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

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

Percolation Tanks & Ponds in India: Recharging Groundwater at Plot, Farm and Watershed Scale

A professional guide to percolation tanks — impounding monsoon runoff in a shallow surface basin so it slowly infiltrates and recharges the aquifer. How they work, where they suit, siting, indicative sizing, silt management and how they relate to check dams.

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