Rainwater Collection Calculator — Roof Yield and Tank Size
Calculate how many litres or US gallons a roof catchment can collect from one storm, a month, or a year. Add water demand to estimate a practical rain barrel or cistern size, demand coverage, overflow, and reserve.
Planning estimates · Private by design — calculations run locally in your browser.
Calculator inputs
Results
Enter the roof area, rainfall period and depth, roof material, and collection efficiency, then select Calculate.
100 m² tile roof; Heathrow normals; 90% runoff; 85% filter
Annual formula
100 × 614.98 × 0.90 × 0.85
Gross roof water
61,498 L
Roof + filter losses
14,452 L
Final annual yield
47,046 L before storage
Demand
150 L/day; 54,788 L/year; 21-day dry spell
Storage result
3,200 L recommendation; about 69.7% of demand covered in the monthly balance
Methodology, sources, and limits
Author: Starlight RoboticsMethodology sources reviewed: 17 July 2026
The yield calculation follows the standard depth × horizontal catchment area × runoff × collection-efficiency method. In metric, 1 mm on 1 m² equals 1 litre. In US customary units, 1 inch on 1 ft² equals 0.623 US gallons. The EPA uses that 0.623 conversion and monthly rainfall to represent seasonality. US EPA NEWR methods.
Roof presets are screening values, not material certifications. Official New Mexico guidance supplies common values for metal, asphalt, and tar/gravel; Oregon State guidance explains plan-area measurement and material effects. New Mexico roof guidance; Oregon State Extension.
Tank recommendations combine demand × dry-spell days with seasonal deficit in monthly mode. A tested capacity is then evaluated with a running monthly balance. Government and research guidance emphasizes that demand pattern, rainfall frequency, catchment, and storage interact; coarser monthly or annual inputs can introduce substantial uncertainty. Australian Building Codes Board research report.
First flush and tank safety: a diverter removes initial runoff; storage, top-up, backflow, and maintenance requirements depend on local rules. Council design guidelines.
Water quality: harvested roof runoff should not be assumed potable; an EPA-indexed study found sanitary quality limitations even where physicochemical results were satisfactory. US EPA HERO record.
Limitations: Storm mode does not predict when the next storm occurs. Annual mode spreads rainfall evenly only for a rough storage screen. Monthly mode is more useful for seasonality but still hides daily storm timing, roof wetting thresholds, leaks, pump dead volume, climate variability, and changing demand. Obtain local rainfall records and qualified design, plumbing, structural, overflow, and water-quality advice before purchasing or installing a tank.
Rainwater harvesting FAQ
How much water does one inch of rain produce?
One inch of rain produces 0.623 US gallons per square foot before losses. A 2,000 ft² catchment therefore receives about 1,246 US gallons; roof and filter losses reduce the collectable amount.
How do I measure roof catchment area?
Use the horizontal plan area that drains to the selected downpipe, including the relevant overhang: usually length × width. Split L-shaped roofs into rectangles and add them. Do not use the larger sloped roof-surface area.
Does roof slope matter?
Use plan area, not sloped surface area. Slope can affect how quickly water sheds and how much a textured roof retains, but it does not increase the rainfall falling on the roof footprint; the runoff coefficient represents those collection differences.
Which runoff coefficient should I use?
Start with the roof preset, then use a supplier, local authority, or measured value when available. Smooth metal roofs are commonly near 0.95; tile and asphalt near 0.90; gravel, flat, and green roofs generally retain more water. Presets are planning assumptions, not guarantees.
How many rain barrels do I need?
Divide the recommended storage by 55 US gallons or 200 litres for an equivalent container count, then round up. Linked barrels need secure bases, screened inlets, and an overflow route; many sites are better served by one purpose-built cistern.
How is tank size determined?
The recommendation covers the intended daily demand through the entered dry spell and, in monthly mode, checks seasonal inflow against monthly demand. The tested tank uses a running opening storage + inflow − demand balance with overflow and shortage recorded. Annual mode is only a rough screening estimate.
How does first-flush volume work?
First flush diverts the initial roof runoff from each rain event. The calculator subtracts the entered volume once per event, so event count matters. Size and maintain the diverter using its manufacturer’s instructions and local water-quality guidance.
Should multiple downpipes be calculated separately?
Yes, when downpipes feed different barrels or tanks. Enter only the plan area draining to that downpipe. If several downpipes are connected to one correctly sized cistern, their contributing plan areas can be added.
Does snow count as rainfall?
Use the measured or forecast liquid-water equivalent only; do not enter snow depth as rain depth. Actual collection can be delayed or reduced by roof snow storage, freezing gutters, evaporation, and overflow during a thaw.
Is harvested rainwater potable?
Do not assume roof-collected rainwater is safe to drink. Potable use may require approved catchment materials, treatment, testing, backflow protection, permits, and ongoing maintenance. Follow local rules and obtain qualified water-quality guidance.
