Rooftop rainwater harvesting: how much water your roof can collect

Rooftop rainwater harvesting means catching the rain that falls on your roof and either storing it for use or sending it into the ground. The Central Ground Water Board (CGWB) works out a roof's yield with a short formula, and the arithmetic fits on the back of an electricity bill.

The CGWB formula

The CGWB's Manual on Artificial Recharge of Ground Water gives the harvest from a roof as:

Water harvested (litres) = rainfall (mm) × roof area (m²) × runoff coefficient

The formula works because one millimetre of rain falling on one square metre is exactly one litre of water. Multiply the millimetres by the square metres and you have litres. The runoff coefficient then takes away the share that never reaches the downpipe: rain that splashes off, wets the surface, sits in hollows or evaporates.

Two details matter when you put in your own numbers.

• Use the plan area of the roof. That is the area seen from directly above: the length times the breadth of the footprint. A sloping tiled roof catches the same rain as a flat slab of the same footprint, so do not measure along the slope.
• Use the coefficient for your roof material. Smooth, hard surfaces shed more of the rain than rough ones.

Table 7.2 of the manual gives these runoff coefficients for roofs:

The manual's ready reckoner offers a quick check. It shows a 20 m² roof receiving 100 mm of rain giving 1.6 m³ of water, using a coefficient of 0.8. In the formula: 100 × 20 × 0.8 = 1,600 litres, which is 1.6 cubic metres.

A worked example for an Indian house

The numbers below are an example, chosen to keep the arithmetic easy. They do not describe any particular town or house.

Take a house with a flat concrete roof measuring 10 metres by 10 metres, so the plan area is 100 m². Assume the town receives 1,000 mm of rain in a year. The coefficient for concrete is 0.7.

1,000 mm × 100 m² × 0.7 = 70,000 litres a year, or 70 cubic metres.

If the same house had a GI sheet roof, the result would be 1,000 × 100 × 0.9 = 90,000 litres. The roof material alone changes the yield by 20,000 litres a year.

The yearly total hides how unevenly rain arrives. Over much of India, most of the year's rain falls in the monsoon months, often in heavy bursts. A single storm that drops 50 mm on the example roof sends 50 × 100 × 0.7 = 3,500 litres down the pipes within hours. How much of that the house keeps depends on the size of its tank or the capacity of its recharge pit, not on the roof.

For your own house, use the rainfall figure for your district. The India Meteorological Department publishes rainfall records, and the state groundwater department or a CGWB regional office can advise on local conditions.

First flush: keep the dirtiest water out

The first rain after a dry spell washes dust, dry leaves, bird droppings and soot off the roof. That water should not go into a storage tank or straight into the ground. The CGWB manual recommends a first-flush arrangement (section 7.2.5) that diverts the start of each rain away before collection begins.

In practice, households do this in a few simple ways:

• a valve or removable cap on the downpipe, opened at the start of the rain and closed once the water runs clear;
• a separate vertical pipe that fills with the first water and is emptied after the rain;
• a mesh or grating at each roof outlet to stop leaves, plus a sweep of the roof before the monsoon starts.

A filter of sand and gravel between the roof and a recharge structure adds a second line of protection, especially where the water will go down into an aquifer that also feeds borewells.

Store it or recharge the ground

There are two things to do with harvested rain, and many homes do both.

Storage puts the water in a sump or tank for later use. It suits washing, gardening and toilet flushing. Drinking it needs treatment and a water test first. A tank gives you water you can draw on, but it fills quickly in heavy rain, and once full, the rest overflows.

Recharge sends the water into the ground through a recharge pit, a trench, or a filter feeding an existing or disused borewell. You cannot draw on it directly, but it adds to the groundwater that wells and borewells in the area depend on. Recharging through a borewell carries a risk of contaminating the aquifer, so it needs a proper filter and a clean catchment. Ask the state groundwater department whether the method suits your site.

A common arrangement is to send the roof water to a tank first and lead the overflow to a recharge pit, so nothing is wasted once the tank is full.

Jal Sanchay Jan Bhagidari and Catch the Rain

Household recharge sits inside a much larger national effort. According to a PIB backgrounder from the Ministry of Jal Shakti dated 19 July 2026, Jal Sanchay Jan Bhagidari (JSJB) 2.0 was launched on 1 June 2025, with a special focus on groundwater recharge in Over-Exploited and Critical districts. Against a target of at least one crore groundwater recharge structures by 31 May 2026, more than 1.5 crore structures were reported nationwide.

The same backgrounder states that Jal Sanchay Jan Bhagidari: Catch the Rain was launched on 1 June 2026, with a focused nationwide campaign from 4 July to 4 August 2026.

A later PIB feature dated 3 September 2026 reported that, as of that date, more than 1.58 crore artificial groundwater recharge structures had been constructed.

These are counts of structures, not of water recharged. For a single house, the useful number is still the one from the formula: rainfall, times roof area, times the coefficient for the roof. The glossary has a short entry on rainwater harvesting and groundwater recharge.