How to Use the Rainwater Harvesting for Greywater Irrigation System Calculator

A Rainwater Harvesting for Greywater Irrigation System Calculator helps estimate how much rainwater you can collect monthly and annually from your roof, which can be redirected for non-potable uses like garden irrigation, flushing toilets, or washing clothes.

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What It Does

This calculator takes your roof catchment area, the average monthly rainfall in your region, and your system’s efficiency, then calculates:

• How much rainwater can be harvested in a typical month
• Your total annual water collection potential

This gives you insight into how self-sufficient your household irrigation system can be when using harvested rainwater.

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How to Use It

1. Enter your Roof Area in square meters (m²).
   • You can find this from a building plan or by measuring the length × width of your roof.
2. Input the Average Monthly Rainfall in millimeters (mm).
   • Check your local weather service for this data.
3. Input your System Efficiency as a percentage.
   • This accounts for losses due to evaporation, splashing, or imperfect filtration (usually 70%–90%).
4. Click “Calculate Collected Water”.

The results will show you how many liters of water you can expect to collect each month and year.

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Rainwater Harvesting FAQ

Q1: What is system efficiency?
A: It represents the percentage of rainwater captured and stored after accounting for losses. For example, a 90% efficient system loses about 10% to splashing, leaks, or overflow.

Q2: Can I use this calculator for flat roofs or sloped roofs?
A: Yes, as long as you use the correct horizontal area in square meters. Roof shape does not affect the math if you measure area correctly.

Q3: How accurate is this calculator?
A: It gives a useful estimate based on average rainfall and roof size. Real-world output may vary slightly due to seasonal rain patterns or system maintenance.

Q4: What can harvested rainwater be used for?
A: Common greywater uses include:

• Irrigating plants/lawns
• Toilet flushing
• Washing cars or outdoor surfaces
• Laundry (with filtration)

Q5: Does this include greywater from inside the house (like showers)?
A: No. This calculator only estimates rainwater collection. Greywater from indoor sources like sinks and showers is a separate system.