When you decide to collect rainwater, the first big question is: How big should my system be?
Sizing a rainwater harvesting setup is about finding the sweet spot between what falls from the sky, what you use the water for, and how much storage you need to bridge the dry spells. Let's break it down step by step.
Step 1: Measure Your Catchment Area.Your roof is your collection site. Measure the length and width of the roof sections that drain into your gutters, so that you can determine an approximate square footage of the catchment area. Only include the portions that feed your tank. A 2,000 sq ft home with two main rooflines might give you roughly that much catchment area.
Step 2: Estimate How Much Rainfall to Expect.Look up your local annual rainfall through the NOAA Climate Data Center. For example, Greenville, SC, averages about 50 inches per year - a healthy amount for rain harvesting.
Step 3: Calculate How Much Rainwater You Can Collect.Armed with your catchment area and the amount of rainfall you can reasonably expect in your area, you can easily calculate the amount of harvestable rainwater using this simple equation:
Harvestable Rainwater (gal) = 0.623 x Rainfall (in.) x Catchment Area (sq ft.) x Runoff Coefficient
0.623 = Gallons per sq ft per inch of rain
Runoff Coefficient: Efficiency Factor for Roof Type
Metal Roof = 0.95
Asphalt Shingles = 0.85
Tile Roof = 0.75
As an example, if I live in Greenville and can confidently expect an average of 50 inches of rainfall per year, and I have 2,000 sq ft of catchment area on my metal roof, I can expect to harvest 59,000 gallons of rainwater per year. That is nearly 5,000 gallons per month... all from one roof!
Step 4: Estimate Your Water Use.For this step, you must first decide how the rainwater will be used. Here is a quick table that breaks down water use by application.
Application |
Average Water Use |
| Garden Irrigation | 5-10 gal/day per 100 sq ft |
| Laundry & Toilets | 10-20 gal/person/day |
| Whole House (Potable) | 40-60 gal/person/day |
Estimate your total water use by application and compare it with the available harvestable water supply to determine how best to use this precious resource.
Step 5: Choose Your Tank Size.Your collection tank(s) need to be able to bridge dry periods. In the Southeast, plan for 1 to 1.5 months of storage demand. As an example, if your family uses 3,000 gallons per month, install a 3,000 - 4,500-gallon tank. In areas with longer dry seasons, you might double that.
Step 6: Manage Overflow and First Flush.Always include an overflow line to route excess water safely away from the foundation. A first-flush diverter discards the initial few gallons of water to keep debris-filled roof run-off from gunking up your storage tank.
Step 7: Filter and Treat the Water.For non-potable use (garden, laundry, toilets), run harvested rainwater through a leaf screen and a 5-micron sediment filter before diverting to your storage tank.
Rain Harvesting Water for Potable Use: In addition to a leaf screen and a 5-micron sediment filter, you should also add carbon filtration plus UV sterilization (or chlorination), and reverse osmosis for drinking water.
Step 8: Understand Costs of Rainwater Harvesting.Up-front expenses vary with system size. Check out the table below for a breakdown. Most systems pay for themselves within 5-10 years through reduced city-water bills, compared with the opportunity cost of well-digging and maintenance.
| Component | Typical Range |
| Storage Tanks | $0.50-$1.50 per gallon capacity |
| Gutters + First Flush Diverter | $300 - $1,000 |
| Rainwater Treatment & Filtration Equipment | $500 - $2,000 |
| Pumps and Plumbing | $500 - $2,000 |
Main Takeaways.
Sizing your rainwater system is about balance - catch enough to meet your needs, store enough to ride out dry weeks, and filter enough to stay safe. Rainwater harvesting offers:
- Independence from chemically-treated municipal water,
- Backup supply when wells or city systems fail,
- Real resilience for households, farms, and gardens.
Ready to begin? Try our Rain Harvest Estimator to see how much water your roof could collect each year. Give us a call, or shoot us a message. Our team can help design a system that is tailored to your rainfall amounts, roof type, and lifestyle, so that you can enjoy clean, reliable, sustainable water straight from the sky.
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