Água de Chuva Captada
- Created by
- Renato Passos, Eng. de Software
- Reviewed by
- Renato Passos, Eng. de Software
Last updated: Apr 18, 2026
About this calculator
The Collected Rainwater Calculator estimates stored water volume using catchment area, precipitation in millimeters, and system efficiency. It's ideal for planning rooftop, cistern, or reservoir rainwater harvesting systems. The formula multiplies area (m²) by precipitation (converted from mm to meters) and adjusts the result by efficiency (70-90%), depending on system quality.
To calculate, input the area in square meters, local rainfall average (in mm), and efficiency percentage (e.g., 85%). Example: a 100m² roof in an area with 400mm annual rainfall and 80% efficiency stores 100 × 0.4 × 0.8 = 32m³ (32,000 liters) yearly. Efficiency should reflect real factors like roof material and pipe maintenance.
Use this tool to size cisterns, optimize urban/rural collection, or design off-grid systems. Note that climatic variations and leaks may reduce effective volume. Regions with irregular rainfall should prioritize historical local precipitation data for accurate estimates.
Important: precipitation must be in millimeters, not liters. Efficiency below 70% indicates potential system losses like leaks or blockages. Adjust parameters for specific needs, such as potable use or irrigation.
Frequently asked questions
How to convert rainfall in mm to water volume?
Multiply area in m² by rainfall in mm (divided by 1000 to convert to m³) and efficiency factor. Example: 50m² × 20mm ÷ 1000 × 0.8 = 0.8m³ (800L).
What efficiency to consider for clay tiled roofs?
Clay tiles typically have 85-90% efficiency, while concrete tiles vary between 75-85% due to higher porosity.
Does the calculator account for evaporation losses?
No calculation includes evaporation. For long-term projects, reduce calculated volume by 5-10% to compensate for real losses.
Can I use daily rainfall data instead of annual?
Yes, but multiply by the average number of rainy days yearly for more realistic estimates.
How to adapt for regions with high seasonal variations?
Use historical average rainfall and add a 20-30% safety margin to the calculated volume.