Rainwater Harvesting Calculator

How much rainwater can I collect from my roof?

An inch of rain on a square foot is 0.623 gallons — 144 cubic inches of water against 231 in a gallon. Multiply by the roof's footprint, the storm's depth, and the share the roof actually delivers to the gutter, and that's the catch. Everything else on this page is that one line applied to storms, years, barrels and garden beds.

Gallons per inch of rain, by roof size

Shingle at 80% efficiency, first flush diverted at 10 gal per 1,000 sq ft. Barrels are 55 gal. "Fills a barrel" is the rain needed to fill one 55 gal barrel from that roof.

How many rain barrels do I need?

Enough to carry what you water through the longest dry stretch you get, because that's the only time you'll be drawing on it. A vegetable bed wants about an inch a week in summer — 0.623 gallons per square foot — so two 4 × 8 beds draw 40 gallons a week, and a single barrel is 1.4 weeks of water for them. Size for the dry spell, not the storm:

Smallest standard size — barrels (55 / 110 / 165), IBC totes (275 / 330), then cisterns — that holds an inch a week for the stretch. 32 sq ft is one 4 × 8 bed. Refills between dry spells are a bonus, not a plan.

The roof only has to be big enough to refill that tank in one ordinary rain. From the first table, even a 500 sq ft garage roof refills three barrels in a ¾ in storm — the roof is never the constraint; storage and demand are.

How much rain falls where you live

The annual figure sets the year's catch. NOAA's 1991–2020 normals for twenty cities, and what a 1,000 sq ft shingle roof would deliver over the year — if you had somewhere to put it:

NOAA NCEI 1991–2020 U.S. Climate Normals, annual precipitation. Gallons at 80% efficiency, gross of first flush. The city select above fills the annual field; any other town's normal is on NOAA's site or your local NWS office page.

Is a rain barrel worth it?

Not as a water bill play, and the table says why: municipal water is cheap. At $5 per 1,000 gallons — a typical rate — a full 55 gallon barrel holds 28¢ of water, and the entire year's catch off a 1,000 sq ft roof in Seattle is worth about $98, most of which you couldn't store anyway. A $100 barrel filled and emptied twenty times a season saves $5.50.

The honest reasons to do it are different, and they're good ones. Watering restrictions — a barrel is exempt from every odd/even-day rule, and it's the only water some gardens get in an August ban. Plants that prefer it — seedlings, blueberries, carnivorous plants and anything sensitive to chlorine or hard water. Stormwater — every gallon in the barrel isn't scouring a downspout splash zone or running into a combined sewer, which is why many cities and water districts rebate or give away barrels; check yours before buying one. And convenience: a spigot beside the beds beats dragging a hose.

Can I water my lawn from a rain barrel?

No — and the calculator's "what a full tank waters" panel will say so in your own numbers. An inch over a 5,000 sq ft lawn is 3,115 gallons: 57 barrels. One barrel puts 0.018 in on it, which is a heavy dew. Lawns are watered from the tap, or not at all — the watering guide makes the case for "not at all" in a dry August — and rain barrels are for beds, pots and new plantings, where the gallons go exactly where they're needed through a hose or a gravity drip line.

First flush, screens, overflow — the parts that matter

First-flush diverter: the first gallons off a roof carry the dust, pollen and bird droppings of the dry spell before them. The rule of thumb is to waste 10 gallons per 1,000 sq ft of roof per storm — a length of capped downpipe with a slow drain does it — and the calculator nets it out of every storm. Screen: a fine mesh over every opening, or the barrel is a mosquito nursery in a week. Overflow: route it away from the foundation with a hose or a splash block — the barrel fills in minutes and then the whole roof's water is coming out of that fitting, exactly where the downspout used to discharge before you cut it.

Rainwater off an asphalt roof is not drinking water. Vegetables are fine watered at the soil; rinse produce as you would anyway. Some states regulate collection — Colorado allows two barrels and 110 gallons per household, most states encourage it — so check the rule where you are.

Sources: Texas Water Development Board, The Texas Manual on Rainwater Harvesting, 3rd ed. (2005) — conversion factor, system efficiency, first-flush guidance; NOAA National Centers for Environmental Information, U.S. Climate Normals 1991–2020; EPA, Soak Up the Rain: Rain Barrels; Colorado Senate Bill 16-005 (rain barrel allowance).