The Rainwater Harvesting Formula
| Annual gallons | Area (sq ft) × Rainfall (in) × 0.623 × Efficiency |
| Per-storm gallons | Area (sq ft) × Storm depth (in) × 0.623 × Efficiency |
| 0.623 constant | Gallons per sq ft per inch of rain |
Annual Catchment by Roof Size & Rainfall
| Roof Area | 20" Rain/yr | 30" Rain/yr | 40" Rain/yr | 50" Rain/yr |
|---|---|---|---|---|
| 1,000 sq ft | 10,591 gal | 15,886 gal | 21,182 gal | 26,477 gal |
| 1,500 sq ft | 15,886 gal | 23,828 gal | 31,771 gal | 39,714 gal |
| 2,000 sq ft | 21,182 gal | 31,771 gal | 42,361 gal | 52,951 gal |
| 3,000 sq ft | 31,771 gal | 47,657 gal | 63,542 gal | 79,428 gal |
| 5,000 sq ft | 52,952 gal | 79,428 gal | 105,904 gal | 132,380 gal |
| Based on 85% collection efficiency (asphalt shingles). Actual yield will vary. | ||||
Tank Sizing Rule of Thumb
Size your storage tank to capture the runoff from a single significant storm — typically 1–2 inches on your catchment area. Beyond that, the tank overflows and additional storage has diminishing returns unless you're filling from accumulated smaller rains over time.
For a 2,000 sq ft roof: 1 inch of rain = 2,000 × 1 × 0.623 × 0.85 = 1,059 gallons. A 1,500-gallon tank captures most 1-inch storms. A 3,000-gallon tank handles a 2-inch storm event — appropriate for areas where large storm events are common and you want to maximize capture before the ground saturates.
Legality by State
Rainwater harvesting is legal in most US states. Key facts:
- Texas: No restrictions. State actively encourages it with a sales tax exemption on harvesting equipment.
- Arizona: No restrictions. State funded a rebate program for cistern installation.
- Colorado: Now allows up to 110 gallons (two 55-gallon barrels) for outdoor use only, after years of prohibition.
- California: Rain barrels permitted; larger cisterns generally allowed for non-potable use. No statewide permit requirement.
- Oregon: Allows collection from residential rooftops for outdoor use.
- Washington: No permit required for residential roof collection.
Always verify your specific county and municipal regulations — some HOAs and localities have additional restrictions beyond state law.