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Rainwater Tank Size Formula:
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The rainwater tank size calculation estimates the optimal storage capacity needed to capture rainfall from a roof area. It considers the roof catchment area, local rainfall patterns, and the efficiency of water collection (runoff coefficient).
The calculator uses the rainwater harvesting formula:
Where:
Explanation: The formula calculates the volume of rainwater that can be collected from a specific roof area during a rainfall event, accounting for collection efficiency losses.
Details: Correctly sizing a rainwater tank is crucial for maximizing water capture, ensuring adequate supply during dry periods, and optimizing the return on investment. Undersized tanks may not store enough water, while oversized tanks can be unnecessarily expensive.
Tips: Measure your roof's catchment area in square meters. Use local rainfall data (average monthly or annual rainfall in mm). The runoff coefficient typically ranges from 0.6-0.9 for most roofing materials (0.8 is a common default for standard roofs).
Q1: What is a runoff coefficient?
A: The runoff coefficient represents the efficiency of rainwater collection from a surface. It accounts for losses from evaporation, splash-out, and first-flush diversion. Smooth, impervious surfaces like metal roofs have higher coefficients (0.8-0.9) than porous surfaces.
Q2: Should I use average monthly or annual rainfall?
A: For tank sizing, it's best to use the average rainfall for the driest month to ensure adequate supply year-round, or use seasonal patterns based on your water usage needs.
Q3: How do I measure my roof area?
A: Calculate the footprint area of your roof (length × width) and adjust for roof pitch if necessary. For complex roofs, consider consulting a professional or using satellite imagery tools.
Q4: What factors affect runoff coefficient?
A: Roof material, slope, temperature, rainfall intensity, and the presence of debris or contaminants can all affect the runoff coefficient. Metal roofs typically have the highest coefficients.
Q5: Should I consider water usage in tank sizing?
A: Yes, for a complete system design, you should also consider your household water demand, the number of dry days between rainfall events, and whether the tank is for partial or full water supply.