Turn rainfall into reliable supply for your site. Enter roof size, losses, and goals today. See harvest volume, tank sizing, and savings instantly here.
White theme, single column layout. Results appear below after you calculate.
| Rainfall (mm/yr) | Area (m²) | C | Eff (%) | Losses (%) | First Flush (L/event) | Events/yr | Net (L/yr) (approx.) |
|---|---|---|---|---|---|---|---|
| 800 | 120 | 0.85 | 90 | Convey 5, Filter 2, Overflow 0 | 20 | 60 | ~61,000 |
| 500 | 80 | 0.80 | 85 | Convey 7, Filter 3, Overflow 5 | 15 | 45 | ~21,000 |
| 1200 | 200 | 0.90 | 92 | Convey 4, Filter 1, Overflow 0 | 25 | 80 | ~180,000 |
Construction note: For potable use, add proper treatment and local approvals. For non-potable uses, plan separate plumbing, labeling, and backflow protection.
Roof runoff can replace part of non-potable water demand during construction and building occupancy. Even a modest roof can yield significant volume: 800 mm/year over 120 m² equals 96 m³ gross before losses. Capturing that water reduces supply interruptions, tanker trips, and operating cost.
Use annual rainfall depth from a reliable local source, then match units. This tool accepts millimeters or inches and converts using 1 in = 25.4 mm. For design, treat rainfall as a long-term average, then add safety margin for dry years if water security is critical.
Enter the roof plan area (horizontal projection), not the sloped surface. If you measure in feet, the calculator converts using 1 ft² = 0.092903 m². Accurate area is often the largest driver after rainfall, so verify drawings and include only connected roof sections.
The runoff coefficient estimates what fraction of rain becomes flow into gutters. Typical values: metal roofs 0.85–0.90, tiles 0.80–0.85, smooth concrete about 0.70–0.80, and vegetated or gravel roofs near 0.40–0.60. Use “Custom value” for project-specific testing.
Real systems lose water through leaks, splashing, filter bypass, and overflow. Many well-built setups achieve 70–95% overall collection efficiency before first flush. Conveyance losses of 3–10% and filter losses of 1–5% are common. Overflow loss depends on tank size and storm pattern.
First flush diversion improves water quality by discarding the initial runoff that carries dust and bird droppings. Practical settings range from 10–50 L per event for small roofs, or roughly 0.5–2.0 L per m² of roof per event. Multiply by rain events per year to see the annual impact.
Storage should match how the water will be used. The calculator estimates daily demand as people × liters per person per day, then recommends storage as demand × autonomy days. For non-potable uses, 20–60 L/person/day is a typical planning band. Larger autonomy improves resilience but increases capital cost and footprint.
If you enter a tariff (per m³), the tool estimates annual savings as harvested m³ × tariff. This supports basic payback screening alongside tank cost per liter. Export CSV for spreadsheets and PDF for site reports, approvals, and client updates. Always document assumptions: C, losses, first flush, and demand profile.
Net harvested is the usable annual volume after efficiency and loss factors, minus first flush diversion. It represents the water you can realistically store and use.
Use long-term average for baseline planning. For critical supply, test a lower rainfall scenario and increase storage or backup supply to reduce risk.
If first flush total exceeds usable volume, the calculator sets net to zero. Reduce first flush per event, reduce event count, or improve efficiency and losses.
Only with proper treatment and local approvals. For drinking, plan filtration, disinfection, safe storage, testing, and separate plumbing from non-potable lines.
Use local climate summaries, weather station records, or project hydrology reports. If unsure, start with 40–80 events and run sensitivity checks.
For non-potable uses, 20–60 L/person/day is common. Use higher values for washing and cleaning-heavy sites, and lower values for toilet-only supply.
It is a planning estimate based on average demand and chosen autonomy days. Detailed sizing should model seasonal rainfall, storm variability, overflow behavior, and operational constraints.
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.