Rainwater Harvesting Calculator — Potential Collection from Roof & Rainfall

What’s Being Calculated?

• Potential (before losses): rain depth × roof area. In metric, 1 mm on 1 m² ≈ 1 L.
• Runoff & filter: multiply by runoff coefficient and filter/screen efficiency.
• First-flush: subtract a fixed amount per rain event (editable).
• Tank capacity (optional): simple per-event cap assuming rainfall is spread across a number of events.

Tips

• Typical runoff coefficients: metal/pitched 0.9–0.95; tile/asphalt 0.8–0.9; rough/green roofs lower. Everything is editable.
• If you don’t track events, leave the default (e.g., 20/year) or enter your local estimate.

How Rainwater Harvesting Works (and What This Calculator Assumes)

Rainwater harvesting is the straightforward idea of catching rainfall from a roof, filtering out debris, and storing it for later use. The core math is simple: rain depth × roof area gives the raw volume hitting your roof. From there we apply a runoff coefficient (how much actually makes it to the gutter after surface wetting and splash) and a filter/screen efficiency (losses at leaf screens, mesh, or pre-filters). We also optionally subtract a first-flush amount per storm to divert initial dirty water. If you enter a tank size here, we use a simple per-event cap (not a full time-series model) to show how storage might limit captures in practice.

Typical Inputs and Sensible Ranges

• Roof area: Plan-view area (shadow on the ground), not the sloped surface. Adjust if you have multiple downpipes.
• Runoff coefficient (RC): Metal/pitched roofs often 0.90–0.95; tile/asphalt shingles 0.80–0.90; rough or green roofs can be lower. Everything is editable.
• Filter efficiency: Basic leaf screens may be ~0.80–0.90; finer filters can reduce flow slightly more. Use your system’s spec if available.
• First-flush volume: A small diversion per storm helps remove roof dust, pollen, or bird droppings. Common values range from a few litres to a bucketful per downpipe—set what matches your hardware.
• Events per year: If unknown, start with a round number (e.g., 20) and refine later. Monthly mode lets you reflect wet and dry seasons.
• Units: In metric, 1 mm of rain on 1 m² yields ~1 litre before losses. In imperial, 1 inch on 1 ft² ≈ 0.623 gallons.

Worked Example (Awareness-Level)

Suppose a 100 m² roof in a 800 mm/year climate with RC 0.9, filter 0.85, first-flush 50 L per event, and 20 events per year:
Raw: 800 mm × 100 m² ≈ 80,000 L.
After RC & filter: 80,000 × 0.9 × 0.85 ≈ 61,200 L.
First-flush (50 L × 20) subtracts 1,000 L → ≈ 60,200 L potential.
If your tank can only accept, say, 500 L per event after losses, the per-event cap would yield ~10,000 L (500 × 20). This illustrates how storage sizing and storm frequency both matter.

What This Tool Does (and Doesn’t)

• Does: Convert your rainfall and roof size into potential collection with transparent losses; let you compare scenarios and sensitivities quickly.
• Doesn’t: Simulate day-by-day weather, tank drawdown, overflow routing, or pressure/flow. For design approvals, consult local codes and a professional.

Good Uses for Harvested Rain

Garden and landscape watering, toilet flushing, and tool washing are common. If you plan to use water for laundry or potable purposes, you’ll need appropriate treatment, backflow protection, and to follow local regulations. Always label outlets and keep roof/filters maintained.

Tips for Better Yield

• Keep gutters and screens clear; small blockages can reduce effective efficiency.
• Match tank size to both seasonality and demand. A slightly larger tank can bridge dry spells.
• If your area has a defined dry season, switch to Monthly mode and adjust rainfall and events per month.
• Consider capturing from multiple downpipes and routing to a shared tank to smooth uneven roof contributions.

Reminder: These are planning-level estimates. Real-world performance depends on specific roof materials, slope, gutter layout, storm timing, first-flush hardware, filtration stages, and how often you draw water from the tank. When in doubt, add safety margins.