# Rainwater Harvesting Calculator

URL: https://speedycalculator.com/calculators/ecology/rainwater-harvesting/
Last updated: 2026-10-25

> The rainwater harvesting calculator estimates how much rain you can collect from a roof from its area, the rainfall and a runoff coefficient for the roof material.

## Inputs

- Roof area (footprint) (m², ft², cm², in², yd², acre, ha)
- Rainfall (mm, in)
- Roof type

## Results

- Water collected

## Formula

```
Volume (L) = roof area (m²) × rainfall (mm) × runoff coefficient
```

## Explanation

One millimetre of rain on one square metre of roof equals one litre of water, before losses.

### How to use the Rainwater Harvesting Calculator

- **Roof area (footprint)**
- **Rainfall**
- **Roof type**

Results update as you type, and every field accepts the units shown in its menu.

### How it works

Multiply roof area by rainfall depth and by the share of rain that actually reaches the gutter.

### Inputs explained

FieldWhat to enterRoof area (footprint)Units: m², ft², cm², in², yd², acre …RainfallUnits: mm, inRoof typeChoose one: Metal (0.9), Tiles / shingles (0.85), Concrete / flat (0.8), Green roof (0.4)

### Example values

How the main result changes when **Roof area** changes and everything else stays at the example values above:

Roof area (footprint) (m²)Water collected (L)5025,5007538,25010051,00012563,75015076,500200102,000

## Worked example

Inputs — Roof area (footprint): 100 m²; Rainfall: 600 mm; Roof type: Tiles / shingles (0.85).

Results — **Water collected = 51,000 L**.

## FAQ

**Should I use roof surface or footprint area?**
Footprint (plan) area; the slope does not change how much rain falls on it.

**Is rainwater drinkable?**
Not without proper treatment; it is fine for gardens and toilets.

**What formula does the Rainwater Harvesting Calculator use?**
Volume (L) = roof area (m²) × rainfall (mm) × runoff coefficient.