Rainwater harvesting basics: how much you can collect and how to store it

How much water your roof sheds

One inch of rain over one square foot is 144 cubic inches of water. A US gallon is 231 cubic inches, so each square foot of roof gives 0.623 gallons per inch of rain. In metric, 1 mm of rain on 1 m² is exactly 1 litre.

gallons = roof footprint (sq ft) × rainfall (in) × 0.623
litres = roof footprint (m²) × rainfall (mm)

Use the footprint — the area covered by the roof as seen from above — not the sloped surface area. And use only the part of the roof that drains to the downspout you are collecting from. Most houses have four or more downspouts, each draining a portion of the roof.

The same formula, worked through for common cases: 1 inch of rain on a 1,000 sq ft roof, a whole year on a 2,000 sq ft roof with 40 inches of rain, a small 10×12 shed roof, and in metric, 25 mm of rain on a 100 m² roof.

Losses: why you won’t catch all of it

Some water wets the roof and evaporates, some splashes out of gutters in heavy rain, and a first-flush diverter deliberately sends the dirtiest first runoff away. Planners handle the first two with a runoff coefficient for the roof surface and the third as a fixed volume per storm.

For the first flush, the Texas Water Development Board’s rainwater manual gives a rule of thumb of diverting at least 10 gallons per 1,000 sq ft of roof, or 1–2 gallons per 100 sq ft. It is lost at every storm, so it matters more in a climate of many small showers than one of a few big storms. The rain barrel calculator uses both: a roof type (asphalt shingles, 0.90, by default) and 10 gallons per 1,000 sq ft per storm. One inch on a 1,000 sq ft shingle roof then comes to about 551 gallons, down from 623.

Storage is the real limit

The mismatch between supply and storage is the key fact of home rainwater harvesting. A standard 55-gallon barrel under a downspout draining 500 sq ft is full after about a quarter inch of rain. Everything after that overflows. That is fine — a barrel that refills in every modest storm can still supply a lot of garden water over a season — but it means:

• Link barrels. Connecting two or three barrels in series with overflow hoses multiplies storage cheaply.
• Use it. An empty barrel before a storm catches water; a full one doesn’t. Water the garden from the barrel, not the tap, during wet spells too.
• Go bigger if it matters. Tanks and cisterns in the hundreds or thousands of gallons need a level, strong base and often a pump.

Check the rules where you live

In most of the US, rain barrels are legal and often encouraged, and some utilities offer rebates. But rules vary. Some states have historically limited collection because of water-rights law; Colorado, for example, allows at most two rain barrels with 110 gallons of combined storage per household, for outdoor use on the same property (Colorado State University Extension summarizes the rule). Some places require permits for larger cisterns, set rules on overflow, or restrict indoor use. Outside the US, rules vary by country and region. Check with your state or local water authority before building anything beyond a barrel or two.

Setting up a rain barrel

1. Choose a downspout near the garden, draining as much roof as practical.
2. Level a base and set the barrel on a stand; raising it 1–2 ft improves pressure at the spigot.
3. Cut the downspout and fit a diverter, or run the downspout into the barrel’s screened inlet.
4. Route the overflow at least several feet away from the foundation, or into a second barrel.
5. Keep the lid on and inlets screened to keep out mosquitoes, debris, children and animals.

Mosquitoes and water quality

Standing water breeds mosquitoes within about a week in warm weather. Fine mesh screens on every opening are the main defense; some people also use mosquito dunks labelled for use in water barrels. Clean the barrel and gutters once or twice a year.

Roof runoff picks up bird droppings, dust and material from the roof itself. It is fine for ornamental plants and generally used for vegetable gardens too, but Clemson’s guidance for edibles is to apply it to the soil around the plant rather than over the leaves, not to use it on the day of harvest, and to wash produce in potable water — sensible caution, especially with older or treated roofs. It is not drinking water without proper treatment.

How much will it help the garden?

Vegetable gardens commonly need about an inch of water a week in the growing season, including rain — about 62 gallons per 100 sq ft. A 200 sq ft garden uses roughly 125 gallons a week. Two linked barrels on a busy downspout can cover much of that between storms in a typical summer, less during a long drought.

That weekly figure for a typical plot is worked out on How much water does a 10×20 vegetable garden need.

Using the water

A barrel’s gravity pressure is low. It works well with a watering can, a short hose, or a soaker hose, which is designed for low pressure. Sprinklers generally won’t run from a barrel without a pump. Drip systems can, if designed for low pressure; check the emitter specs.

Beyond barrels

If you have a lot of roof and a lot of garden, the next steps are larger tanks, rain gardens that let runoff soak in where it lands, and redirecting downspouts onto lawns or beds instead of driveways. Rain gardens don’t store water, but they keep it on your property and in your soil — and they don’t overflow.