Rainwater Harvesting Calculator
Use the plan area. A steep roof collects no more.
Work out Rainwater Harvesting. Use the plan area. A steep roof collects no more. States the assumption instead of hiding it.
Collected per year
64.8 m³
64,800 litres from 100 m² of roof · 100% of demand
One millimetre of rain on one square metre is exactly one litre. That makes roof yield the cleanest arithmetic in the whole subject — and it is why the losses, not the rainfall, are where estimates go wrong. Use the plan area, not the sloped surface. Rain falls vertically, so a steep roof and a flat one covering the same footprint collect identically. Using the sloped area overstates yield by a third on a 45° pitch. Tank size is set by the dry spell, not by the annual total. A tank sized to the yearly yield is enormous and pointless; one sized to bridge the longest gap between useful rain is what actually determines how much you use.
How the Rainwater Harvesting Calculator works
Annual and monthly yield from a roof, with runoff and filter losses, peak-day overflow, coverage of your demand and payback. One millimetre on one square metre is exactly one litre.
Also known as: how much rain can i collect from my roof · rainwater tank size calculator · litres per mm of rain · does roof pitch affect rainwater collection
Frequently asked questions
How much water can I collect from my roof?
Plan area in m² × rainfall in mm × runoff coefficient × filter efficiency, in litres. A 100 m² roof at 800 mm with typical losses yields around 65,000 litres a year.
Does a steeper roof collect more rain?
No. Rain falls vertically, so collection depends on the footprint the roof covers, not its sloped surface. Using the sloped area overstates yield by a third on a 45° pitch.
What is a runoff coefficient?
The share of rain that reaches the gutter rather than evaporating or soaking in. Around 0.9 for metal or tile, 0.8 for asphalt shingle, and much lower for a green roof.
What size tank do I need?
Sized to bridge the longest dry spell, not to the annual total. A tank sized to the yearly yield is enormous and pointless; what matters is how many days of demand it covers.
Is rainwater harvesting worth it financially?
It depends entirely on your water price and system cost, which is why both are inputs. In many places the payback runs to decades, and the case is about resilience rather than money.
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