Rainwater Harvesting Calculator
Rainwater Harvesting is evaluated from Roof Catchment Area, Annual Rainfall and Roof Surface Type. The calculation reports Annual Collection Potential, Average Monthly Collection and Recommended Tank Size.
Results
About the Rainwater Harvesting Calculator
The calculator uses a multi formula configuration. Each reported value is read as a direct evaluation of the stored rules with the declared field formats and units.
Formula basis:
Collection = roof area (sq ft) x rainfall depth (ft) x 7.48 gal/cu ft x runoff coefficient
Tank size = 30% of monthly average (match peak collection period)
Savings = annual gallons x water rate
Interpret the outputs in the order shown by the result fields. Optional inputs affect only the outputs that depend on those variables.
Formula & How It Works
The calculation applies the following relations exactly as recorded in the metadata: Collection = roof area (sq ft) x rainfall depth (ft) x 7.48 gal/cu ft x runoff coefficient Tank size = 30% of monthly average (match peak collection period) Savings = annual gallons x water rate Each output field is produced by substituting the supplied inputs into the relevant relation and then applying the declared rounding or text format.
Worked Examples
Example 1: 1,500 sq ft roof, Atlanta, GA (50 inches/year), asphalt shingles, $4/1,000 gal
Inputs
With Roof Catchment Area = 1,500, Annual Rainfall = 50, Roof Surface Type = 0.85 and Your Water Rate = 4 as the stated inputs, the result is Annual Collection Potential = 39,740 gallons/year, Average Monthly Collection = 3,312 gallons/month and Recommended Tank Size = 1,000 gallons. Each value corresponds to the declared output fields.
Example 2: 2,000 sq ft metal roof, Austin, TX (33 inches/year), $5/1,000 gal
Inputs
With Roof Catchment Area = 2,000, Annual Rainfall = 33, Roof Surface Type = 0.95 and Your Water Rate = 5 as the stated inputs, the result is Annual Collection Potential = 39,086 gallons/year, Average Monthly Collection = 3,257 gallons/month and Recommended Tank Size = 1,000 gallons. Each value corresponds to the declared output fields.
Example 3: 800 sq ft roof, Phoenix, AZ (8 inches/year), tile roof, $7/1,000 gal
Inputs
With Roof Catchment Area = 800, Annual Rainfall = 8, Roof Surface Type = 0.8 and Your Water Rate = 7 as the stated inputs, the result is Annual Collection Potential = 3,192 gallons/year, Average Monthly Collection = 266 gallons/month and Recommended Tank Size = 100 gallons. Each value corresponds to the declared output fields.
Example 4: 3,000 sq ft roof, Seattle, WA (38 inches/year), asphalt, $6/1,000 gal
Inputs
With Roof Catchment Area = 3,000, Annual Rainfall = 38, Roof Surface Type = 0.85 and Your Water Rate = 6 as the stated inputs, the result is Annual Collection Potential = 60,405 gallons/year, Average Monthly Collection = 5,034 gallons/month and Recommended Tank Size = 1,500 gallons. Each value corresponds to the declared output fields.
Common Use Cases
- Calculate rainwater collection potential from roof area
- Size a rain barrel or cistern for rainwater harvesting
- Estimate water bill savings from rainwater harvesting