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Rainwater Harvesting Calculator: What It Does, How It Works, and a Step-by-Step Example
This calculator estimates how much rainwater you can collect from your site, and recommends a tank size based on standards-informed methods (such as the simplified BS 8515 approach and days-of-storage). It supports multiple catchment segments (different roofs/yards), SI/Imperial units, and optional costing for India, USA, and UK.
What this calculator estimates
- Annual harvestable volume from all selected catchments (roofs, paved areas) after first-flush and system losses.
- Daily and annual demand for non-potable uses (WC flushing, laundry, cleaning, irrigation).
- Tank sizing recommendations via:
- BS 8515 simplified method — storage = lesser of 5% of annual yield or 5% of annual non-potable demand (user must verify locally).
- Days-of-storage — storage = daily demand × chosen days.
- Reverse check — given a tank size, how many days of demand it can cover.
- Budgetary costing — editable unit rates with presets for India (INR), USA (USD), and UK (GBP).
How it works (the simple math)
Annual capture (SI units): For each segment i:
Vi = Ai (m²) × P (m/yr) × Ci × η × (1 − fseg) × (1 − ℓ)
Sum across all segments: Vannual = Σ Vi
Where: A = area, P = annual rainfall, C = runoff coefficient (0–1), η = utilisation factor (0–1), f = first-flush fraction, ℓ = losses fraction.
US/IP shortcut (common rule-of-thumb):
Captured gallons = Area(ft²) × Rain(in/yr) × 0.623 × C × η × (1 − f) × (1 − ℓ)
Tank sizing methods used here:
- BS 8515 simplified: Storage = min(5% of annual yield, 5% of annual non-potable demand). (User must verify with their edition and local authority.)
- Days-of-storage: Storage = daily non-potable demand × chosen days of autonomy.
Typical coefficients (starting points, edit to suit):
- Metal roof (smooth): C ≈ 0.95
- Tiled roof: C ≈ 0.85
- Concrete/terrace: C ≈ 0.80
- Paved surface: C ≈ 0.70
Standards consulted: BS 8515, ARCSA/ASPE 63 (US), IS 15797 (India). Exact clause/annex varies by edition—verify for compliance.
What you need before you start
- Annual rainfall for your location (mm/yr or in/yr). You can use a preset and then tweak.
- Catchment areas (roof plans, patio, paved zones), and a sensible runoff coefficient for each surface.
- Non-potable demand estimate: occupants × L/p/d (or gal/p/d) + irrigation/other uses.
- Policy/standard you wish to align with (BS 8515, ARCSA/ASPE 63, IS 15797).
How to use the calculator (quick steps)
- Choose units (SI or Imperial). The widget converts inputs on the fly.
- Select a standard profile to auto-populate defaults (utilisation, typical loss allowances, etc.).
- Pick a city rainfall preset close to your site, then refine the annual rainfall value.
- Add segments for each roof or paved area. Pick surface type, confirm area and C, and (optionally) a first-flush % for that segment.
- Enter demand: occupants × per-capita non-potable use + any other daily uses.
- Adjust losses and first-flush (system-wide) if you have better project data.
- Pick a sizing approach: (a) BS 8515 simplified, (b) days-of-storage, or compare both.
- Optionally use reverse mode — enter a tank volume you already have to see how many days it covers.
- Use costing: choose India/USA/UK presets or directly edit unit rates; the summary updates instantly.
- Download the HTML report for a clean record with KPIs, detailed tables, assumptions and change-log.
Sample project (fully worked)
Scenario
- Location: Bengaluru, India (use rainfall ≈ 970 mm/yr)
- Standard profile: IS 15797 (utilisation η ≈ 0.90; first-flush ≈ 2%; losses ≈ 5%)
- Segments:
- Roof A — 120 m², metal roof, C = 0.95
- Roof B — 60 m², tiled roof, C = 0.85
- Paved — 40 m², paver finish, C = 0.70
- Demand: 6 occupants × 50 L/p/d + 60 L/d irrigation = 360 L/day
- Alternative sizing: 10 days storage (owner wants ~1.5 weeks autonomy)
1) Annual harvest (SI)
P = 970 mm = 0.97 m; η = 0.90; system losses ℓ = 5%; first-flush per segment f = 2%.
| Segment | Area A (m²) | C | V = A×P×C×η×(1−f)×(1−ℓ) | Result |
|---|---|---|---|---|
| Roof A | 120 | 0.95 | 120×0.97×0.95×0.90×0.98×0.95 | ≈ 92.65 m³/yr |
| Roof B | 60 | 0.85 | 60×0.97×0.85×0.90×0.98×0.95 | ≈ 41.45 m³/yr |
| Paved | 40 | 0.70 | 40×0.97×0.70×0.90×0.98×0.95 | ≈ 22.76 m³/yr |
| Annual harvest (sum) | ≈ 156.86 m³/yr (156,863 L) | |||
2) Demand & sizing
- Daily demand = 6×50 + 60 = 360 L/day
- Annual non-potable demand = 360 × 365 = 131,400 L/yr
- BS 8515 simplified: storage = min(5% of yield, 5% of demand) = min(7,843 L, 6,570 L) → 6,570 L (≈ 6.57 m³)
- Days-of-storage (10 days) = 360 × 10 = 3,600 L (3.6 m³)
- Recommended tank: choose the more robust of the two for resilience → ≈ 6.6 m³
3) Reverse check (if an owner already has a 5,000 L tank)
Days covered = 5,000 ÷ 360 ≈ 13.9 days.
4) Costing snapshots (editable in the widget)
The calculator includes editable unit-rate presets. Example totals below assume 6.57 m³ recommended tank, 15 m piping allowance, first-flush/filter fixed cost, and 15% install markup:
| Region | Tank Cost | Piping | Filter/FF | Subtotal | Total (with 15%) |
|---|---|---|---|---|---|
| India (INR) | ₹78,840 | ₹3,000 | ₹5,000 | ₹86,840 | ₹99,866 |
| USA (USD) | $5,913 | $180 | $250 | $6,343 | $7,294 |
| UK (GBP) | £4,927.50 | £135 | £200 | £5,262.50 | £6,051.88 |
Jargon buster
- Runoff coefficient (C): fraction (0–1) of rainfall that becomes collectable runoff from a surface.
- Utilisation factor (η): practical capture allowance considering gutters, screens, timing, etc. (0–1).
- First-flush (% or volume): the initial portion of rain diverted to wash off dust/debris.
- System losses (ℓ): evaporation, minor leaks, downtime.
- Days-of-storage: autonomy target—how long the tank should cover demand without rain.
- Non-potable demand: uses that don’t require drinking-water quality (WC, laundry, irrigation).
Tips & common pitfalls
- Don’t double count areas. A roof over a paved zone shouldn’t be counted twice.
- Match C to material (metal vs tile vs concrete) and adjust if surfaces are rough/dirty.
- Irrigation is seasonal. Consider adding a higher summer “other demand” in scenarios and compare.
- Check local rules on backflow prevention and potable/non-potable separation.
- Overflow routing: plan safe discharge or infiltration when the tank is full.
Where to go next
After you pick a tank size, you might also need:
- Water Sump & Tank Estimator — capacity & RCC cost planning.
- Pipe Design Calculator — quick checks for downpipes and conveyance to storage.
- Cement Concrete Calculator — pads, plinth beams, or chamber works around your tank.
- Septic Tank Calculator — plan separation between wastewater and harvested water networks.
Notes on standards (verify locally)
This article aligns the calculator with widely used guidance:
• BS 8515 (UK) — simplified storage method often expressed as 5% of annual yield vs. 5% of annual non-potable demand.
• ARCSA/ASPE 63 (US) — common US practice for roof runoff uses; includes the 0.623 gal rule-of-thumb (ft² × in).
• IS 15797 (India) — national guidance; rooftop C values commonly in the 0.8–0.95 range.
Regulations differ by state/municipality; consult your authority, and always verify clause numbers in the specific edition you’re using.
