Turn rainfall data into reliable capture estimates. Model first-flush diversion, overflow, and tank volume needs. See results instantly, then export CSV or PDF files.
| Scenario | Area (m2) | Rain (mm/yr) | Coeff | Eff | Losses (%) | Tank (L) | Demand (L/day) |
|---|---|---|---|---|---|---|---|
| Small house | 80 | 600 | 0.80 | 0.88 | 10 | 3000 | 200 |
| Medium building | 120 | 800 | 0.85 | 0.90 | 8 | 5000 | 300 |
| Warehouse | 500 | 900 | 0.90 | 0.92 | 6 | 20000 | 1200 |
Start by confirming the effective catchment area, not the building footprint. Roof pitch, parapets, and drainage splits change where water actually flows. Smooth metal roofs typically produce higher runoff coefficients than rough concrete or green roofs. Use field observation during rain to validate downpipe routing. If several roof zones exist, calculate each zone separately and sum the annual yield for clearer sizing decisions.
Annual rainfall gives a planning baseline, but collection reliability depends on seasonality. In monsoon climates, large volumes arrive in short periods, increasing overflow risk if storage is small. In dry climates, long gaps require more autonomy days. Use long term local station averages where possible, and compare at least wet, normal, and dry years. This helps you stress test expected coverage. Where meter data is unavailable, use regional maps, then document assumptions so reviewers can verify the rainfall basis during permitting and audits future.
First flush diversion reduces pollutants from dust, bird droppings, and roof debris. Represent it as a depth equivalent over the catchment, then subtract it from gross yield. Losses also occur through filter backwash, evaporation from open tanks, leaks, and overflow during intense storms. Conservative loss percentages provide safer estimates. If water will be used indoors, include disinfection and comply with applicable codes.
Tank sizing is a balance between capture opportunity and end use demand. A minimum autonomy volume equals daily demand times target days without rain. Larger tanks reduce overflow during heavy storms, yet may increase cost and footprint. When demand is low, prioritize right sized storage with a clear overflow route. When demand is high, focus on maximizing collection and maintaining filtration performance.
Use net harvestable water to estimate annual savings and payback, then use coverage percent to communicate performance. A high coverage rating suggests the system can offset a meaningful share of non potable demand. If coverage is low, consider expanding catchment, improving efficiency, or reducing demand through fixtures. Confirm structural loads, access for maintenance, and safe separation from potable plumbing.
It represents how much rainfall becomes runoff from a surface. Smooth roofs shed more water, so values are higher. Rough or absorbent surfaces lose more to wetting and retention, so values are lower.
Annual rainfall is fine for early planning. For detailed design, monthly or daily data improves reliability checks, because it reflects dry spells and seasonal peaks that affect tank refill and overflow.
Use a small depth to divert initial dirty runoff, then refine using site conditions. Heavily dusty roofs or overhanging trees may need more. Keep it realistic so diversion does not dominate annual yield.
Gross uses area, rainfall, and coefficient only. Net harvestable subtracts first flush and then applies efficiency and losses. These adjustments reflect practical capture limits, filtration performance, leaks, and overflow.
Start with daily irrigation demand and autonomy days to set a minimum volume. Then check whether expected annual yield can refill the tank. If rainfall is seasonal, consider a larger tank plus an overflow plan.
Only if you add robust treatment and meet local health requirements. Most projects use harvested water for toilets, cleaning, and irrigation. Separate plumbing, backflow protection, and disinfection are essential.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.