Rainwater harvesting calculator
Estimate litres and gallons captured from a roof for a storm and a year, plus how many barrels you need.
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Tool guide
Rainwater harvesting calculator: how it works
Practical information to understand the result and use this tool correctly.
About Rainwater harvesting calculator
One millimetre of rain on one square metre is one litre. Collected volume is roof area times rainfall depth times a runoff coefficient, then minus an optional first-flush diversion. The gutter calculator orders sections and hangers; the roof calculator orders shingles. This one sizes the tank.
A metal shed of 18 m² in a 20 mm storm at 0.90 runoff captures 324 litres before first flush — about two 200 L barrels if you round up. The same roof at 800 mm a year is about 12,960 litres, a planning figure for irrigation, not a weather forecast. Tile and asphalt roofs lose more to absorption and splash, so their presets use 0.75–0.80.
Storm volume answers “will this barrel overflow on Saturday?” Annual volume answers “how much can the roof supply over a year?” If you enter litres used per day, the tool divides the storm capture by that rate and shows days covered. Gallons are US gallons (litres × 0.264172). Barrel count rounds up so the tank is not short.
Coefficients are planning values: metal near 0.90, asphalt shingle near 0.80, clay tile near 0.75. First flush is a fixed litre diversion for the dirty start of a storm, not a filter model. Nothing is uploaded. Empty values, areas outside 0.5–5,000 m², rainfall outside 0.1–4,000 mm, and coefficients outside 0.30–1 are rejected.
How to use it
- 1Pick a roof preset or enter the catchment area in square metres.
- 2Set storm rainfall and annual rainfall in millimetres, plus the runoff coefficient.
- 3Optional: first-flush litres, barrel size, and daily irrigation use.
- 4Read storm and annual litres, gallons, barrels, and days covered, then copy the estimate.
Frequently asked questions
- How many litres does 1 mm of rain collect?
- One millimetre over one square metre is one litre before losses. Multiply area (m²) by rainfall (mm) by the runoff coefficient, then subtract first flush.
- What runoff coefficient should I use?
- Metal is often about 0.90, asphalt shingles about 0.80, and clay tile about 0.75. Use the supplier or local guide if you have one. The tool does not model wind-driven loss.
- Is annual rainfall the same as a design storm?
- No. Storm millimetres size the barrel for one event. Annual millimetres estimate a year of supply. A 25 mm storm and 650 mm a year are separate inputs.
- Does first flush change the barrel count?
- Yes. First flush is subtracted from the storm volume before barrels are rounded up. It is a fixed litre allowance, not a timed diverter.
- Does the calculation leave the browser?
- No. Area, rainfall, and tank size stay on your device. The result is a planning estimate, not a plumbing design or a forecast.