Inputs
White theme • client-side
Results
0 L
Gross harvest (before coefficients)
0 L
Net available (after all adjustments)
0 m³
Net available in cubic meters
0 gal
Net available in US gallons
0 L
Recommended tank for target autonomy
0 days
Autonomy with your tank and demand
- Area used: m²
- Rainfall used: mm
- Runoff coefficient:
- Collection efficiency: %
- First flush volume: L
- Additional losses: %
- Daily demand: L/day
- Target autonomy: days
Example Scenarios (illustrative)
The following table shows example yields for a 100 m² roof with 0.9 runoff coefficient, 95% collection efficiency, 0.5 mm first flush, and 5% losses.
| Scenario | Rainfall (mm) | Gross (L) | After coefficients (L) | After first flush (L) | Net after losses (L) |
|---|
Season-by-Season Storage Simulation
Mass-curve / sequent peak sizing
Switch between monthly or 4-season view.
0 L
Required tank (no shortages)
0 L
Total overflow with chosen tank
0 L
Total unmet demand (shortage)
0 %
Reliability (demand served)
| Period | Days | Rainfall | Daily demand (L/day) | Inflow net (L) | Demand (L) | End storage (L) | Overflow (L) | Shortage (L) |
|---|---|---|---|---|---|---|---|---|
| Totals | 0 | 0 | 0 | 0 | 0 | |||
Notes: Inflow per period uses your main inputs (area, runoff, collection efficiency, first flush once per period, other losses). Required tank is computed by sequent peak analysis using period inflows and demands.
Runoff Coefficient Reference by Roof Type
| Roof material / condition | Runoff coefficient (Cr) range | Notes |
|---|---|---|
| Smooth metal (painted) | 0.90 – 0.95 | High yield, minimal absorption and texture. |
| Metal, weathered/rough | 0.85 – 0.92 | Slight texture lowers runoff a bit. |
| Clay tile | 0.80 – 0.90 | Joints and porosity reduce runoff. |
| Concrete tile | 0.75 – 0.85 | More porous; conservative design advisable. |
| Asphalt shingle | 0.75 – 0.85 | Granular surface increases losses. |
| Bitumen/membrane | 0.85 – 0.95 | Similar to smooth roofs; check ponding. |
| Green roof (extensive) | 0.50 – 0.70 | Vegetation/media retain rainfall. |
| Green roof (intensive) | 0.30 – 0.50 | Deep substrate, higher retention. |
Choose the low end of the range for conservative planning and for dirty roofs.
First Flush Sizing Guide (Rule‑of‑Thumb)
Convert mm to liters as: mm × roof area (m²) = liters. For a 100 m² roof, 1.0 mm ≈ 100 L.
| Environment / debris load | Suggested diversion (mm) | Equivalent per 100 m² (L) | Notes |
|---|---|---|---|
| Rural, clean roof | 0.3 – 0.5 | 30 – 50 | Minimal dust; infrequent traffic/industry. |
| Urban, moderate pollution | 0.5 – 1.0 | 50 – 100 | More particulates, leaf litter. |
| Industrial/near highways | 1.0 – 2.0 | 100 – 200 | Higher contaminants; increase diversion. |
| Arid/dust season onset | 1.0 – 1.5 | 100 – 150 | Heavier accumulated dust before first rains. |
| High tree cover/leaf fall | 0.8 – 1.2 | 80 – 120 | Use leaf guards; clean gutters often. |
End‑Use Demand Benchmarks (Non‑potable)
| Use case | Unit | Typical value | Planning note |
|---|---|---|---|
| Toilet flush (dual‑flush) | L/flush | 3 – 6 | Assume ~5 uses/person/day for estimates. |
| Clothes washing (HE) | L/load | 50 – 70 | Modern front‑loaders use less water. |
| Clothes washing (conventional) | L/load | 70 – 120 | Older top‑loaders; vary by model. |
| Garden irrigation | L/m²/day | 2 – 6 | Climate, soil, and crop drive actual needs. |
| Vehicle wash (bucket) | L/event | 60 – 100 | Hose‑only methods use substantially more. |
| Outdoor cleaning | L/event | 20 – 40 | Consider pressure‑reducer nozzles. |
Values are indicative for planning—verify against local guidance and appliance specs.
Formulas Used
- Unit conversions
- Area: \( A_{m^2} = A_{ft^2} \times 0.092903 \)
- Rain: \( R_{mm} = R_{in} \times 25.4 \)
- 1 mm over 1 m² = 1 L
- Gross harvest: \( V_g = A_{m^2} \times R_{mm} \)
- After coefficients: \( V_c = V_g \times C_r \times \eta_c \), where \( C_r \) is runoff coefficient and \( \eta_c = \frac{\text{collection %}}{100} \).
- First flush volume: \( V_f = \begin{cases} R_{ff(mm)} \times A_{m^2}, & \text{if mm} \\ V_{ff(L)}, & \text{if L} \\ 3.78541 \times V_{ff(gal)}, & \text{if gal} \end{cases} \)
- After first flush: \( V_{af} = \max(0, V_c - V_f) \)
- Net after losses: \( V_n = V_{af} \times (1 - \frac{L\%}{100}) \)
- Tank sizing for autonomy: \( S_{rec} = \min(V_n, D \times d) \), where \( D \) is demand (L/day) and \( d \) is target dry days.
- Autonomy with existing tank: \( d_{tank} = \frac{S_{tank}}{D} \)
- Sequent peak capacity (variable demand): Let \( I_i \) be period inflow (net) and \( D_i \) period demand. Define \( Z_0=0 \) and \( Z_i = \max(0, Z_{i-1} + D_i - I_i) \). Required storage is \( \max(Z_i) \).
These are planning approximations; actual performance depends on distribution of rainfall over time and on system maintenance.
How to Use This Calculator
- Enter roof catchment area and choose your preferred area unit.
- Enter a rainfall depth for a month, season, or event, with unit.
- Set a runoff coefficient based on roofing material and cleanliness.
- Set collection efficiency to reflect gutters, screens, and conveyance.
- Provide a first flush diversion as millimeters, liters, or gallons.
- Add any additional losses as percent of volume.
- Enter daily demand and desired dry days to size storage.
- For Season Simulation, switch to months or seasons, edit rainfall and days, and click Simulate.
- Click Calculate. Review results and export as CSV or PDF.
Tip: Use your city’s climatology to populate monthly rain; explore tank sizes for reliability.
FAQs
Smooth metal roofs often range 0.9–0.95, tiles 0.8–0.9, concrete/asphalt 0.75–0.85, vegetated surfaces lower. Choose conservative values if uncertain.
Many designs divert 0.3–1.0 mm of initial rainfall over the catchment. Increase if pollution or debris loads are high.
Losses from gutter overshoot, screens, leaks, and evaporation reduce yield. Also, rainfall depth may be uneven; a single storm with overflow will limit capture.
The projected plan area is used, not the sloped surface. Material and cleanliness are captured via the runoff coefficient input.
Evaluate month-by-month rainfall and demand. The limiting dry season controls required storage and supplemental supply. This tool sizes with sequent peak analysis.
Yes. Enter rainfall in inches and first flush in gallons if preferred. The results panel also shows equivalent US gallons.
For potable use, follow local codes and public-health guidance, including roof selection, pretreatment, filtration, disinfection, and backflow protection. Consult a qualified professional for final design.