Roof Rainwater Harvest Calculator
Estimate how much rainwater your roof can capture, how full your tank will get, and how much may overflow — based on your local rainfall and system setup.
Quick Answer
A 1,000 sq ft roof with 1 inch of rain and a runoff coefficient of 0.85 can collect roughly 530 gallons. Multiply by annual rainfall and efficiency for yearly totals.
System inputs
Footprint or sloped surface area
Per event or period
Metal/tile ~0.9, asphalt ~0.8
Accounts for gutter & filter losses
Total usable storage volume
Used for annual estimate
Potential
0
gallons
Effective
0
gallons
Tank fill
0%
of capacity
Overflow
0
gallons
Based on 12 rainfall events per year
Potential = roof area × rainfall × runoff coefficient. Effective = potential × efficiency. Overflow = max(0, effective − tank capacity). Results update live.
How It Works
The calculator estimates rainwater harvesting in four logical steps:
- Potential collection — the theoretical maximum water available from your roof surface.
- Effective collection — after accounting for gutter losses, first-flush diversion, and filtration.
- Tank fill — how much of your storage capacity is filled per event.
- Overflow — water that exceeds tank capacity and is lost.
The animated diagram updates in real time: rain intensity, flow paths, and tank water level respond to your inputs.
Visual Flow: Rain → Roof → Gutter → Filter → Storage Tank
Each stage introduces losses. The runoff coefficient models roof surface losses; collection efficiency models the rest of the system. The tank fill bar and overflow value show how storage capacity interacts with supply.
Formula
Potential
Area × Rainfall × Runoff
Effective
Potential × Efficiency
Overflow
max(0, Effective − Tank)
All volumes in gallons. 1 inch of rain on 1 sq ft = 0.623 gallons.
Example
A homeowner has a 1,200 sq ft tile roof, receives 1.2 inches of rain, runoff coefficient 0.85, collection efficiency 85%, and a 1,500 gallon tank.
763 gal
649 gal
43%
0 gal
With 12 events per year, annual collection could reach about 7,788 gallons (effective).
Factors That Affect Results
Roof catchment area
Larger roofs capture more rain. Measure the footprint or sloped surface accurately.
Rainfall
Intensity and frequency vary by season and region. Use local averages for annual estimates.
Runoff coefficient
Smooth metal (0.9–0.95) vs. asphalt shingle (0.75–0.85) vs. green roof (0.3–0.5).
Collection efficiency
Gutter losses, first-flush, filter clogging, and evaporation reduce practical yield.
Gutter losses
Splash-out, overflow during heavy rain, and debris reduce captured volume.
Storage capacity
Tank size limits how much you can hold; overflow occurs when supply exceeds demand between events.
Seasonal rainfall variation means annual estimates should use monthly averages for best accuracy.
Practical Tips
- • Install a first-flush diverter to improve water quality and protect your tank.
- • Use a fine mesh gutter guard to reduce leaf litter and filter clogging.
- • Size your tank to cover the longest dry spell, not just average rainfall.
- • Clean gutters before the rainy season to maximize collection efficiency.
- • Consider multiple smaller tanks in series to reduce overflow and increase resilience.
- • Measure actual roof area (not just building footprint) if the roof has significant slope.
Frequently Asked Questions
How much rainwater can I collect from my roof? ▼
It depends on roof area, rainfall, runoff coefficient, and system efficiency. Our calculator gives a precise estimate. As a rule of thumb, 1 inch of rain on 1,000 sq ft yields about 623 gallons before losses.
What is a runoff coefficient? ▼
The runoff coefficient represents the fraction of rainfall that becomes runoff. Smooth surfaces like metal or tile roofs have high coefficients (0.8–0.95), while gravel or green roofs have lower values.
How does collection efficiency affect my harvest? ▼
Collection efficiency accounts for losses from gutters, first-flush diversion, filtration, and evaporation. A typical efficient system has 80–90% efficiency.
Why does tank size matter? ▼
Tank capacity determines how much water you can store between rainfall events. If collection exceeds capacity, overflow occurs. Proper sizing balances supply and demand.
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