How does the rainwater harvesting calculator work?

It multiplies your roof or catchment area in square metres by your local annual rainfall in millimetres and by a runoff coefficient. Because one millimetre of rain falling on one square metre equals one litre, this gives the litres your roof can catch in a year. That figure is then divided by 1000 to show the potential in kilolitres.

What inputs do I need and what values should I use?

You need three things: the plan area of the roof that drains to your collection point, your city's long-term average annual rainfall in millimetres, and a runoff coefficient. Look up your local rainfall from IMD data — roughly 550 mm in Jaipur, 900 mm in Delhi, 1200 mm in Bengaluru, 2200 mm in Mumbai. For smooth RCC or metal roofs a coefficient of about 0.80 to 0.90 is sensible; rougher or sloped surfaces are lower.

How accurate is the result and what should I verify?

The number is the catchable potential for concept planning, not a guarantee. Actual usable water is lower because it depends on tank storage size, your demand pattern, first-flush diversion and filtration losses. Rainfall also varies year to year, so a dry year yields far less. Confirm your local rainfall, and design the full system with a qualified consultant per NBC 2016 Part 9 and any local municipal rules before you build or buy tanks.

Interactive Calculator · 2026

Rainwater Harvesting Calculator

Estimate how much rain your roof can harvest in a year. Enter the catchment area, your local annual rainfall and a runoff coefficient — get the annual catchable potential in kilolitres and litres.

Annual harvesting potential (at 0.85 coefficient)0.00 kL≈ 0 litres a year · ≈ 85 L per mm of rain

Annual harvest at your roof & coefficient across a few rainfall figures

Your catchment

Roof / catchment area (m²)

Plan (horizontal projected) area of the roof or paved surface that drains to your collection point.

Annual rainfall (mm)900 mm

Look up your city's long-term average — roughly 550 mm (Jaipur), 900 mm (Delhi/Hyderabad), 1200 mm (Bengaluru), 2200 mm (Mumbai). Verify your local figure.

Runoff / collection coefficient0.85

Fraction of rain you actually capture after losses to first-flush diversion, evaporation and spillage. Smooth RCC / metal roofs ≈ 0.80–0.90; rougher or sloped surfaces are lower.

Annual harvest

0.00 kL

Annual harvest

0 L

Litres per mm of rain

0 L

Yield across rainfall figures

Harvest scales linearly with rainfall — the same roof collects far more in a wet year than a dry one.

At your 100 m² catchment and a 0.85 coefficient, your local rainfall of 900 mm yields about 76.5 kL a year.

This is the catchable potential — how much water actually lands on the roof and reaches your intake. How much you can store and reuse depends on tank size; work that out with the rainwater tank sizer before you buy a tank.

How this is calculated

Annual harvest (litres) = area × rainfall × coefficient = 100 m² × 900 mm × 0.85 = 76,500 L. One millimetre of rain over one square metre is exactly one litre.
Annual harvest (kilolitres) = litres ÷ 1000 = 76,500 ÷ 1000 = 76.5 kL.
Litres per mm of rain = area × coefficient = 100 × 0.85 = 85 L — a quick way to read any single storm off a rain gauge.

Indicative sizing for concept planning. Actual usable water depends on storage, demand pattern and filtration — confirm with a qualified consultant and NBC 2016 Part 9 before procurement.

Understand the number you just got

Rooftop Rainwater Harvesting in IndiaHow a rooftop system is put together — catchment, first-flush, filtration and storage.The Rainwater Harvesting GuideMethods, recharge vs storage, regulation and cost for Indian homes.

Frequently asked questions

How does the rainwater harvesting calculator work?: It multiplies your roof or catchment area in square metres by your local annual rainfall in millimetres and by a runoff coefficient. Because one millimetre of rain falling on one square metre equals one litre, this gives the litres your roof can catch in a year. That figure is then divided by 1000 to show the potential in kilolitres.
What inputs do I need and what values should I use?: You need three things: the plan area of the roof that drains to your collection point, your city's long-term average annual rainfall in millimetres, and a runoff coefficient. Look up your local rainfall from IMD data — roughly 550 mm in Jaipur, 900 mm in Delhi, 1200 mm in Bengaluru, 2200 mm in Mumbai. For smooth RCC or metal roofs a coefficient of about 0.80 to 0.90 is sensible; rougher or sloped surfaces are lower.
How accurate is the result and what should I verify?: The number is the catchable potential for concept planning, not a guarantee. Actual usable water is lower because it depends on tank storage size, your demand pattern, first-flush diversion and filtration losses. Rainfall also varies year to year, so a dry year yields far less. Confirm your local rainfall, and design the full system with a qualified consultant per NBC 2016 Part 9 and any local municipal rules before you build or buy tanks.

Related Guides — Deep-dive reading

Rooftop Rainwater Harvesting in India: Catching Rain Off Your Roof for Storage or Recharge

A homeowner's guide to rooftop rainwater harvesting — using the roof as a catchment, sizing gutters and downpipes for your area and rainfall, first-flush and filtration, and where the water finally goes: a storage sump for use or a recharge structure for the ground.

Plumbing

Rooftop Rainwater Harvesting

The rain that lands on your roof is a free water supply and, in most Indian cities, a legal duty. What a rooftop rainwater harvesting system is, the two paths (store it for reuse or recharge the groundwater), the parts that make it work, how much a roof can actually collect, and the maintenance that keeps it honest. Plain language, India-grounded.

Roofing

Rainwater Harvesting Guide for India: Catchment, Storage, Recharge & Bye-laws

The section pillar for rainwater harvesting — why it matters for water security and groundwater, the full catchment-to-store-or-recharge chain, the two paths of using rain versus recharging aquifers, indicative yield and sizing, first-flush and filtration, maintenance, and the mandatory-RWH reality in Indian cities.

Plumbing

Related Tools — You may also find these useful

First Flush Diverter Calculator

Size a rainwater first-flush diverter — the initial dirty roof-wash to divert before storage — from roof area and divert depth.

Plumbing Calculator

Recharge Pit Size Calculator

Size a rainwater recharge pit from catchment area, rainfall intensity and detention time, with a suggested footprint.

Plumbing Calculator

Drain Slope Calculator

Work out the fall a drain pipe needs from its length and slope ratio, in mm per metre and total drop.

Plumbing Calculator