (US, simplified)
| State category | Status |
|---|---|
| Encouraging (most states) | Legal; many offer rebates |
| Restricting (CO, UT, WA) | Allowed but with limits (e.g., CO: 110 gal residential max in 2 barrels) |
| Highly regulated (NV, AZ for large systems) | Permits required for systems above thresholds |
| HOA / private | HOAs may restrict regardless of state law |
Always check your state and local rules. Western water-rights states historically restricted catchment due to senior surface-water rights, though most have liberalized.
First-flush diversion
The first ~1 gallon per 100 sq ft of roof carries dust, debris, bird droppings, and pollutants from previous dry weeks. A first-flush diverter dumps this initial flow before the rest reaches the tank — dramatically improves stored water quality.
- Sizing: ~1 gal first flush per 100 sq ft roof
- Maintenance: Empty after each rain event (slow-drip drain at bottom)
- Cost: $40-150 commercial; can DIY with PVC standpipe + drain
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