Rainwater Harvesting Calculator
Estimate how many gallons of rainwater your roof can collect annually and what tank size you need to meet your water goals. Useful for sizing cisterns, rain barrels, and off-grid water systems.
About this calculator
Rainwater yield depends on collection area, local rainfall, and how much water is lost to evaporation, absorption, and first-flush diversion — captured in a roof runoff coefficient. The core formula is: Annual yield (gallons) = roofArea × annualRainfall × 0.623 × runoffCoefficient / 12 × storageMonths. The factor 0.623 converts square feet at one inch of rain to gallons; dividing by 12 converts annual inches to a monthly figure. Runoff coefficients vary by material: metal roofing retains the least water (coefficient 0.90), asphalt shingles 0.80, clay or concrete tile 0.75, and wood shake 0.70. Multiplying by storageMonths gives the tank capacity needed to store that many months of average yield. Comparing this storage figure to your monthlyUsage goal shows whether the system can meet demand.
How to use
Example: 1,500 sq ft metal roof, 30 inches of annual rainfall, 2 months of storage needed. Step 1 — Base volume: 1,500 × 30 × 0.623 = 28,035 gal/year. Step 2 — Runoff coefficient (metal = 0.90): 28,035 × 0.90 = 25,231.5 gal/year. Step 3 — Monthly average: 25,231.5 / 12 = 2,102.6 gal/month. Step 4 — Storage required: 2,102.6 × 2 = 4,205 gallons. You would need a cistern of at least 4,205 gallons to store two months of average yield.
Frequently asked questions
How much rainwater can I collect from my roof per inch of rain?
The quick rule of thumb is that one inch of rain falling on 1,000 sq ft of roof produces about 600–625 gallons of collectible water, assuming a runoff coefficient near 1.0. Real-world collection is lower once you account for roof material losses and first-flush diverters, which discard the first 10–15 gallons to flush contaminants off the roof surface. A metal roof at 0.90 efficiency over 1,000 sq ft yields roughly 561 gallons per inch of rain. Tracking local monthly rainfall averages lets you project seasonal yields and plan storage accordingly.
What size rainwater storage tank do I need for my home?
Tank size depends on how much rain you can collect, how long dry periods last between significant rain events, and how much water you intend to use from the system. A general starting point is to size the tank to hold one to three months of your intended monthly usage. For a household aiming to offset 2,000 gallons per month of outdoor irrigation over a two-month dry season, a 4,000-gallon cistern is the minimum practical size. Above-ground polyethylene tanks in the 1,000–5,000 gallon range are the most cost-effective for residential use.
Is rainwater harvesting legal in my state and worth the investment?
Rainwater harvesting is legal in most U.S. states, though a handful of western states historically restricted it due to prior appropriation water law; many of those restrictions have since been relaxed or eliminated. Texas and North Carolina actively incentivize it with rebates and tax exemptions. The payback period depends on local water rates and rainfall reliability, but systems typically pay for themselves in 5–10 years when used for irrigation, toilet flushing, or laundry. Always check your county or municipal codes before installing a cistern, as some areas require permits for tanks above a certain capacity.