Rainwater Harvesting Calculator & Litres & Tank Size
Estimates collection from concrete roofs
Find how many litres of rainwater you can collect from a roof or catchment — from its area, your annual rainfall and the surface runoff — with a suggested tank size, days of supply and the money it saves.
A 100 m² concrete / rcc roof can capture about 85,000 litres (85 m³) a year at 1,000 mm of rainfall — roughly 7,083 L a month on average.
Next: a storage tank of about 8,500 L suits this catchment. Add a first-flush diverter and a leaf filter at the inlet, and divert any overflow to a recharge pit so it tops up groundwater.
Estimate based on average annual rainfall — actual yield depends on rain distribution, evaporation and first-flush losses.
Rainwater harvesting — key facts
- Core formula
- Area (m²) × rainfall (mm) × runoff
- 1 mm on 1 m²
- = 1 litre
- Metal roof
- runoff ≈ 0.90
- Concrete roof
- runoff ≈ 0.85
- Tiled roof
- runoff ≈ 0.75
- Per m² yield
- ≈ 0.8 L per mm of rain
- First flush
- discard the dirty first rain
- Privacy
- Runs in your browser; nothing uploaded
How the harvest is worked out
The yield comes from one reliable equation: catchment area × annual rainfall × runoff coefficient. Since a millimetre of rain on a square metre is exactly one litre, the maths is simple — the only judgement is the runoff coefficient, which captures how much rain a given surface actually delivers to the tank after wetting and splash losses. This tool carries tested coefficients for common roof and ground surfaces so you don't have to guess.
From the yearly total it also estimates a practical tank size, the days of household demand the harvest covers (when you add people and per-person use), and the money saved against your water price. Because rain is seasonal, storage and a recharge pit for overflow turn a once-a-year number into water you can actually use through dry spells.
Size your harvest
Enter roof area and local rainfall to see litres and cubic metres you can capture each year and month.
Pick the right tank
Get a suggested storage capacity for your catchment, so you neither overflow nor run dry too soon.
Days of supply
Add household size and daily use to see how many days the harvested water can cover.
Money & groundwater
Estimate the yearly saving versus piped/tanker water, and divert overflow to recharge the aquifer.
Frequently Asked Questions
How do you calculate rainwater harvesting potential?+
Multiply the catchment area (m²) by the annual rainfall (mm) by the runoff coefficient of the surface. Because 1 mm of rain on 1 m² equals 1 litre, a 100 m² concrete roof (coefficient 0.85) in a 1000 mm rainfall area collects 100 × 1000 × 0.85 = 85,000 litres a year. This tool does it for any area, rainfall and surface.
What is a runoff coefficient?+
It's the fraction of rain falling on a surface that actually reaches your tank, after losses to wetting, splashing and evaporation. Metal roofs are highest (~0.9), concrete ~0.85, tiled ~0.75, paved ~0.8, and bare ground much lower (~0.3). The tool applies the right coefficient for the surface you pick.
How much rainwater can I collect from my roof?+
It depends on roof area and local rainfall. As a rule of thumb, every 1 m² of roof yields about 0.8 litres per millimetre of rain. So a 120 m² roof in a place with 800 mm of rain collects roughly 80,000 litres a year — enter your own figures to get an exact number.
What size rainwater tank do I need?+
Tank size depends on rainfall pattern and how long you must bridge between rains. A common starting point is around one month of average harvest; the tool suggests a sensible tank capacity (rounded to 500 L) for your catchment. In monsoon climates with long dry spells you may want more.
Where do I get my annual rainfall figure?+
Use the long-term average annual rainfall for your town or district from a local meteorological department, weather service or almanac. For planning, the multi-year average is more reliable than a single year.
How many days of water will it cover?+
Add your household size and litres-per-person-per-day (135 L is a common domestic figure) and the tool divides the yearly harvest by daily demand to estimate the days it can cover. Remember rain isn't evenly spread, so storage and timing matter.
What is a first-flush diverter?+
It's a device that discards the first few litres of each rain — which carry dust, droppings and debris washed off the roof — before water enters the tank. It noticeably improves stored-water quality and is recommended on every system.
Is harvested rainwater safe to drink?+
Rooftop rainwater is generally clean but not automatically potable. With a first-flush diverter, fine filtration and disinfection (boiling, UV or chlorination) it can be made drinkable; without treatment, use it for irrigation, washing, flushing and livestock.
Does harvesting help recharge groundwater?+
Yes — directing overflow or runoff into a recharge pit, trench or well lets water percolate down and replenish the aquifer, which raises nearby water tables and improves borewell yields over time. It's a key benefit alongside direct storage.
How accurate is this estimate?+
It's a solid planning estimate from the standard area × rainfall × coefficient method. Real yield varies with how rain is distributed through the year, evaporation, first-flush losses and overflow when the tank is full, so size storage with a margin.