Rain Volume Calculator
One inch of rain sounds small until you spread it over a roof: on 1,000 square feet it is 623 gallons of water. This calculator multiplies rain depth in inches by area in square feet to give the total rain volume in gallons, so you can size a rain barrel, check a drainage plan, or put a real number on a storm.
This is the gross volume that fell on the area. What you can actually collect is lower after splash, overflow and first-flush losses; the losses section below explains the gap. The scale badge is a reading of the size, not a second calculation.
Quick Answer
1 inch of rain on 1,000 square feet is 623.00 gallons. The formula is rain depth (inches) × area (square feet) × 0.623, and those worked-example values are pre-filled above. Double the depth or double the area and the volume doubles with it: 2 inches on the same roof is 1,246.00 gallons.
Key takeaways
- Rain volume = depth × area × 0.623. The 0.623 packs two unit conversions into one number: a square foot is 144 square inches, and a US gallon is 231 cubic inches, and 144 ÷ 231 = 0.6234.
- The answer is the water that fell, not the water you keep. Real collection systems lose 10 to 30 percent to splash, overflow and first-flush diversion.
- For a roof, enter the horizontal footprint, not the sloped roof surface. Rain falls vertically, so a steep roof and a flat roof over the same footprint catch the same volume.
- An acre puts the scale in perspective: 1 inch on one acre is 27,137.88 gallons by this calculator, within 0.1 percent of the US Geological Survey figure of 27,154 gallons.
- Quick Answer
- How to use the rain volume calculator
- The rain volume formula and where 0.623 comes from
- Worked example: one inch on a 1,000 square foot roof
- Benchmarks: what an inch of rain delivers
- What competing calculators miss
- Gross rain vs water you can actually keep
- Frequently asked questions
- Add This Calculator to Your Website
- References
How to use the rain volume calculator
Two inputs, one multiplication. The result stays hidden until you calculate.
- Enter the rain depth in inches: the depth from a rain gauge or a storm total. The worked example has 1 inch pre-filled. If your figure is in millimetres, divide by 25.4 first.
- Enter the area in square feet: the flat area the rain fell on. A 25 ft by 40 ft roof footprint is 1,000 square feet. If your area is in square metres, multiply by 10.764 first.
- Click Calculate rain volume: the gallons figure appears with a scale badge (barrel, cistern, or stormwater scale), and you can copy or share it from the results box.
Both numbers must be positive. A blank, zero, or negative entry in either field triggers the message “Please enter valid, positive numbers for rain depth and area.” and no new result is calculated (a previous result, if any, stays on screen). Decimals are fine: 0.5 inches on 800 square feet is a valid, common run.
The rain volume formula and where 0.623 comes from
Rain volume is depth times area, converted into gallons. Depth in inches times area in square feet is not a volume in any single unit, so the 0.623 factor does the unit repair work in one step.
Here is the derivation, with nothing hidden. One square foot is 144 square inches, so 1 inch of rain on 1 square foot is 144 cubic inches of water. A US gallon is defined as 231 cubic inches. Dividing, 144 ÷ 231 = 0.6234 gallons per inch per square foot. The calculator uses 0.623, the same rounded factor the US Geological Survey and university extension programs publish. The rounding costs about 0.06 percent: on an acre-inch the calculator gives 27,137.88 gallons against the exact-factor figure of 27,154 gallons, a difference of roughly 16 gallons in 27,000. For barrel sizing, drainage checks, and garden planning, that gap is smaller than the error in any rain gauge.
The formula assumes the rain fell evenly over the whole area and that the area is measured flat. Those two assumptions are why the roof rule matters: a pitched roof sheds rain from its horizontal footprint, and measuring the sloped surface instead inflates the area. If you need a roof footprint and only know the slope, a gable roof area calculator can help you work back to the plan dimensions. For odd-shaped yards or lots, split the shape into rectangles and triangles, run each piece, and add the volumes. The same depth-times-area logic, run in reverse, is what an ice to water volume calculator uses when frozen water melts back to liquid.
Worked example: one inch on a 1,000 square foot roof
A storm drops 1 inch of rain on a house with a 1,000 square foot roof footprint. Those are the pre-filled values above, and running the calculator gives 623.00 gallons.
Six more runs of the same calculator, covering a drizzle on a patio up to an acre of land. Type any row in and you will get the same figure back.
| Scenario | Depth (in) | Area (sq ft) | Volume (gal) | Scale |
|---|---|---|---|---|
| Light shower on a patio | 0.5 | 800 | 249.20 | Rain-barrel scale |
| One inch on a small roof | 1 | 1,000 | 623.00 | Cistern scale |
| Heavy storm on a house roof | 1.5 | 1,200 | 1121.40 | Cistern scale |
| Wet day on a large roof | 2 | 2,000 | 2492.00 | Cistern scale |
| Summer storm on a garden plot | 3.25 | 750 | 1518.56 | Cistern scale |
| One inch on a full acre | 1 | 43,560 | 27137.88 | Large tank or stormwater scale |
Benchmarks: what an inch of rain delivers
One inch is the standard yardstick because storm totals and annual rainfall are both quoted in inches. At 1 inch, every 100 square feet yields 62.30 gallons, and the volume scales in a straight line from there.
| Area | Gallons per inch | Relative size |
|---|---|---|
| 100 sq ft (small patio) | 62.30 | |
| 500 sq ft (garage roof) | 311.50 | |
| 1,000 sq ft (small house roof) | 623.00 | |
| 2,000 sq ft (large house roof) | 1246.00 | |
| 5,000 sq ft (small commercial roof) | 3115.00 |
The practical reading: a single 55-gallon rain barrel under one downspout fills after roughly a tenth of an inch of rain on a 1,000 square foot roof, which is why serious harvesting setups chain barrels or move up to a cistern. Penn State Extension makes the same point from the runoff side: barrels capture only a slice of roof runoff, so overflow routing matters as much as capacity. If you are costing a system, the annual water cost calculator shows what the captured gallons are worth against your water bill, and a drain flow rate calculator covers the related question of how fast that water has to move away.
What competing calculators miss
The formula is public and every rival gets it right, so the differences are in what surrounds the multiplication:
- Two-step tools that stop at cubic feet (BraveCalculator and similar): they show depth ÷ 12 × area, then a separate gallon conversion. Correct, but the reader does two conversions to reach the number they actually wanted. This page folds both into the single 0.623 factor and shows the derivation, so the shortcut is checkable rather than magic.
- Harvesting tools with an efficiency dial (CalcExp, Calculator City, CalculatorsHub): they add a collection-efficiency or runoff-coefficient input, typically 0.80 to 0.95, which is genuinely useful for tank sizing. The cost is a false sense of precision: the coefficient is a guess dressed as a measurement. This calculator reports the honest gross volume and explains the loss range in prose, where a guess belongs.
- Metric-first rainfall tools (CalculatorLib): excellent conversion tables and the neat rule that 1 mm on 1 m² is exactly 1 litre, but US readers working in inches and square feet have to convert before they can start. This page is imperial-native and gives the mm and m² conversions in the how-to steps.
- The USGS rainfall calculator: authoritative and the source of the 27,154-gallon acre benchmark, but it works in lot sizes and acres for a classroom audience. It will not give a homeowner a barrel-versus-cistern reading of a 623-gallon result, which is the decision most visitors here are making.
- The gap this page fills: the standard formula, an explained factor, verified runs at six scales, per-inch benchmarks for common areas, and an explicit gross-versus-collectible warning, on one page.
Gross rain vs water you can actually keep
The calculator reports the volume that fell. No gutter system keeps all of it, and treating the gross figure as stored water is the most common planning error with this tool.
Three losses do most of the damage. Splash and wind blow a share off the roof edge, especially in hard rain. A first-flush diverter deliberately discards the first few gallons of each storm, because that water carries dust, pollen, and roof grit. And once a barrel or tank is full, every further gallon is overflow by definition. Published harvesting guides put total collection efficiency at roughly 70 to 90 percent for a well-built roof system, so a realistic keep from the 623-gallon worked example is about 435 to 560 gallons, not 623.
Two more boundaries are worth stating plainly. The tool does not model ground absorption: rain on soil partly soaks in rather than running off, so for stormwater sizing on paved areas the gross figure is the right input, while for garden soil it overstates surface water. And the tool is for rain only. Snow needs a melt-water conversion (fresh snow is roughly a tenth water by depth, and it varies a lot), so a snow depth typed into this calculator gives a wrong answer with a confident face. Size storage and drainage from the gross number, then discount it by your system, not the other way round.
Frequently asked questions
Multiply the rain depth in inches by the area in square feet, then multiply by 0.623. For example, 1 inch of rain on 1,000 square feet is 1 × 1,000 × 0.623 = 623.00 gallons. The calculator above runs the same multiplication and adds a scale badge for the result.
623.00 gallons by this calculator, which uses the standard rounded factor of 0.623 gallons per inch per square foot. The US Geological Survey quotes the same benchmark as about 623 gallons, and 27,154 gallons for a full acre-inch using the unrounded factor.
One square foot is 144 square inches, so 1 inch of rain on 1 square foot is 144 cubic inches of water. A US gallon is 231 cubic inches, and 144 ÷ 231 = 0.6234. The calculator rounds that to 0.623, which costs about 0.06 percent, far less than the error in a typical rain gauge reading.
Use the footprint, meaning the ground area the roof covers. Rain falls vertically, so a steep roof and a flat roof over the same footprint receive the same volume. Measuring the sloped surface overstates the area and inflates the result.
No. It returns the gross volume that fell on the area. Real harvesting systems typically keep 70 to 90 percent of that after splash, first-flush diversion, and overflow, so discount the result before you size a tank or promise the water to a garden.
Convert first, then calculate. Divide millimetres by 25.4 to get inches, and multiply square metres by 10.764 to get square feet. A handy metric check: 1 mm of rain on 1 square metre is exactly 1 litre.
A depth or area of zero means no rain volume to compute, and negative values have no physical meaning here. Either case shows the message “Please enter valid, positive numbers for rain depth and area.” Enter two positive numbers and calculate again.
Not directly. The tool assumes liquid rain. Snow is mostly air: as a rough guide, 10 inches of fresh snow melts to about 1 inch of water, but the ratio swings with temperature and compaction, so a snow depth entered here would overstate the water volume several times over.
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References
- U.S. Geological Survey, Water Science School. Water Q&A: How much water falls during a storm? (1 inch of rain deposits 27,154 gallons on one acre and 13,577 gallons on a half-acre lot). Accessed October 24, 2026.
- U.S. Geological Survey, Water Science School. Rainfall calculator (English units): How much water falls during a storm? (rainfall volume by area size and storm depth, the method this page cross-checks). Accessed October 24, 2026.
- Stephen La Rocque, University of Regina, MathCentral. Calculating rainwater volume (1 inch on 1,000 sq ft is 144,000 cubic inches, divided by 231 cubic inches per US gallon gives 623 gallons; pitched roofs use ground coverage). Accessed October 24, 2026.
- Penn State Extension. Why Use a Rain Barrel? (roof runoff capture, overflow routing, barrel maintenance, and mosquito prevention). Accessed October 24, 2026.
- U.S. Geological Survey, Water Science School. Rainfall calculator, metric units (metric rainfall volume method behind the millimetre and square metre conversions in the FAQ). Accessed October 24, 2026.