Rainwater Harvesting Calculator
Home & ConstructionHow much rain a roof can collect, after runoff losses and first flush, with storage sizing.
Runs entirely in your browser — nothing is uploaded
The yield is one multiplication — a millimetre of rain on a square metre is a litre — but two adjustments decide whether the number is honest. Not all of it runs off: a smooth roof gives up most, a planted surface almost none. And the first few millimetres of every storm carry the dust and leaves off the roof and have to be discarded, which is a real loss and the reason unfiltered systems turn foul.
What people do next
Features
- Runoff coefficients for seven surfaces, from metal sheet to lawn.
- First flush discarded per rain event, not once a year.
- Annual rainfall for twenty Indian cities, all editable.
- Storage sized to a dry spell rather than the annual yield.
- Demand coverage, tanker-load equivalent and recharge pit size.
- Runs entirely in your browser — nothing is uploaded.
How to use the Rainwater Harvesting Calculator
- 1Enter the roof area and choose the surface.
- 2Pick your city, or type in your own rainfall figure.
- 3Set the daily demand you want to meet.
- 4Read the harvestable yield and the storage it justifies.
Frequently asked questions
How much rainwater can I harvest from my roof?
Area in square metres × annual rainfall in millimetres × the runoff coefficient, and the answer is in litres — because a millimetre of rain on a square metre is exactly one litre. A 120 m² RCC roof in Bengaluru, at 970 mm a year and a coefficient of 0.85, yields about 99,000 litres gross before first-flush losses. That is roughly twenty tanker loads.
What is a runoff coefficient?
The proportion of rain landing on a surface that can actually be collected. Nothing collects all of it — some evaporates off a warm roof, some wets the surface and never runs, some is lost at the gutter. A metal sheet roof gives about 0.9, RCC about 0.85, tiles 0.75, a paved yard 0.7, and a lawn only 0.15.
What is first flush and why discard it?
The opening few millimetres of each storm, which wash off the dust, leaves, droppings and grit that have accumulated on the roof since the last rain. Diverting them keeps the stored water usable; systems without a first-flush diverter turn foul quickly and need far more filtration downstream. It is a real loss — two millimetres over sixty rain events on a 100 m² roof is 12,000 litres a year.
What size storage tank do I need?
Size it to the dry spell you want to cover, not to the annual yield. At 400 litres a day and a fifteen day gap between rains, that is 6,000 litres. A tank big enough to hold a whole year of rain would cost more than the water it saves and stand empty most of the time — surplus is better sent to a recharge pit than stored.
Is harvested rainwater safe to drink?
Not without treatment. Roof runoff picks up dust, bird droppings, and whatever the roof itself sheds, so it needs filtration and disinfection before drinking. For flushing, washing, gardening and construction it is fine with basic filtration, and those uses are most of a household's consumption anyway.