Monthly Collection
3,495 gal
Barrels Needed
64
Garden Weeks
28
Annual Savings
$419
Catchment Area: 1,200 sq ft
Runoff Coefficient: 0.85 (85% efficiency)
Monthly: 3,495 gal
64 barrels needed
3,495
gal
64
needed
0
gal
28
Based on $0.01/gallon average water cost
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Inputs
Result
Collects 2,542 gallons monthly (1200 sq ft x 4 in x 0.623 x 0.85), needs 47 barrels for full capacity, covers 20.4 weeks of garden watering (demand: 124.6 gal/week), saves $305/year.
Inputs
Result
Collects 6,510 gallons monthly (2000 sq ft x 5.5 in x 0.623 x 0.95), needs 66 barrels (using 2 means significant overflow), covers 23.2 weeks of garden watering (demand: 280.4 gal/week), saves $781/year.
Inputs
Result
Collects 467 gallons monthly (500 sq ft x 3 in x 0.623 x 0.50) due to low runoff coefficient on flat roof, needs 10 barrels for full capacity, annual collection 5,607 gallons, saves $56/year.
The amount depends on your roof area, rainfall, and roof type. For every inch of rain, a 1,000 sq ft roof with asphalt shingles can collect approximately 530 gallons (1000 x 1 x 0.623 x 0.85). A typical 40x30 ft roof (1,200 sq ft) with asphalt shingles would collect about 2,542 gallons with 4 inches of monthly rainfall (1200 x 4 x 0.623 x 0.85).
Most residential applications use 50-55 gallon barrels. Calculate your monthly collection potential and divide by your desired number of barrels. For higher rainfall areas or larger roofs, consider 100+ gallon barrels or multiple connected barrels to maximize collection.
Different roofs have different runoff coefficients: Metal roofs (95% efficiency), Asphalt shingles (85%), Tile roofs (80%), and Flat/rubber roofs (50%). Metal roofs collect the most water while flat roofs collect the least due to absorption and evaporation.
| Roof Type | Coefficient | Gal/1,000 sq ft/inch | Water Quality |
|---|---|---|---|
| Metal (standing seam) | 0.95 | 592 | Excellent |
| Asphalt shingle | 0.85 | 530 | Good |
| Clay/concrete tile | 0.80 | 498 | Good |
| Flat/rubber (EPDM) | 0.50 | 312 | Fair |
This depends on your garden size and water needs. A 200 sq ft garden needing 1 inch per week requires about 125 gallons weekly. If you collect 500 gallons monthly, that covers about 4 weeks of watering. The calculator shows how many weeks your collection will support.
Savings vary by location and water rates. At an average of $0.01 per gallon, a 1,200 sq ft roof in a moderate rainfall area (4 inches/month) could save approximately $300-400 annually. Areas with higher water rates or more rainfall see greater savings.
A first flush diverter automatically diverts the first runoff (which contains roof debris, bird droppings, and pollutants) away from your barrel. It's highly recommended for cleaner water collection, especially if you plan to use the water on edible plants.
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Last Updated: Mar 26, 2026
This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.
