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
| Field | What to enter |
|---|---|
| Roof area (footprint) | Units: m², ft², cm², in², yd², acre … |
| Rainfall | Units: mm, in |
| Roof type | Choose 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) |
|---|---|
| 50 | 25,500 |
| 75 | 38,250 |
| 100 | 51,000 |
| 125 | 63,750 |
| 150 | 76,500 |
| 200 | 102,000 |
Formula
Volume (L) = roof area (m²) × rainfall (mm) × runoff coefficientWorked example
Inputs — Roof area (footprint): 100 m²; Rainfall: 600 mm; Roof type: Tiles / shingles (0.85).
Results — Water collected = 51,000 L.
Frequently asked questions
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.