Rainwater Harvesting
Enter catchment area, rainfall and runoff coefficient to compute collectible rainwater V = area × rainfall × runoff coefficient (litres and m³), estimating rainwater-recycling potential.
Input Data
Results
At a glance:Rainwater harvesting intercepts, diverts and stores rainfall falling on roofs or impervious ground for reuse — irrigation, toilet flushing, cleaning, landscape topping up, even treated non-potable supply. It cuts reliance on mains water and cost, and eases storm runoff and drainage load, making it central to 'sponge city' and sustainable-building strategies. The most practical estimate of how much a catchment yields over a period or a storm is V = catchment area × rainfall × runoff coefficient. Catchment area is the horizontal projection that catches rain (m²; a sloped roof uses its horizontal projection, not slope length); rainfall is millimetres (mm), the depth rain would form if it stayed on the ground; the runoff coefficient (0–1) is the fraction truly collected — some rain is lost to surface wetting, evaporation, or the initial 'first-flush' that washes off dust, so the tank always receives less than the theoretical value. Roof materials differ: smooth metal or tile roofs ~0.8–0.9, concrete or brick ~0.6–0.8, green roofs much lower (0.2–0.5) due to absorption. Units are intuitive: 1 mm of rain over 1 m² is exactly 1 litre (1 mm × 1 m² = 1 L), so 'area(m²) × rainfall(mm)' is directly the theoretical litres, times the runoff coefficient gives actual collectible litres; ÷1000 gives cubic metres (m³). Example: a 100 m² roof, a 100 mm storm, runoff 0.85 → 100 × 100 × 0.85 = 8,500 L = 8.5 m³. For annual recovery, substitute annual rainfall: Hong Kong ~2,400 mm/yr → same roof ≈ 100 × 2400 × 0.85 = 204,000 L (204 m³), very substantial (actual capture limited by tank capacity and usage rhythm). Design notes: (1) tank size should match rainfall pattern and demand — too small overflows and wastes in big storms, too large raises cost and footprint; (2) add a first-flush diverter, screen and lid to keep out leaves and mosquitoes, and clean the catchment and filter media regularly; (3) for human-contact use, treat properly and meet local water standards — most harvested rainwater is non-potable; (4) use local representative runoff and rainfall values, adjusting the default 0.85 as needed. In short, the volume formula links area × rainfall × reduction, letting users quickly estimate recovery potential, size storage and assess water savings.
Formula
Collectible rainwater: V(L) = catchment area(m²) × rainfall(mm) × runoff coefficient.
Convert to m³: V(m³) = V(L) ÷ 1000, since 1 mm × 1 m² = 1 L.
Runoff coefficient 0–1: metal/tile roof ~0.8–0.9, concrete ~0.6–0.8, green roof lower.
$$V_{\text{L}} = A_{\text{m}^2} \times R_{\text{mm}} \times C$$$$V_{\text{m}^3} = \dfrac{V_{\text{L}}}{1000}$$How to Use
- Measure the rain-catching catchment area (roof horizontal projection, m²).
- Enter rainfall over the period (mm, single storm or annual) and roof runoff coefficient (0–1).
- The tool returns collectible rainwater (litres) and converts to m³ for tank sizing and recovery estimates.
Runoff coefficient reference by roof material
| Roof material | Runoff coefficient | Note |
|---|---|---|
| Metal sheet / tile | 0.8–0.9 | Smooth, impervious, high capture |
| Concrete / brick | 0.6–0.8 | Some absorption and roughness loss |
| Bitumen waterproofing | 0.7–0.85 | Depends on age and slope |
| Green roof | 0.2–0.5 | Vegetation absorbs and holds water, low capture |
| Example: 100 m² × 100 mm × 0.85 | = 8500 L | = 8.5 m³ |
1 mm rain over 1 m² = 1 L; the runoff coefficient already discounts wetting, evaporation and first-flush losses.
Case Studies
Roof collection from a single storm
A 100 m² metal roof meets a 100 mm storm; runoff coefficient 0.85.
V = 100 × 100 × 0.85 = 8,500 L = 8.5 m³.
If the tank is only 5 m³, about 3.5 m³ overflows this storm — tank size must match the rainfall pattern.
Annual recovery potential
Same 100 m² roof, Hong Kong annual rainfall ~2,400 mm, runoff 0.85.
V = 100 × 2400 × 0.85 = 204,000 L ≈ 204 m³/yr.
This is a theoretical ceiling, limited by tank capacity and usage; used for irrigation and flushing it saves substantial mains water.
FAQ
Should catchment area be the actual roof area or the projection?
Use the horizontal projection. Rainfall is a vertical depth (mm), and a sloped roof catches rain according to its horizontal projection, not its slope length. If you know the roof's plan dimensions, multiply them directly; a sloped roof does not need slope scaling.
How do I choose the runoff coefficient?
By roof material: metal or tile ~0.8–0.9, concrete or brick ~0.6–0.8, green roof lower (~0.2–0.5). It already discounts wetting, evaporation and first-flush losses. For a rough estimate use 0.8–0.85; for precision use local measured or code values.
Why is 1 mm of rain equal to 1 litre per square metre?
1 mm is 0.001 m deep; over 1 m² the volume is 1 × 0.001 = 0.001 m³ = 1 litre. So 'area(m²) × rainfall(mm)' gives the theoretical litres directly; times the runoff coefficient is the actual collectible litres, ÷1000 gives m³.
Is harvested rainwater safe to drink?
Generally not recommended directly. Rainwater picks up dust, bird droppings and microbes on the catchment, and is mostly used non-potably — irrigation, toilet flushing, cleaning. For higher-grade use, add first-flush diversion, filtration and disinfection and meet local water standards.
How large should the storage tank be?
It depends on rainfall pattern and demand: too small overflows and wastes in big storms, too large raises cost and footprint. Estimate from the largest single-storm collection and daily usage rate so storage supports dry periods without sitting idle. Add an overflow pipe to divert excess safely.
Related Tools
References
Content review: Calculatorism Science Team. Results are for reference only; please refer to the relevant authorities for the official figures.