How much water could you harvest — and how much should you store?
Whether you're planning for restrictions, outages or just want to capture the rain, The Water Vault gives you honest numbers from your own roof and household — no guesswork, no sales pitch.
Planning guide only. Estimates use the standard rainwater formula and stated assumptions; actual yield and needs vary. Harvested rainwater must be filtered and treated before drinking. Guidance last reviewed 2026-06-24.
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How much rainwater can I harvest from my roof?
Use the standard formula: litres ≈ roof area (m²) × annual rainfall (mm) × 0.8 (the runoff factor that allows for splash and first-flush losses). One millimetre of rain on one square metre of roof is about one litre of water. So a 120 m² roof in a town that gets 700 mm of rain a year could capture roughly 67,000 litres over the year — enough to make a real dent in irrigation and non-drinking use.
Work out your own above, or read the full harvesting & storage guide.
Transparent maths
Every number comes from your inputs and the standard formula — we show the assumptions and invent no brands or prices.
Plan it properly
First-flush, filtration, tank placement and water quality all matter. The guide covers them.
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