EcoTrack

Rainwater Harvesting Calculator

How much water can your roof catch?

Roof area (m²)

Rainfall (mm)

Runoff coefficient

Harvestable volume

3,187.5 L

What you entered

Roof area (m²): 150Rainfall (mm): 25Runoff coefficient: 0.85

1
Volume = roof area × rainfall × runoff coefficient
150 m² × 0.025 m × 0.85= 3187.5 L

Result

Harvestable volume: 3,187.5 L

A single rainfall of 25mm on this roof could yield about 3,188 liters of water.

Rainwater collected by roof area and rainfall

Volume is roof area times rainfall depth times a runoff coefficient. One millimetre of rain on one square metre is one litre before losses.

Roof area	Annual rainfall	Runoff coefficient	Water collected
50 m2	600 mm	0.90	27,000 L
100 m2	500 mm	0.85	42,500 L
100 m2	1,000 mm	0.85	85,000 L
150 m2	1,200 mm	0.75	135,000 L
200 m2	800 mm	0.85	136,000 L

Use the roof's plan area, the footprint seen from above, not the sloped surface area. The runoff coefficient covers evaporation, splashing, and first-flush diversion: around 0.9 for metal, 0.8 to 0.85 for tile, lower for rough or porous surfaces.

Roof area times rainfall times efficiency

The harvestable volume is roof area × rainfall depth × a runoff coefficient (typically 0.8-0.9 for a hard roof, since some water evaporates or is lost to first-flush diversion) — even a modest roof and rainfall combine to surprisingly large volumes.

Why this is worth calculating

Rainwater harvesting sizing decisions (tank size, gutter capacity) all start from this number — knowing how much a single storm event can deliver prevents both undersized tanks (wasted runoff) and oversized ones (wasted cost).

Frequently asked questions

My roof is 150 m² and we get 800 mm of rain per year — how much water can I collect?

Volume = 150 × 0.800 × 0.85 (typical runoff coefficient) = 102 m³ = 102,000 liters per year. That covers a substantial portion of a household's non-potable water needs.

What is a runoff coefficient?

It accounts for water lost to evaporation, splash, and first-flush diversion. A metal roof has a coefficient of about 0.90 (90% captured); a flat concrete roof is about 0.80; a green roof is about 0.50 due to plant absorption.

How does this relate to the water footprint calculator?

The water footprint calculator shows how much water goes into producing goods. Rainwater harvesting shows how much free water your roof can supply. Comparing the two reveals how much of your indirect water use you could offset with collected rainwater.

How big a tank do I need?

Size depends on your usage rate and dry-spell length. A common rule: tank capacity = 2-4 weeks of expected usage. If you use 500 liters/week for garden irrigation, a 1,000-2,000 liter tank handles most dry spells.