Rainwater Collection Calculator
Roof harvest in gallons or liters per storm, month or year — with a recommended tank size for the capture you actually get.
Roof harvest in gallons or liters per storm, month or year — with a recommended tank size for the capture you actually get.
Rainfall period
Roof material
Overrides the material preset. 1 = perfect capture, 0 = nothing.
Enter your roof area and a rainfall figure to size the harvest.
Frequently asked questions
How much rainwater can I collect from my roof?
One inch of rain on 1,000 square feet of roof yields about 623 gallons before losses — with a typical asphalt-shingle efficiency of 85%, you actually collect about 529 gallons. Scale linearly from there: a 1,500 ft² asphalt roof catches roughly 794 gallons per inch of rain, and the same roof in metric terms collects 4,250 liters from 25 mm falling on 200 m². Serious harvests add up fast in wet climates, which is why tank choice matters as much as roof size.
What is the 0.623 factor?
It is the volume of one inch of rain spread over one square foot, expressed in US gallons. One square foot is 144 square inches, so a one-inch depth over that footprint is 144 cubic inches of water; a US gallon is 231 cubic inches; 144 ÷ 231 = 0.623 gallons. The metric system needs no such factor: one millimeter of rain on one square meter is exactly one liter, which is why the calculator's metric path multiplies straight through.
Which roof material collects the most?
Metal, by a clear margin. Its smooth surface sheds water quickly, so less soaks into pores, evaporates on the surface, or is trapped in texture — the calculator credits 95%. Asphalt shingle (85%) loses some water to its gritty surface, especially at the start of a rain when the roof is dry. Tile (80%) loses more to texture and capillary gaps, and flat gravel roofs (70%) are the worst collectors: water pools, soaks in, and evaporates again before it reaches a downspout.
What size rain tank do I need?
Size for the storm you want to capture completely, then check the dry season. A 1,500 ft² asphalt roof fills about 794 gallons in a one-inch storm — the calculator rounds that up to a 1,000-gallon tank so the next cloudburst does not overflow. If you irrigate through dry months, also compare the monthly average against your draw: a tank that empties weekly between rains is doing its job even if it never fills in one event. Standard tiers run 250, 500, 1,000, 1,500, 2,500 and 5,000 gallons.
Is collected rainwater safe to drink?
Not without treatment. Roof runoff picks up dust, pollen, bird droppings, leaf tannins and — on older shingle roofs — stormwater residues you do not want in a glass. Collected rainwater is excellent for irrigation, toilet flushing and washing; making it potable requires a first-flush diverter, fine filtration, disinfection (UV or chlorination) and whatever your local plumbing code demands. Treat the calculator's numbers as tank-sizing guidance, never as a water-quality claim.
How the collection formula works
Rainwater harvesting math is one line: volume = catchment area × rainfall depth × runoff efficiency. In US units, gallons = square feet × inches × 0.623 × efficiency, where 0.623 is the gallon content of one inch of rain over one square foot (144 cubic inches ÷ 231 cubic inches per gallon). Efficiency is the fraction that actually reaches your tank — roofs are not gutters-perfect surfaces.
The metric path is cleaner still, because the units were designed to fit: one millimeter of rain over one square meter is exactly one liter. So liters = square meters × millimeters × efficiency, no conversion constant at all. The calculator runs whichever path matches your input units and cross-checks the results, so the same storm reads the same in either system.
Efficiency bundles three losses: surface wetting (the first bit of rain sticks to the roof and never drains), evaporation during light showers, and overflow or splash-out on the path to the tank. Smooth metal roofs lose the least; rough, porous or flat surfaces lose the most. If a manufacturer or a local extension office quotes a runoff coefficient for your exact roof, enter it as a custom efficiency and the preset is overridden.
Worked example — a 1,500 ft² asphalt-shingle roof:
1. Efficiency for asphalt: 0.85. 2. One-inch storm: 1,500 × 1 × 0.623 × 0.85 = 794.3 gallons. 3. Tank tier: the smallest standard size at or above 794 gal is 1,000 gallons. 4. A wet 32-inch year on the same roof: 1,500 × 32 × 0.623 × 0.85 ≈ 25,418 gallons — about 25,000 gallons annually, which is why serious irrigators chain tanks or bury a cistern.
Roof material efficiency
The calculator ships with four presets; pick Custom efficiency to override with a measured runoff coefficient.
- Metal — 0.95. Smooth and fast-shedding; the collector's favorite.
- Asphalt shingle — 0.85. The default for most suburban roofs.
- Tile — 0.80. Texture and capillary paths hold water back.
- Flat gravel — 0.70. Pooling, soaking and evaporation take the rest.
Sizing a tank: storm capture vs monthly average
There are two defensible sizing philosophies, and the calculator supports both through the rainfall period you choose. Storm capture asks: what is the biggest single event I want to swallow without overflow? Enter that storm's inches, and the recommended tank is the capture rounded up to the next standard tier. Monthly average asks a slower question: if I collect the month's rain and draw it down steadily, what buffer do I need? Enter the monthly figure and the same tiering applies to that basis.
Tanks are cheapest per gallon at the standard sizes — 250, 500, 1,000, 1,500, 2,500 and 5,000 gallons — because those are the molds manufacturers run. Between tiers, round up: a tank that is slightly too big costs a little more, while one that is slightly too small dumps your overflow down the storm drain. Above 5,000 gallons the calculator flags the cap; at that scale, two tanks or a buried cistern usually beats one oversized poly unit.
Two refinements worth the money on any installation: a first-flush diverter, which discards the dirtiest initial runoff (commonly the first gallon or two per hundred square feet of roof), and an overflow route that directs surplus to where it does useful work instead of against your foundation. Neither changes the math above, but both change what ends up in the tank.
Estimates assume uniform roof geometry and standard runoff coefficients. Verify tank siting, plumbing and backflow requirements against local codes.
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