Enter your roof area, local annual rainfall, and current water rate to instantly see how many litres you can collect and how much money rainwater harvesting saves you per year.
Enter your roof size, rainfall data, and water tariff
Catchment & Rainfall Inputs
Roof / Catchment Area
Annual Rainfall
Water Rate & System
Runoff Coefficient
0–1
Water Rate
System Cost (optional)
USD
Rate Is Per
Share You Actually Use
%
⚠ Please enter valid values for Roof Area, Annual Rainfall, Runoff Coefficient (0.1–1.0), and Water Rate.
Your Savings Results
Annual Rainwater Collected
—
Litres per Year
Kilolitres / Year
— kL
US Gallons / Year
— gal
Cubic Metres / Year
— m³
Monthly Collection
— L/mo
Annual $ Savings
—
Monthly $ Savings
—
10-Year Savings
—
Payback Period
— yrs
CO₂ Offset Est.
— kg/yr
Collection Breakdown
📖 How To Use
How to Use This Rainwater Savings Calculator
It takes less than a minute to see your full annual rainwater harvesting potential:
Enter your roof or catchment area
Measure the horizontal footprint of your roof — not the sloped surface. A 40 ft × 30 ft house has a 1,200 ft² (111 m²) catchment regardless of roof pitch. Choose ft² or m² to match your tape measure.
Enter your local annual rainfall
Find your city's average annual rainfall from a weather authority or BOM/NOAA website. Enter in mm, cm, or inches — whatever your source uses. Global averages range from under 200 mm (arid desert) to over 3,000 mm (tropical rainforest).
Set the runoff coefficient
This accounts for water lost to evaporation, absorption, and first-flush diversion. Use 0.85–0.90 for metal/tile roofs, 0.75–0.85 for asphalt shingles, and 0.60–0.70 for gravel or green roofs. If unsure, 0.80 is a safe default.
Enter your water tariff
Enter your water price and pick the unit your bill uses: per 1,000 US gallons (most US bills), per 100 cubic feet (CCF or HCF, about 748 gallons), per US gallon, or per kilolitre / m³. If sewer charges are based on water use, add them too — but outdoor water often isn't charged for sewer.
Set the share you actually use
Savings only come from rainwater that replaces water you would otherwise buy. If your tank overflows in wet months or sits full in winter, that water saves nothing — lower this percentage to match what you really use.
Enter system cost (optional)
If you know your installation cost for tanks, pumps, and guttering, enter it here to get a payback period estimate. Leave blank or zero to skip.
Tip: Only count roof area that drains into your collection system. If one side of your roof drains to the street, use only the area that feeds your gutters and downpipes.
📐 The Formula
Rainwater Savings Formula Explained
The calculation uses two chained equations — one for collection volume, one for financial savings:
Annual Collection (L) = Roof Area (m²) × Annual Rainfall (mm) × Runoff Coefficient
Annual Savings = (Collection ÷ 1,000) × Share Used × Water Rate per kL
US units: Collection (gal) = Area (sq ft) × Rainfall (in) × 0.623 × C; Savings = gal ÷ 1,000 × Share Used × Rate per 1,000 gal
Payback Period (yrs) = System Cost ÷ Annual Savings
The roof area × rainfall gives the theoretical maximum rainfall volume in litres (1 mm of rain on 1 m² = exactly 1 litre). The runoff coefficient scales this down to account for real-world losses.
Runoff Coefficient Reference
Roof / Surface Type
Runoff Coefficient
Notes
Colorbond / Metal
0.90–0.95
Very high efficiency, minimal absorption
Clay / Concrete Tile
0.85–0.90
Good efficiency when wet
Asphalt Shingles
0.75–0.85
Some absorption, especially when dry
Slate
0.85–0.90
Similar to tile
Green / Living Roof
0.30–0.60
High absorption by growing media
Gravel Roof
0.60–0.70
Significant retention in aggregate
Paved Courtyard
0.70–0.85
Depends on surface smoothness
Annual Rainfall by Region
City
Annual Rainfall (mm)
Classification
London, UK
601
Temperate maritime
Sydney, Australia
1,213
Humid subtropical
Mumbai, India
2,167
Tropical monsoon
New York, USA
1,257 (49.5 in)
Humid continental
Dubai, UAE
94
Hot desert
Karachi, Pakistan
196
Hot arid
Cape Town, S. Africa
515
Mediterranean
São Paulo, Brazil
1,450
Humid subtropical
🏠 Use Cases
When to Use This Rainwater Savings Calculator
🏡
Residential Households
Check whether a domestic rainwater tank pays off before committing. Payback depends heavily on your water price: where water is cheap, as in much of the US, water savings alone can take decades to repay a large system; where water is expensive or delivered by tanker, it can be a few years.
🏗️
New Builds & Renovations
Size a tank correctly from the start. Knowing annual collection lets you pick a tank that won't overflow in wet months or run dry in summer.
🌾
Agricultural & Irrigation
Estimate how much irrigation water a farm shed or greenhouse catchment can supply before you invest in bores or mains water connections.
🏢
Commercial & Industrial
Large roof areas make commercial rainwater harvesting highly attractive. A 2,000 m² warehouse in a 700 mm rainfall zone collects 1.19 million litres annually.
📊
Green Building Assessment
Quantify water savings for LEED, BREEAM, or Green Star certification submissions. The annual savings figure is exactly what these schemes require.
🌍
Water Scarcity Planning
In water-stressed regions, knowing collection potential helps plan supplementary storage capacity before dry seasons hit.
🧮 Worked Example
Worked example: a 1,500 sq ft roof on a US water bill
A 1,500 sq ft roof (coefficient 0.85) gets 40 in of rain a year. The household pays $9.00 per 1,000 gallons for water and reckons it will use about 70% of what it collects (the rest overflows in wet spells). The system cost $3,000.
Collected: 1,500 × 40 × 0.623 × 0.85 ≈ 31,790 gallons a year (about 120,300 L).
Used: 70% ≈ 22,250 gallons.
Saving: 22.25 × $9.00 ≈ $200 a year.
Payback: $3,000 ÷ $200 ≈ 15 years — before pump electricity and maintenance. Rebates, higher water rates or replacing trucked water shorten it.
❓ FAQ
Frequently Asked Questions
How do I calculate rainwater savings from my roof?
In US units, multiply roof area (sq ft) × annual rainfall (in) × 0.623 × runoff coefficient to get gallons a year; then multiply the gallons you actually use by your water rate per gallon (rate per 1,000 gallons ÷ 1,000). In metric, area (m²) × rainfall (mm) × coefficient gives litres; divide by 1,000 and multiply by your rate per kL. This calculator does all of that automatically.
What is a runoff coefficient and what should I use?
The runoff coefficient (0 to 1.0) represents what fraction of rainfall actually reaches your tank after accounting for evaporation, absorption by the roof material, and first-flush diversion losses. Use 0.85–0.90 for metal roofs, 0.80–0.85 for tiles, 0.75–0.80 for asphalt shingles. When in doubt, 0.80 is a conservative and reasonable default for most residential roofs.
How much water can a typical house collect per year?
A typical 150 m² home in a city with 700 mm annual rainfall and a 0.85 runoff coefficient collects about 89,250 litres (89 kL) per year. That's about 23,600 US gallons, or roughly 245 litres (65 gallons) a day on average — a useful share of garden watering, toilet flushing and laundry, if the tank is big enough to carry it through dry spells. In high-rainfall cities like Mumbai or Sydney, the same roof could collect 200,000+ litres annually.
How long does a rainwater harvesting system take to pay back?
It varies enormously with your water price. At typical US rates of a few dollars to about $15 per 1,000 gallons, a system has to supply a lot of water to save even $200–400 a year, so payback from water savings alone is often 10–30 years or more. Where water is expensive, metered steeply or delivered by tanker, payback can be a few years. Rebates, stormwater fee credits and avoided well or tanker costs can change the picture. Enter your system cost into the calculator to see your personal payback period.
Does roof pitch affect how much rainwater I can collect?
No — roof pitch does not affect collection volume. You should use the horizontal footprint area (plan area), not the actual sloped roof surface. The rainfall measurement already represents vertical depth of water falling, so horizontal area × vertical rainfall gives the correct volume regardless of pitch angle.
What is the CO₂ offset calculation based on?
The estimate assumes mains water takes roughly 0.35 kWh per kilolitre to treat and deliver, at about 0.5 kg CO₂ per kWh — roughly 0.19 kg CO₂ per kilolitre replaced. Both figures vary widely by region, and a rainwater pump also uses electricity, which can cancel out the saving. Treat it as indicative only.
Can I use this calculator for rainwater harvesting ROI?
Yes. Enter your total system installation cost (tanks, pumps, filters, guttering) in the System Cost field. The calculator will show your payback period in years and 10-year cumulative savings. For a more detailed return-on-investment analysis including maintenance costs, see our Rainwater Harvesting ROI Calculator.
What size tank do I need to store the rainwater I collect?
Tank sizing depends on collection rate, consumption rate, and the dry season gap between rainfalls. A quick rule of thumb is one to two months of average collection; design guides advise sizing storage for the longest expected dry spell. Use our Rainwater Harvesting Calculator for a starting tank size.
✅ Sources & Review
Sources and review
The collection formula is standard physics; the savings depend on your own water rate. The efficiency and storage guidance comes from these references:
United States — Texas Water Development Board, The Texas Manual on Rainwater Harvesting (3rd ed.), one of the most widely used US design guides: about 0.62 gallons per sq ft per inch; 75–90% collection efficiency; storage to cover the longest interval without rain.
Australia — enHealth, Guidance on Use of Rainwater Tanks (2010): first-flush diversion of the first 20–25 litres for an average roof, tank inspection for sediment every 2–3 years, and drinking-water safety.
Europe and UK — BS EN 16941-1:2018, On-site non-potable water systems — Systems for the use of rainwater: design, sizing and installation of rainwater systems.
NOAA National Centers for Environmental Information — U.S. Climate Normals (e.g. New York Central Park about 49.5 in a year).
Unit conversions from NIST SP 811: 1,000 US gallons = 3.785 m³; 100 cu ft = 2.832 m³ = 748 gallons.
Reviewed by John Mac, PE, Water Systems Specialist
Texas-licensed Professional Engineer with 13+ years in residential water storage and 180+ tank projects. John writes and reviews every calculator on this site.
