Rain barrel calculator

Where 0.623 comes from

One inch of water over one square foot is 144 cubic inches. A US gallon is 231 cubic inches, so each square foot of roof sheds 144 ÷ 231 ≈ 0.623 gallons per inch of rain — the same factor Clemson Cooperative Extension’s rain barrel guide uses. In metric it is simpler: 1 mm of rain on 1 m² is exactly 1 litre.

gallons = roof area (sq ft) × rainfall (in) × 0.623
litres = roof area (m²) × rainfall (mm)
collected = gallons × roof coefficient − first flush (per storm)

Use the roof footprint — the area the roof covers seen from above. A steep roof has more surface area, but it doesn’t catch more rain than a flat roof over the same footprint.

Worked examples: How much water from 1 inch of rain on a 1,000 sq ft roof, How much rainwater a 2,000 sq ft roof collects in a year, How much rainwater from a 10×12 shed roof and, in metric, How much rainwater from 25 mm on a 100 m² roof.

Roof type and the first flush

Not every drop reaches the gutter. A roof’s runoff coefficient is the share that does: New Mexico’s Office of the State Engineer lists metal at 0.95, asphalt shingles at 0.90 and flat tar-and-gravel at 0.80–0.85, and Texas A&M AgriLife’s planning manual puts roofs between 0.75 and 0.95, rough surfaces at the low end. Tile sits in between. A planted green roof is a different animal: the Minnesota Stormwater Manual reports green roofs holding back 50–70% of a year’s rain, so half or less reaches the gutter.

A first-flush diverter then sends the dirtiest start of each storm away. The Texas Water Development Board’s rainwater manual gives a rule of thumb of at least 10 gallons for every 1,000 sq ft of roof (1–2 gallons per 100 sq ft), so the calculator takes 10 gal per 1,000 sq ft off every storm. Set it to 0 if you have no diverter. For a month or a year of rain, set the number of rain events: the diverter fills once per storm.

Why a barrel is never enough (and why that’s fine)

A 1,000 sq ft roof produces about 60 gallons in the first tenth of an inch of rain. A standard 55-gallon barrel is full almost immediately. That doesn’t make it pointless: one barrel refilled by every small storm can cover a good share of a small vegetable bed’s watering between rains. For bigger goals, link barrels in series or look at a larger tank.

Sizing storage against what you use

Storage only helps up to what you actually use between storms. Put a month of rain in the calculator, set the rain events, and enter the water you would use in that month under More options (the garden watering calculator gives that figure). It shows how much of the use the roof can cover, what still comes from the tap, and how many barrels are worth having: anything beyond your use in the period just sits full and overflows. Run it for your driest growing-season month, which is when storage matters most.

Overflow doesn’t have to be wasted. A garden pond is a good place to send it: an 8 × 6 ft pond holds a few hundred gallons, and the pond liner calculator works out its volume — see What size liner for an 8×6 pond, 2 ft deep.

Tips

• Screen the inlet against mosquitoes and debris, keep a lid on the barrel, and give it an overflow that sends water away from the foundation (Clemson’s guide calls for both the screen and the overflow).
• A first-flush diverter sends the dirtiest first runoff away from the barrel.
• Raise the barrel on a sturdy stand — Clemson suggests 12–36 in for enough pressure to fill a watering can or run a hose. A full 55-gallon barrel weighs over 450 lb, so the stand has to be solid and level.
• Roof runoff isn’t drinking water. For edible crops, Clemson recommends watering the soil rather than the leaves, not watering on harvest day, and washing produce in tap water.

More in rainwater harvesting basics.