Calculadora de Precipitação
- Created by
- Renato Passos, Eng. de Software
- Reviewed by
- Renato Passos, Eng. de Software
Last updated: Apr 18, 2026
Formula
L = mm × m² (1 mm = 1 L/m²)
About this calculator
The Precipitation Calculator converts rainfall data into collected water volume. You input precipitation in millimeters (mm) and catchment area in square meters (m²). The result is the total volume in liters, calculated by the formula L = mm × m², where 1 mm of rain equals 1 liter per square meter.
This tool is useful for estimating rainwater volume on roofs, cisterns, gardens, or agricultural areas. For example, if 30 mm of rain falls on a 100 m² roof, it collects 3,000 liters. The calculation is simple and fast, without complex conversions.
Use the calculator whenever you need to size rainwater harvesting systems, plan irrigation, or assess rainfall impact on construction. Remember that precipitation is measured in millimeters, corresponding to the height of water accumulated on a flat, impermeable surface.
Caution: the formula assumes all rainwater is collected without losses from evaporation, leaks, or absorption. For real systems, consider a runoff coefficient (typically 0.8 to 0.9 for roofs). Also, the area should be the horizontal projection of the catchment surface.
Frequently asked questions
How do I convert mm of rain to liters per square meter?
1 mm of rain equals 1 liter per square meter. Multiply precipitation in mm by area in m² to get total volume in liters.
Do I need to consider losses in the calculation?
Yes, for real systems, use a runoff coefficient (between 0.8 and 0.9) to account for losses from evaporation, leaks, or absorption.
Does the calculator work for any area?
Yes, as long as the area is the horizontal projection of the catchment surface, such as roofs or slabs.
What is the difference between precipitation and volume?
Precipitation is the height of the water layer (in mm). Volume is the total amount of water (in liters) collected over a specific area.
Can I use it to calculate water in cisterns?
Yes, if you know the catchment area and precipitation. The calculator gives the potential volume, but ignore losses and cistern capacity.