Rainwater Harvesting Calculator: Litres from Your Roof

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Formula at a glance

Rainwater Harvesting Calculator: Water harvested (litres) — Area (m²) × rainfall (mm) × runoff coefficient × (1 − losses). Example: 1,000 sq ft RCC roof in Delhi → Rainwater you can harvest: 55,009 litres a year — How the Rainwater Harvesting Calculator works: Water harvested (litres) — Area (m²) × rainfall (mm) × runoff coefficient × (1 − losses)

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📘 About the Rainwater Harvesting Calculator

Rainwater harvesting either stores roof water in a tank or sends it down a recharge pit or borewell to refill groundwater. In a city like Delhi with about 775 mm of rain a year, a 1,000 sq ft concrete roof collects around 55,000 litres — enough for weeks of a family's water.

Choose your city (or enter your own rainfall), the roof area and surface. The calculator applies the runoff coefficient and losses, and suggests a storage tank and recharge-pit size.

🧮 Formula

Water harvested (litres)

Area (m²) × rainfall (mm) × runoff coefficient × (1 − losses)

Where

Runoff coefficient = share of rain that runs off: RCC 0.85, metal 0.9, tiles 0.75, garden 0.2

✏️ Worked examples

1,000 sq ft RCC roof in Delhi

You enter
Roof / catchment area	1000
Area in	square feet
City (average rainfall)	Delhi / NCR (774 mm)
Roof surface	Concrete (RCC) roof
Losses (first flush, filter, spills)	10%
People in the home	4
Water each person uses a day	135 litres

You get
Rainwater you can harvest	55,009 litres a year
That is	55 cubic metres (5 water tankers of 12,000 L)
Rainfall used	774 mm a year (Delhi / NCR)
In the wettest month (about 30% of the year)	16,503 litres
Storage tank to aim for	3,000 litres (or send the rest to a recharge pit)
Covers your household's use for	102 days (4 people at 135 litres each a day)
Recharge pit size (approx.)	13.8 m³ — for example 2.6 m × 2.6 m × 2 m deep

55,009 litres a year.

2,000 sq ft roof in Bengaluru

You enter
Roof / catchment area	2000
Area in	square feet
City (average rainfall)	Bengaluru (970 mm)
Roof surface	Concrete (RCC) roof
Losses (first flush, filter, spills)	10%
People in the home	5
Water each person uses a day	135 litres

You get
Rainwater you can harvest	137,878 litres a year
That is	137.9 cubic metres (11 water tankers of 12,000 L)
Rainfall used	970 mm a year (Bengaluru)
In the wettest month (about 30% of the year)	41,363 litres
Storage tank to aim for	7,000 litres (or send the rest to a recharge pit)
Covers your household's use for	204 days (5 people at 135 litres each a day)
Recharge pit size (approx.)	34.5 m³ — for example 4.2 m × 4.2 m × 2 m deep

137,878 litres a year.

🧭 How to use this calculator

What you enter

Roof / catchment area
Area in
City (average rainfall)
Or your yearly rainfall (optional) (mm)
Roof surface
Losses (first flush, filter, spills) (%)
People in the home
Water each person uses a day (litres)

What you get

Rainwater you can harvest
That is
Rainfall used
In the wettest month (about 30% of the year)
Storage tank to aim for
Covers your household's use for
Recharge pit size (approx.)

💡 Simple system

Gutters and downpipes from the roof.
A first-flush diverter to waste the first dirty rain.
A filter (sand, gravel, charcoal or mesh).
A tank, a recharge pit or an old borewell for the overflow.

❓ Frequently asked questions

How much rainwater can I collect from 1,000 sq ft?

About 93 m² × rainfall. In Mumbai (2,200 mm) roughly 1.5 lakh litres a year; in Delhi about 55,000 litres; in Jaipur about 43,000.

Is rainwater harvesting compulsory?

Many cities require it for plots above a set size, including Delhi (100 m² and above), Bengaluru, Chennai and Hyderabad. Rules and rebates vary by city.

Is harvested rainwater safe to drink?

Roof rainwater needs filtering and disinfection before drinking. It is ideal for gardening, washing, flushing and recharging groundwater.

🌧️ Rainwater Harvesting Calculator: Litres from Your Roof