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Rainwater collection calculator

How much water can you catch from a roof, shed or tarp in a rain shower?

How it is worked out

One millimetre of rain falling on one square metre is exactly one litre of water. In US units, an inch of rain on 1,000 square feet is about 623 gallons. Use the flat (plan) area the roof covers, not the sloping surface. Some water is always lost to splashing, evaporation and the first, dirty flush off the roof, so set the share you capture to suit your gutters — 80% is a reasonable starting point for a good gutter system.

Is rainwater safe to drink?

Not straight off a roof. It can pick up bird droppings, leaves, moss and chemicals from roofing. Let the first few minutes of rain wash the roof, collect into clean containers, and filter and boil or disinfect it before drinking. For washing and flushing, it can be used as it is.

For planning and general information. In an emergency, follow the instructions of your local authorities and call your emergency number (911, 112, 999 or 000).

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Source: arithmetic (1 mm × 1 m² = 1 litre); water safety: US EPA, Emergency Disinfection of Drinking Water.