Plan, size, and optimise your rainwater collection system with seven free tools. From estimating how much water your roof can catch, to calculating long-term savings and payback periods — everything you need in one place.
Estimate how much rainwater your roof can collect in a year — 0.623 gallons per sq ft per inch of rain, less runoff losses — with the share of your demand it could cover and a starting tank size.
Work out your roof's horizontal catchment area — the number used in every rainwater formula — and its effective area after losses, with gallons per inch of rain and the sloped surface for reference.
Project your full-year rainwater yield from roof area, annual rainfall, runoff coefficient and system efficiency (first-flush, gutter and filter losses), with monthly and daily averages.
Determine the correct volume for your first flush diverter based on roof area and the recommended flush rate. Properly sized diverters remove the most contaminated initial runoff before water enters your tank.
Turn your annual collection into water-bill savings at your rate per 1,000 gallons, CCF or m³, counting only the share you actually use — with monthly, annual and 10-year figures and payback.
Find how many years of water-bill savings it takes to recover your system's cost, after annual maintenance and counting only the share of the harvest you use, with 10- and 25-year net savings.
Rainwater harvesting calculators help homeowners, architects, farmers, and engineers design and evaluate systems that collect rainfall from rooftops and store it for domestic, agricultural, or industrial use. Whether you are sizing a tank for a new home, assessing the financial case for a retrofit system, or optimising first flush diversion for a commercial building, these tools translate real-world inputs — roof dimensions, local rainfall data, water tariffs — into actionable numbers.
This category covers the full lifecycle of a rainwater system: from understanding how much water your catchment area can realistically produce, to modelling the savings and financial return over 10 or more years. All seven calculators are free, work in US and metric units, and show their formula, a worked example and their sources. They were reviewed by John Mac, PE.
New to rainwater harvesting? Start with the Rainwater Harvesting Calculator to get a system-wide yield estimate, then use the Savings Calculator to quantify the financial benefit before committing to installation.
🧭 Choosing the Right Tool
How to Choose the Right Calculator
Each tool in this category answers a specific question at a different stage of the system planning process. Here is a guide to picking the right one for your situation.
Starting from scratch
Rainwater Harvesting Calculator
If you want a single headline number — "how much rainwater can I collect?" — this is your first stop. It takes roof area and local rainfall and returns an annual yield estimate.
Before you can estimate yield, you need the correct effective catchment area. This tool measures the horizontal footprint that actually catches rain (not the sloped surface) so you feed accurate numbers into every other calculator.
Use this for a year-total estimate that includes system losses. Where rain is seasonal, run it with each month's rainfall to see what each month delivers.
A correctly sized first flush diverter is the single most cost-effective water quality upgrade for any rainwater system. Use this before specifying your diverter to get the right volume.
Use the Savings Calculator to model annual water bill reductions, and the ROI Calculator to see the 10-year net return with rising water prices — useful for grant applications or comparing against other home investments.
If you have a specific system cost in mind and want to know how many years of savings it takes to recover that cost — after maintenance and counting only the water you actually use — the Payback Calculator gives you a clear answer.
Every calculator in this category uses the same physics — rain depth × catchment area × efficiency — and shows its working, so you can check the numbers. The design guidance draws on US, Australian and European 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% efficiency; first flush of 1–2 gallons per 100 sq ft; storage to cover the longest interval without rain; filtration and disinfection before drinking.
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: 30-year average rainfall by station.
NIST SP 811 — unit conversions (1 ft³ = 7.48 US gallons, 1 in = 25.4 mm, 1,000 gallons = 3.785 m³).
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.
Last updated
❓ Frequently Asked Questions
Rainwater Harvesting — Common Questions
How much rainwater can I collect from my roof?
In US units: Gallons = Roof Area (sq ft) × Rainfall (in) × 0.623 × Runoff Coefficient; in metric, Litres = Area (m²) × Rainfall (mm) × Coefficient. Rainwater design guides commonly assume 75–90% overall efficiency. A 1,500 sq ft roof with 30 in of rain collects about 24,000 gallons a year at 0.85; a 100 m² roof with 600 mm collects about 54,000 litres at 0.90. Our Rainwater Harvesting Calculator handles this automatically.
What size rainwater tank do I need for a household?
Tank sizing depends on your daily household water demand, your roof catchment area, and local rainfall patterns. Design guides advise sizing storage for the longest expected dry spell, and a common rule among US installers is about three months of demand. A quicker starting point is a few weeks of use: a family using 150 gallons (570 L) a day needs about 2,100–4,200 gallons (8,000–16,000 L) for 2–4 weeks. Our Rainwater Harvesting Calculator can recommend a tank size based on your specific inputs.
What is a first flush diverter and why do I need one?
A first flush diverter automatically discards the initial flow of rainwater from your roof — typically the first 1–2 mm of rainfall — which carries the highest concentration of dust, bird droppings and atmospheric pollutants. A widely used US guideline is to divert one to two gallons per 100 sq ft of roof (about 41–81 litres per 100 m²), and at least 10 gallons per 1,000 sq ft. Australia's national enHealth guidance suggests discarding the first 20–25 litres from an average roof. Use our First Flush Diverter Size Calculator to get the right specification for your roof.
How do I calculate the ROI on a rainwater harvesting system?
ROI is calculated by comparing annual water bill savings against total system costs (tank, pump, filtration, installation, and ongoing maintenance). Over a period, ROI (%) = (Total Net Savings − System Cost) ÷ System Cost × 100, where net savings are water-bill savings minus maintenance. Payback is simply cost ÷ annual net savings. Enter your local water tariff, estimated annual collection, system purchase price, and maintenance costs into our ROI Calculator for a personalised result.
How long does it take for a rainwater system to pay for itself?
It ranges from a few years to never. Where mains water is cheap — as in much of the US, often a few dollars to about $15 per 1,000 gallons — water savings alone can take 20+ years to repay a full system. Where water is expensive, restricted or delivered by tanker, payback can be a few years. Rebates and rising water prices shorten it; pump replacement and maintenance lengthen it. Use our Payback Calculator to generate a personalised estimate.
What roof materials are best for rainwater collection?
Smooth metal roofs (painted steel, Galvalume/Zincalume) collect the most — around 0.85–0.95 of the rain — and concrete tile is close behind. Terracotta tiles perform slightly lower (0.75–0.85). Fibrous cement, slate, and asphalt shingles are also suitable. Roofs with lead flashing, bituminous coatings, or treated timber may introduce contaminants and should be tested before the water is used for drinking or food preparation.
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Other Tools You Might Find Useful
Rainwater harvesting sits at the intersection of several other planning decisions. These tools from other categories complement your system design.
