Enter your roof area, annual rainfall, and runoff coefficient to calculate your harvestable rainwater yield, ideal tank size, and potential water savings — instantly.
Calculate your roof's annual rainwater collection potential in under a minute:
Measure your roof catchment area
Enter the horizontal length and width of your roof footprint — use the plan (top-down) dimensions, not the sloped surface. For example, a roof 40 ft × 30 ft (12.2 m × 9.1 m) has a 1,200 sq ft (111 m²) catchment area regardless of roof pitch.
Enter your annual rainfall
Look up your location's average annual precipitation — in the US, NOAA's climate normals give it in inches for every weather station; elsewhere, use your national weather service. Enter it in inches or millimetres. Averages range from under 4 in (100 mm) in deserts to over 120 in (3,000 mm) in the wettest tropics.
Select your roof type
The dropdown sets the runoff coefficient — the share of rain that actually reaches the tank after splash, evaporation, wetting the roof and gutter overflow. Smooth metal roofs deliver the most (0.90); rough and planted roofs the least. Rainwater design guides commonly assume 75–90% overall.
Enter your daily water usage (optional)
Inputting your household's daily water consumption lets the calculator show how many days per year your harvested rainwater covers your demand and what percentage of your annual usage it can supply.
Read your results
The primary result shows annual harvestable litres. The secondary grid shows the monthly and daily averages, a starting tank size (1.5 months of average collection), the share of your demand it could cover, and more. The note under the results gives the figures in gallons.
Tip: For multi-section roofs, calculate each section separately and add the yields together. Only connect gutters that drain toward your collection point — a section draining to the street adds nothing to your harvest.
📐 The Formula
Rainwater Harvesting Formula Explained
The standard rainwater yield formula, in metric and US units:
Where:
Roof Area = Length (m) × Width (m) [horizontal footprint]
Annual Rainfall = mm/year from local weather data
Runoff Coefficient (Cr) = 0.3 – 0.95 depending on roof surface
Note: 1 mm of rain on 1 m² of roof = 1 litre of water (before losses)
US units: Annual Yield (gal) = Roof Area (sq ft) × Rainfall (in) × 0.623 × Cr
1 inch of rain on 1 sq ft = 0.623 US gallons (1/12 ft³ × 7.48 gal/ft³)
This formula works because 1 mm of rainfall depth across 1 square metre equals exactly 1 litre. Multiplying by the runoff coefficient accounts for real-world collection losses.
Runoff Coefficients by Roof Type
Roof Material
Runoff Coefficient (Cr)
Notes
Metal / Tin / Zincalume
0.90 – 0.95
Best for harvesting; minimal absorption
Concrete / Flat Tile
0.80 – 0.90
Common residential; good efficiency
Asphalt Shingle
0.70 – 0.80
Some absorption; first-flush critical
Clay / Terracotta Tile
0.65 – 0.75
Porous surface absorbs initial rain
Green / Living Roof
0.50 – 0.65
High absorption; significant retention
Gravel / Rough Membrane
0.25 – 0.40
Significant losses; not recommended
Recommended Tank Size
As a quick starting point, this calculator suggests a tank that holds about 6 weeks of average collection:
Recommended Tank (L) = (Annual Yield ÷ 12) × 1.5
[i.e., 1.5 × average monthly collection]
That is only a first guess. Design guides advise sizing storage for the longest expected dry spell, and a common rule among US installers is about three months of demand. Where rain falls in one season, work through your monthly rainfall against your monthly demand: the tank has to carry you from the end of the wet season to the start of the next.
📏 Reference Data
Rainwater Yield by City — Example Calculations
Using a 100 m² roof with a 0.85 runoff coefficient (concrete tile):
City
Annual Rainfall (mm)
Yield (100 m², Cr=0.85)
Yield (US gal)
London, UK
601
51,085 L
13,493 gal
Sydney, AU
1,213
103,105 L
27,233 gal
Mumbai, IN
2,167
184,195 L
48,642 gal
Los Angeles, US
381
32,385 L
8,550 gal
Cape Town, ZA
515
43,775 L
11,561 gal
Karachi, PK
196
16,660 L
4,401 gal
Singapore
2,340
198,900 L
52,527 gal
Berlin, DE
570
48,450 L
12,795 gal
Note: Rainfall figures are approximate long-term averages and vary by weather station and by decade (e.g. downtown Los Angeles has averaged closer to 14 in, 360 mm, in recent normals). Actual yields vary year to year — use your nearest station's data for planning.
🏡 Use Cases
When to Use a Rainwater Harvesting Calculator
🏠
Residential Planning
Size a rooftop collection system for garden irrigation, toilet flushing or laundry. How much it cuts your water bill depends on rainfall, storage and what you use it for — outdoor watering is often the biggest saving.
🌾
Agricultural Use
Estimate how much water a farm shed or greenhouse can collect to supplement drip irrigation tanks during dry spells.
🏢
Commercial Buildings
Calculate LEED-eligible rainwater reuse potential for office buildings, schools, or industrial facilities with large roof areas.
⚡
Off-Grid Systems
Determine whether roof collection alone can meet household water needs for remote properties without mains connection.
💧
Emergency Preparedness
Model how quickly a tank refills after a storm event to ensure water security during supply disruptions.
🌍
Development Projects
NGOs and engineers use this formula to design community rainwater systems in water-scarce regions across Africa and Asia.
🧮 Worked Example
Worked example: a 1,800 sq ft metal roof with 36 inches of rain
A house with a 60 ft × 30 ft metal roof (runoff coefficient 0.90) in a place with about 36 in (914 mm) of rain a year, using about 50 gallons a day of rainwater for the garden and toilets:
Catchment: 60 × 30 = 1,800 sq ft (167 m²).
Yield: 1,800 × 36 × 0.623 × 0.90 ≈ 36,350 gallons a year (about 137,600 L) — roughly 3,030 gallons a month on average.
Demand: 50 gal × 365 = 18,250 gallons a year, so the roof could supply it all over a year — if the tank is big enough to carry water through dry spells.
Starting tank size: 1.5 × 3,030 ≈ 4,570 gallons (about 17,300 L, rounded up to the next 100 L). Check it against the longest dry spell where you live.
❓ FAQ
Frequently Asked Questions
How do I calculate how much rainwater I can collect from my roof?
In US units, multiply the roof footprint (sq ft) by annual rainfall (inches), by 0.623 gallons, and by the runoff coefficient: a 1,500 sq ft concrete-tile roof with 30 in of rain gives 1,500 × 30 × 0.623 × 0.85 ≈ 23,800 gallons a year. In metric, multiply area (m²) × rainfall (mm) × coefficient: 100 m² × 800 mm × 0.85 = 68,000 litres a year. This calculator does this math instantly.
What is a runoff coefficient and why does it matter?
The runoff coefficient (Cr) represents the proportion of rainfall that actually reaches your storage tank after losses from evaporation, splash, initial surface wetting, and first-flush diversion. A metal roof has Cr ≈ 0.90, meaning 90% of rainfall is collectible. An asphalt shingle roof has Cr ≈ 0.75. Using the wrong coefficient can over- or under-size your system by 15–30%.
What size rainwater tank do I need?
Tank size depends on your daily demand and the longest dry spell you need to bridge. Design guides advise sizing storage for the longest expected dry spell, and a common rule among US installers is about three months of demand. This calculator suggests 1.5 times your average monthly yield as a quick starting point. Use our First Flush Diverter Size Calculator for additional guidance on pre-tank filtration volumes.
How much of my roof area should I use for the calculation?
Use the horizontal footprint (plan area) of your roof, not the sloped surface area. A 10 m × 8 m house has 80 m² of catchment regardless of pitch angle — because rainfall is measured vertically. Only include roof sections whose gutters drain to your collection tank. Exclude areas that drain to the street or neighbouring properties.
Is harvested rainwater safe to drink?
Roof-collected rainwater is not automatically safe for drinking. It can pick up dust, bird droppings, atmospheric pollutants, and moss from the roof surface. For potable use, it requires first-flush diversion, filtration (sediment + carbon), and disinfection (UV or chlorination). For non-potable uses — toilet flushing, laundry, irrigation — basic sediment filtration is usually sufficient. Always comply with your local health authority's guidelines.
What is a first flush diverter and do I need one?
A first flush diverter automatically discards the first few litres of rain from each storm event — the most contaminated water that washes accumulated dust, bird droppings, and debris off the roof. A widely used US guideline is to divert one to two gallons for every 100 sq ft of roof (about 0.4–0.8 L per m²), and at least 10 gallons per 1,000 sq ft; Australia's national enHealth guidance suggests the first 20–25 litres from an average roof. Dusty or tree-covered roofs need more. This device significantly improves water quality and is considered best practice for all potable and many non-potable systems. Use our First Flush Diverter Size Calculator to size yours.
How does roof pitch affect rainwater collection?
Roof pitch does not affect the volume of water you can collect — only the horizontal footprint matters, because rainfall is measured as a vertical depth. A steep pitch actually helps by improving drainage speed and reducing standing water, which reduces evaporative loss and algae growth. Very shallow pitches (under 5°) can pool water and accumulate sediment, slightly reducing effective yield.
Can a rainwater harvesting calculator tell me my ROI?
This calculator gives you the yield data needed to calculate ROI. Take your annual yield, multiply by your local water rate (per gallon, per 1,000 gallons or per m³), and compare with the system cost. Payback varies enormously: where water is cheap, as in much of the US, a small system can take decades to pay for itself in water savings alone; where water is expensive or delivered by tanker, it can be a few years. Use our Rainwater Harvesting ROI Calculator for a full financial analysis.
✅ Sources & Review
Sources and review
The yield formula is standard physics; the efficiency, storage and first-flush guidance on this page come 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 of rain; installers assume 75–90% efficiency; storage to cover the longest interval without rain; first flush of 1–2 gallons per 100 sq ft.
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: average annual precipitation by station.
Unit conversions from NIST SP 811: 1 ft³ = 7.48 US gallons; 1 in = 25.4 mm; 1 US gallon = 3.785 L.
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.
