Calculator Inputs
The layout stays single-column overall, while the input fields use three columns on large screens, two on smaller screens, and one on mobile.
Example Data Table
| Roof Area (m²) | Rainfall (mm) | Runoff (%) | First Flush (%) | Filter (%) | Conveyance (%) | Occupants | Demand/Person (L) | Dry Days | Tank (L) | Net Harvest (L) | Recommended Tank (L) |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 180 | 950 | 85 | 3 | 92 | 96 | 5 | 70 | 25 | 10,000 | 124,522 | 8,750 |
Formula Used
1) Gross Roof Collection (L/year)
Roof Area (m²) × Annual Rainfall (mm)
2) Net Harvestable Water (L/year)
Gross Collection × Runoff Coefficient × (1 − First Flush Loss) × Filter Efficiency × Conveyance Efficiency
3) Daily Demand (L/day)
Occupants × Daily Demand per Person
4) Annual Demand (L/year)
Daily Demand × 365
5) Recommended Tank Size (L)
Daily Demand × Design Dry Days
6) Demand Coverage (%)
(Net Harvestable Water ÷ Annual Demand) × 100
7) Annual Savings
(Minimum of Net Harvestable Water and Annual Demand ÷ 1000) × Water Tariff
How to Use This Calculator
- Choose the roof type. The calculator can suggest a runoff value.
- Enter the total roof catchment area that drains into your system.
- Provide the site’s annual rainfall in millimeters.
- Adjust runoff, first flush, filter, and conveyance values to match real conditions.
- Enter household occupancy and daily water use per person.
- Set the design dry period to size storage for low-rain intervals.
- Add current tank volume and local water tariff for gap and savings analysis.
- Press Calculate System to show results above the form.
- Use the chart and exports for reports, client reviews, or planning notes.
FAQs
1. What does the runoff coefficient mean?
It represents how much rainfall actually becomes collectible runoff. Smooth roofs usually have higher values, while rough, vegetated, or absorbent surfaces lose more water before it reaches the storage system.
2. Why is first flush loss included?
The first portion of rainfall often carries dust, leaves, droppings, and roof debris. Diverting it improves stored water quality but slightly reduces the usable volume collected during each rain event.
3. How should I choose filter efficiency?
Use a value based on the filter type, maintenance quality, and clogging risk. Clean, well-maintained filtration systems perform better and preserve usable storage without sacrificing much collection volume.
4. What is conveyance efficiency?
It reflects losses in gutters, downpipes, screens, and transfer pipes. Leaks, poor slope, overflow, and blockages all reduce the water that successfully arrives at the storage tank.
5. Why is storage sized using dry days?
Storage must bridge gaps between useful rain events. A longer dry period needs a larger tank because the system must supply demand for more days without meaningful replenishment.
6. Can annual harvest exceed annual demand?
Yes. A large roof in a wet climate may collect more water than the building uses. In that case, storage, overflow management, and seasonal rainfall distribution become the next design priorities.
7. Does this calculator replace a detailed hydraulic design?
No. It is a planning and estimation tool. Final projects should still review rainfall seasonality, roof drainage details, structural loading, local standards, treatment needs, and overflow routing.
8. When should I increase roof area instead of tank size?
Increase roof area when annual harvest is too low. Increase tank size when annual harvest is adequate but dry-period storage is insufficient. The two upgrades solve different shortages.