# Rainwater Harvesting Tank Size Calculator Size a rainwater storage tank from your roof area, local rainfall, and daily water demand. --- - **Canonical URL:** https://dothecalculation.com/calculators/rainwater-harvesting-tank-size-calculator - **Category:** Eco & Sustainability - **Publisher:** Do The Calculation (https://dothecalculation.com) - **Cost:** Free, no account or sign-up required - **Privacy:** Runs entirely in the browser; inputs are never sent to a server - **Methodology:** https://dothecalculation.com/methodology --- ## Size a Rainwater Harvesting Tank Estimate how many gallons your roof can realistically collect per year and size a storage tank against your actual daily water demand. - Roof-area × rainfall × runoff-coefficient harvest model - Tank size driven by your daily demand and dry-spell buffer - Percent of demand met, so you know if a tank alone is enough ## Quick Answer — How Much Rain Can a Roof Actually Collect? The standard rainwater harvesting formula is: **Gallons = Roof Area (sq ft) × Rainfall (inches) × 0.623 × Runoff Coefficient × System Efficiency**. The 0.623 constant comes from the physical fact that 1 inch of rain falling on 1 square foot of catchment yields 0.623 gallons of water, before any losses. Not all of that water reaches your tank. A **runoff coefficient** accounts for the roof surface: smooth metal roofing sheds about **95%** of what falls on it, asphalt shingle or concrete sheds about **90%**, and rougher built-up roofing like tar-and-gravel sheds closer to **80%** because more water is absorbed or evaporates before reaching the gutter. A **system efficiency** factor then accounts for first-flush diverters, gutter overflow during heavy storms, and filter losses — commonly 75-90% for a well-maintained system. This calculator applies both factors to your roof's catchment area and local annual rainfall to estimate realistic annual and monthly harvest volume, then sizes a tank against your actual daily water demand. ## How to Use This Calculator Enter your roof's catchment area in square feet (footprint area, not sloped surface area — gutters collect based on the horizontal projection), your local average annual rainfall in inches, your roof material, your system's estimated efficiency, and your target daily water demand along with how many dry days you want the tank to buffer against. **Worked example:** A 1,500 sq ft metal roof in a region with 40 inches of annual rainfall, an 85% efficient system, supplying 20 gallons/day of demand with a 14-day dry-spell buffer. Annual harvest = 1,500 × 40 × 0.623 × 0.95 (metal) × 0.85 (efficiency) = **30,184 gallons/year** (about 2,515 gallons/month on average). Annual demand = 20 × 365 = 7,300 gallons, so the roof harvest could theoretically cover **413%** of demand — meaning the limiting factor here is tank storage capacity, not catchment, since rain doesn't fall evenly across the year. Recommended tank size = 20 × 14 = **280 gallons** to bridge a typical two-week dry spell. ## A Second Example: Smaller Roof, Asphalt Shingle An 800 sq ft asphalt shingle roof in a region with 30 inches of annual rainfall, a 75% efficient system, supplying 10 gallons/day with a 10-day buffer. Annual harvest = 800 × 30 × 0.623 × 0.90 (asphalt) × 0.75 (efficiency) = **10,093 gallons/year** (about 841 gallons/month). Annual demand = 10 × 365 = 3,650 gallons, so harvest could cover **277%** of demand on paper. Recommended tank size = 10 × 10 = **100 gallons**. In both examples the annual harvest volume comfortably exceeds annual demand — this is common for supplemental uses like irrigation or a garden spigot. The tank size number is intentionally sized around bridging dry spells, not around storing a full year of rainfall, since a tank that large is rarely practical or necessary. ## Why Tank Size Isn't Just "Annual Harvest ÷ 12" Rain doesn't fall in equal monthly installments, and most systems can't store a full year of harvest anyway — a tank sized for that would be enormous and mostly sit empty. The practical sizing question is: how many consecutive dry days do you need to buffer against before either refilling from a backup source or rationing use? This calculator uses **daily demand × buffer days** as the recommended tank size, which is the standard approach used by agricultural extension and rainwater harvesting guides. If your harvest percentage of demand is well over 100%, that means your roof has more than enough catchment area in an average year — the real design constraint becomes tank storage and how evenly you can spread usage, not collection capacity. If it's under 100%, no tank size will fully replace your primary water source and rainwater harvesting should be treated as a supplemental, not primary, supply. ## Limitations and Local Regulations This is a planning estimate, not an engineering design. Actual harvest varies year to year with real rainfall patterns — a below-average rain year will collect less than this calculator's long-term average suggests, which is exactly the scenario a buffer-days tank size is meant to protect against. Gutter and downspout capacity, first-flush diverter losses, and roof debris can also reduce real-world efficiency below the entered assumption. Many states and municipalities regulate rainwater harvesting, particularly for potable (drinking water) use, and some water-rights states restrict collection volumes. Check local codes before installing a system, especially if you intend to use harvested water indoors or for anything beyond outdoor irrigation. ## Related Calculators If you're weighing rainwater harvesting against other water-saving options, the [rainwater harvesting calculator](/calculators/rainwater-harvesting-calculator) covers the broader collection-system planning question, while the [water conservation calculator](/calculators/water-conservation-calculator) and [greywater recycling calculator](/calculators/greywater-recycling-calculator) quantify indoor fixture and reuse savings that can reduce your overall demand and shrink the tank size you need. To see your full household water picture before sizing a system, start with the [water footprint calculator](/calculators/water-footprint-calculator). ## Frequently asked questions ### What roof area should I enter — the footprint or the sloped surface area? Use the horizontal footprint (roof's shadow on the ground), not the sloped surface area. Rainfall is measured vertically, so a steeply pitched roof doesn't collect more water per square foot of footprint than a flat one. ### Where do I find my local annual rainfall? NOAA's National Weather Service and most state climate offices publish average annual precipitation by location. Use a multi-year average rather than a single year's total for a more reliable planning estimate. ### Why does roof material affect how much water I collect? Smoother, non-porous materials like metal shed rainwater more completely than rougher, more absorbent materials like tar-and-gravel or wood shake roofing, which retain and evaporate a larger share of the rain that falls on them. ### What's a realistic system efficiency to assume? 75-90% is typical for a maintained system with a first-flush diverter and clean gutters. Systems with clogged gutters, no first-flush diversion, or frequent overflow during heavy storms should use a lower estimate. ### Is harvested rainwater safe to drink? Not without treatment. Roof runoff picks up dust, debris, bird droppings, and roofing material residue. Most residential systems are designed for irrigation, washing, or toilet flushing; potable use requires filtration, disinfection, and often local permitting. ### Why is my "percent of demand met" over 100%? That means your roof's annual catchment potential exceeds your annual usage in an average rainfall year. It doesn't mean your tank never runs dry — rain isn't evenly distributed, so storage (buffer days) is still the practical constraint. ### How many dry-spell buffer days should I plan for? 10-14 days is a common starting point for supplemental irrigation use in most US climates. Arid regions or larger gardens may need a longer buffer; check regional dry-spell patterns for your area. ## Related concepts - **Runoff coefficient** — The fraction of rainfall that actually reaches the gutter from a given roof material, versus being absorbed or evaporated. - **First-flush diverter** — A device that discards the initial, most-contaminated runoff from a storm before it reaches the storage tank. - **Catchment area** — The horizontal footprint of a roof surface used to collect rainwater, measured in square feet. ## Related guides - [How to Use Do The Calculation Calculators: A Practical Step-by-Step Guide](https://dothecalculation.com/blog/site-guides/how-to-use-calculators) — Learn the fastest reliable workflow for using Do The Calculation calculators, reading results, checking formulas, and using save, print, share, and export actions correctly. - [Understanding Calculator Formulas: How DTC Turns Inputs into Results](https://dothecalculation.com/blog/site-guides/understanding-calculator-formulas) — Understand how Do The Calculation formulas are presented, what the explanation blocks mean, and how to verify calculator logic before using a result in a real decision. ## Related calculators - [Rainwater Harvesting Calculator](https://dothecalculation.com/calculators/rainwater-harvesting-calculator) — Estimate harvestable rainwater based on roof size and local rainfall, project cistern sizing, and calculate water bill savings. - [Greywater Recycling & Irrigation Savings Calculator](https://dothecalculation.com/calculators/greywater-recycling-calculator) — Estimate daily household greywater generation and calculate municipal water savings when reused for landscape irrigation. - [Solar Battery Storage & Backup Calculator](https://dothecalculation.com/calculators/solar-battery-sizing-calculator) — Size your emergency solar battery storage capacity and calculate off-grid runtime in days, with or without solar recharge during outages. - [Appliance Energy Cost Calculator](https://dothecalculation.com/calculators/appliance-energy-calculator) — Calculate power costs for home appliances based on wattage, usage hours, and local utility rates to find your biggest energy expenses. - [Fuel Cost Calculator](https://dothecalculation.com/calculators/fuel-cost-calculator) — Estimate gasoline expenses from trip distance, annual mileage, fuel economy, and local fuel prices to budget your driving costs accurately. - [Solar Inverter & Charge Controller Sizing Calculator](https://dothecalculation.com/calculators/solar-inverter-sizing-calculator) — Size solar inverter continuous power and charge controller current based on solar panel specs and temperature variations. --- _This calculator provides an educational, formula-based estimate using standard rainwater harvesting conversion factors and disclosed roof-material assumptions. Actual harvest depends on real-world rainfall patterns, gutter condition, and system maintenance. Check local regulations before installing a rainwater harvesting system._ --- _Source: [Do The Calculation](https://dothecalculation.com/calculators/rainwater-harvesting-tank-size-calculator). Quote freely with attribution and a link to this page._