Rainwater Harvesting Calculator

How much rain can a roof really collect?

More than most people expect. The math is simple: every inch of rain that lands on 1,000 square feet of roof is about 623 gallons of water. In metric terms, 1 millimeter of rain on 1 square meter equals exactly 1 liter. A modest 1,000 sq ft roof in a region with 30 inches of annual rainfall theoretically sheds over 18,000 gallons a year — enough to irrigate a serious vegetable garden through the dry months.

That figure is the theoretical maximum. In practice, expect to actually capture 75–90% of it. The rest is lost to the first-flush diverter (which intentionally discards the first dirty runoff of each storm), evaporation, splashing, and gutters overflowing during intense downpours. When sizing storage, plan on roughly 80% of the calculator's number.

The 0.623 rule, explained

The conversion factor comes straight from unit math: 1 square foot × 1 inch of rain = 144 cubic inches = 0.623 gallons. Multiply roof area (sq ft) × rainfall (inches) × 0.623 and you have gallons. The metric version is even cleaner — millimeters × square meters = liters — because the metric system was designed around water.

One subtlety: use the roof's footprint (length × width as seen from above), not the sloped surface area. Rain falls vertically, so a steep roof and a flat roof with the same footprint catch the same rain.

Sizing your storage

The calculator tells you what the sky delivers; storage decides what you keep. A single 55-gallon rain barrel fills from just a tenth of an inch of rain on a 1,000 sq ft roof — one decent storm and it's overflowing. Practical setups for gardeners:

• 55-gallon barrels — fine for watering containers and a few beds; chain 2–4 together with a linking kit.
• 200–300 gallon cisterns/totes — the sweet spot for a typical vegetable garden; IBC totes are the budget favorite.
• 500+ gallons — for serious food growing or dry climates; bury or shade the tank to keep water cool and algae down.

Always give overflow a place to go — a rain garden, a swale, or simply back to the downspout's original drainage path. An overflowing tank next to a foundation is worse than no tank at all.

Keep the water clean

• First-flush diverter: diverts the first 5–10 gallons of each storm, which carry most of the dust, pollen, bird droppings and roof grit.
• Gutter guards and screens: keep leaves out of the tank; a fine mesh on the tank inlet stops mosquitoes from breeding.
• Dark, sealed storage: light grows algae; an open tank breeds mosquitoes. Keep it dark and covered.
• Roof material matters: avoid collecting from roofs with old asphalt shingles shedding granules, treated wood shakes, or copper flashing if you'll water edibles — metal roofs are the gold standard for harvesting.