A rainwater harvesting system is a roof, gutters, a screen, a first-flush diverter, a storage tank and an overflow, sized so the tank carries you through your longest dry spell. Start with the formula, because it usually changes people’s plans:
Gallons = roof area (sq ft) × rainfall (inches) × 0.623 × runoff coefficient
In metric it is simpler: 1 mm of rain on 1 m² of roof is 1 liter, before losses. The 0.623 factor, used in the Texas Water Development Board’s rainwater harvesting manual, is the number of gallons one inch of rain puts on one square foot. The runoff coefficient accounts for splash, evaporation and absorption; the Appropedia calculation guide explains how it is applied. Typical planning values are about 0.95 for metal and 0.85 for asphalt shingle. A 1,500 sq ft (139 m²) roof in one inch (25 mm) of rain yields roughly 1,500 × 1 × 0.623 × 0.85 = 794 gallons (3,000 liters).
That number surprises most people twice: first because a single ordinary rainfall fills a 50-gallon barrel many times over, and second because it means a barrel is not a water supply, it is a bucket that overflows.
Start with what the water is for
Garden irrigation only. The simplest setup: a barrel or two on a downpipe, a screen over the inlet, a tap at the bottom, and an overflow directed away from the foundation. No pump if you can water by gravity, no filtration, and a first-flush diverter helps but is not essential. This is a weekend job. The EPA’s rain barrel guidance estimates that lawn and garden watering makes up nearly 40 percent of household water use in summer, and that a single rain barrel saves most homeowners about 1,300 gallons (4,900 liters) during the peak summer months.
Meaningful outdoor supply through a dry spell. Now you need real storage, hundreds or thousands of gallons, a proper tank, a first-flush diverter, and usually a pump. This is the sizing exercise below.
Indoor non-potable use (toilet flushing, laundry). Add filtration, a pump, pressure control, backflow prevention, and almost certainly a permit and inspection.
Drinking water. Add disinfection and treatment, regular testing, and take local regulation seriously. This is not a DIY guide for that case.
How much water can you collect from your roof?
Measure the roof’s footprint, not its surface: a steep roof catches the same rain as a flat one covering the same ground area. Then multiply by rainfall for the period you care about.
| Roof footprint | 1 inch (25 mm) of rain, asphalt (0.85) | 1 inch (25 mm), metal (0.95) |
|---|---|---|
| 500 sq ft (46 m²) | 265 gallons (1,000 L) | 296 gallons (1,120 L) |
| 1,000 sq ft (93 m²) | 530 gallons (2,000 L) | 592 gallons (2,240 L) |
| 1,500 sq ft (139 m²) | 794 gallons (3,000 L) | 888 gallons (3,360 L) |
| 2,000 sq ft (186 m²) | 1,059 gallons (4,010 L) | 1,184 gallons (4,480 L) |
You rarely need to capture the whole roof. One downpipe draining a quarter of the roof is plenty to fill a couple of barrels; only a large tank justifies connecting every gutter.
How big should a rainwater tank be?
Tank size is a balance between what your roof supplies, what you use, and how long your dry periods run. The practical method:
- Find your monthly rainfall from local climate data (NOAA or your state climatologist publishes monthly normals).
- Calculate monthly yield with the formula above.
- Estimate demand. Watering one inch (25 mm) across a bed uses about 0.62 gallons per square foot (25 liters per m²), the same 0.623 factor in reverse. Multiply by the bed area and the number of waterings a week.
- Size for the dry gap, not the average. The tank needs to carry you through your longest typical dry stretch. In a climate with even year-round rain, a small tank refills constantly and large storage is wasted money. In a climate with a three-month dry season, storage is the entire system.
A worked example. A 1,000 sq ft asphalt roof feeds 200 sq ft (19 m²) of vegetable beds that need an inch of water a week through a 13-week dry summer. Demand is 200 × 0.62 ≈ 125 gallons a week, or about 1,620 gallons (6,100 liters) for the season. The roof yields about 530 gallons per inch of rain, so roughly three inches of spring rain fills a tank of that size. A 55-gallon barrel in the same garden would last less than half a week. Cutting demand also shrinks the tank: a layer of mulch on the beds slows evaporation, so the same storage lasts longer.
A common outcome: people in even-rainfall climates over-buy tank capacity, and people in Mediterranean or monsoon climates under-buy it badly.
What components does a rainwater harvesting system need?
Catchment (roof). Metal sheds water best and leaches least. Asphalt shingle is fine for garden use. Avoid collecting from roofs with treated wood shakes, lead flashing, or moss treatments if the water touches anything edible.
Gutters and leaf screening. Mesh over the gutter or a leaf eater at the head of the downpipe. Leaves in the tank are the main cause of foul water.
First-flush diverter. The first water off a roof carries the dust, bird droppings and debris that accumulated since the last rain. A diverter captures and discards that initial volume before the rest reaches the tank, then drains slowly through a small valve so it is empty for the next storm. The Texas Water Development Board’s manual recommends diverting about 10 gallons per 1,000 sq ft of roof (roughly 0.4 liters per m²), adjusted upward in dusty areas. Built from PVC, a 4-inch (100 mm) pipe holds roughly a gallon per 18 inches (46 cm) of length, so a 1,500 sq ft roof needs about 15 gallons, around 22 feet (6.7 m) of 4-inch pipe, usually split across several downpipes.
Tank. Opaque, to stop algae. Sealed and screened at every opening, to stop mosquitoes breeding. NC State Extension’s guidance on mosquito control points out that a little water always sits in the bottom of a tank, and recommends placing the outlet 1 to 2 inches (2.5–5 cm) above the floor of the tank, and keeping every opening screened, since that standing layer is prime mosquito habitat. Site the tank on a firm, level base, because water is heavy:
| Tank size | Weight of water when full |
|---|---|
| 55 gallons (208 L) | about 460 lb (208 kg) |
| 275 gallons (1,040 L) | about 2,290 lb (1,040 kg) |
| 1,000 gallons (3,785 L) | about 8,340 lb (3,785 kg) |
| 2,500 gallons (9,460 L) | about 20,850 lb (9,460 kg) |
Overflow. Sized at least as large as the inlet and directed well away from the building. An overflowing tank against a foundation causes more expensive damage than the system saves.
Pump, if needed. Gravity works if the tank is uphill of the beds. Otherwise a small pump. Every foot of tank height gives about 0.43 psi (every meter about 9.8 kPa), so raising a barrel on blocks helps a hose flow, though it still will not match mains pressure or run most drip irrigation timers without a pump.
Is harvested rainwater safe for vegetable gardens?
For watering soil around food plants, it is widely used, and the components above are what keep it clean: a metal or shingle roof without chemical treatments, a first-flush diverter, screened inlets, and an opaque, sealed tank. Sensible habits reduce the remaining risk: water the soil rather than the leaves of crops you eat raw, rinse produce before eating it, and do not store water in containers that previously held chemicals. Drinking it is a separate matter that requires treatment and testing.
Is rainwater harvesting legal?
Almost everywhere in the US, yes, and many cities and utilities actively encourage it with rebates. But rules vary more than people expect, mostly in western states with prior-appropriation water law:
- Colorado: HB16-1005, the 2016 law that first allowed residential rain barrels, limits single-family homes and small multifamily buildings to two barrels with a combined 110 gallons (416 liters), for outdoor use on the same property.
- Utah: according to the Utah Division of Water Rights, residents can store up to two covered containers of no more than 100 gallons each without registering. Larger systems, up to 2,500 gallons (9,460 liters), need a free registration with the division.
Elsewhere, some jurisdictions cap barrel volume without a permit, and any indoor plumbing connection triggers building code requirements including backflow prevention. Check with your local water authority and building department before buying tanks, since these rules change.
Where harvesting fits in household water use
Rainwater harvesting is useful in dry-summer climates and for gardeners who irrigate heavily. But it is rarely the largest water saving in a home. A leaking toilet can waste hundreds of gallons a day, more than a typical barrel holds, which is why finding and fixing that is the first move for anyone serious about water use. Harvesting makes most sense once the leaks are gone and the demand is outdoors.