Rainwater Harvest Calculator
Estimate how many gallons your roof can collect each year — plus tank size, days of supply, full system cost, water-bill savings and payback — with U.S. rainfall data, a roof material comparison, and a downloadable PDF.
Uses the standard 0.623 gallons per sq ft per inch of rain with 2024-2025 U.S. pricing. Rainwater harvesting rules vary by state and county — check local codes before installing. For planning purposes only.
| Roof Material | Runoff | Gallons / Year | Monthly Avg | Water Value / Yr |
|---|
For your 1,200 sq ft roof at 30 in/yr rainfall, after 10% collection losses. Water value at $4.50 / 1,000 gal.
How to use the rainwater harvest calculator
Four quick steps turn your roof size and local rainfall into a harvest estimate — gallons per storm and per year — with a roof-type efficiency, a tank-size suggestion, and a downloadable PDF.
Enter the roof area
The horizontal footprint of the roof catching the rain, in square feet.
Add the rainfall
A single storm, a monthly figure, or your area’s annual rainfall in inches.
Set roof & efficiency
Pick the roof type so the collection efficiency matches how it sheds water.
Read results & save a PDF
Get gallons harvested, an annual total, a tank-size guide, and a downloadable summary.
How the rainwater estimate is calculated
It rests on one constant: an inch of rain over a square foot yields about 0.623 gallons. The calculator multiplies that by your roof area, your rainfall, and a collection efficiency for the real-world figure. Here is the exact math.
Worked example — a 1,500 sq ft roof, 1 inch of rain
Theoretical: 1,500 × 1 × 0.623 = 934 gal
With efficiency: 934 × 0.85 ≈ 794 gal
Annual (30 in): 1,500 × 30 × 0.623 × 0.85 ≈ 23,830 gal
≈ 794 gallons from one inch of rain, or ~23,800 gallons a year at 30 in of rainfall.
Rainwater harvesting charts
Handy lookups for the questions people ask most — harvest per inch by roof size, annual yield by rainfall, roof-type efficiency, and tank sizing. Figures use the 0.623 constant at 85% efficiency unless noted.
| Roof area | At 100% | At 85% eff. |
|---|---|---|
| 500 sq ft | 312 gal | 265 gal |
| 1,000 sq ft | 623 gal | 530 gal |
| 1,500 sq ft | 934 gal | 794 gal |
| 2,000 sq ft | 1,246 gal | 1,059 gal |
| 2,500 sq ft | 1,558 gal | 1,324 gal |
| 3,000 sq ft | 1,869 gal | 1,589 gal |
Gallons from a single inch of rain. Multiply by the inches in a storm, month, or year to scale up. Use the roof’s horizontal footprint.
| Annual rainfall | Gallons per year |
|---|---|
| 10 inches | ~7,940 gal |
| 20 inches | ~15,890 gal |
| 30 inches | ~23,830 gal |
| 40 inches | ~31,770 gal |
| 50 inches | ~39,720 gal |
For a 1,500 sq ft roof at 85% efficiency. Scale linearly for other roof sizes — a 3,000 sq ft roof yields double these figures.
| Roof type | Typical efficiency | Notes |
|---|---|---|
| Metal / steel | ~90–95% | Smooth, sheds cleanly |
| Clay / concrete tile | ~85–90% | Some loss in joints |
| Asphalt shingle | ~80–85% | Most common, slight absorption |
| Wood shake | ~75–80% | Absorbs more water |
| Gravel / built-up | ~70–80% | Holds and evaporates water |
| Green roof | ~25–50% | Plants absorb most rain |
Efficiency also covers first-flush diversion, splash, and evaporation. A first-flush diverter improves water quality but slightly lowers yield.
| Daily use | Dry days | Tank size |
|---|---|---|
| 30 gal/day | 14 days | ~420 gal |
| 50 gal/day | 30 days | ~1,500 gal |
| 75 gal/day | 21 days | ~1,575 gal |
| 100 gal/day | 21 days | ~2,100 gal |
| 150 gal/day | 14 days | ~2,100 gal |
Size the tank to bridge dry spells, not to hold a year’s rain. Balance it against your roof’s yield, your budget, and available space.
roof footprint
× 0.623 gal
use × dry days
Harvesting is a chain: the roof catches the rain, the constant turns it into gallons, and a tank stores it for dry days.
- Catchment — the roof’s horizontal footprint is what catches the rain.
- The constant — 0.623 gallons per square foot per inch, times an efficiency.
- Storage — size the tank to your daily use across the dry spells you get.
Everything the calculator works out
One roof size and rainfall gives you a complete harvest picture — the yield per storm, the annual total, and a tank guide.
Key figures behind rainwater harvesting
Built for any rainwater project
From a single rain barrel to a whole-property cistern, the same calculator scales the yield to your roof, your rainfall, and your use.
Wants free water for the garden and to know how many barrels a typical storm will actually fill.
- Enter the shed or house roof
- Use local rainfall figures
- Match barrels to storm yield
Planning a larger cistern for irrigation or livestock and needs annual yield and a tank size.
- Work out the annual total
- Size the tank for dry spells
- Pick a high-efficiency roof
Designing rainwater into a build for toilets, laundry, or irrigation and needs the supply figures.
- Total catchment across roofs
- Compare supply to demand
- Check local rules & rebates
7 tips for accurate rainwater planning
Small habits keep your yield estimate honest and your system working, from a barrel to a cistern.
Rainwater harvest calculator FAQ
The yield, formula, roof-type, and tank-size questions people ask most before harvesting rain.
Multiply your roof’s catchment area in square feet by the rainfall in inches, then by 0.623 gallons, and by a collection efficiency of around 85%. That gives the gallons you can harvest.
For example, a 1,500 sq ft roof in a 1-inch rain collects about 794 gallons at 85% efficiency. Over a year, the same roof in a region getting 30 inches of rain could yield well over 20,000 gallons.
Use the formula: gallons = catchment area (sq ft) × rainfall (inches) × 0.623 × efficiency. The 0.623 is the gallons produced by one inch of rain over one square foot, and efficiency accounts for real-world losses.
Measure the roof’s footprint, not its sloped surface, since the horizontal area is what catches the rain. Use your local rainfall figures and an efficiency of about 0.75 to 0.90 for a realistic estimate.
The 0.623 factor is how many gallons one inch of rain produces over one square foot of area. It comes from the math: one inch over one square foot is 144 cubic inches, and there are 231 cubic inches in a gallon, so 144 divided by 231 is about 0.623.
It’s the core constant of rainwater harvesting. Multiply it by your area and rainfall to get the theoretical yield, then apply an efficiency factor for the real-world number.
Yes. Smooth roofs like metal shed water efficiently and lose little, while rough or absorbent surfaces like asphalt shingles or wood shakes hold and lose more, lowering the collection efficiency.
Metal roofs often run around 90% or higher efficiency, asphalt shingle around 80 to 85%, and gravel or green roofs less. The calculator’s efficiency factor lets you match your roof type for a realistic estimate.
Size the tank to bridge the gap between rain events for your intended use, not to hold a whole year’s rain. Estimate your daily water use, multiply by the number of dry days you want to cover, and use that as a target.
For example, 50 gallons a day of garden watering across a 30-day dry spell suggests about a 1,500 gallon tank. Balance the tank size against your roof’s yield, your budget, and the space you have.
Not without proper treatment. Rainwater picks up dust, bird droppings, and roof debris, so it’s best used untreated only for non-potable uses like garden watering, toilets, and laundry.
Making it safe to drink requires filtration and disinfection, plus first-flush diversion and testing, and it may be subject to local health rules. For drinking water, follow local regulations and use a properly designed potable system.
A first-flush diverter routes the first, dirtiest portion of runoff away from the tank at the start of a storm, so the debris, dust, and droppings washed off the roof don’t enter your storage.
After the diverter fills, the cleaner water flows into the tank. It’s a simple, worthwhile addition that improves water quality, and it’s one reason real collection efficiency is below 100%.
In most places yes, and many areas encourage it, but rules vary. Some regions regulate how much you can collect or what it can be used for, and a few have restrictions tied to water rights, so it’s worth checking local and state rules first.
Many jurisdictions offer rebates or incentives for rainwater systems. Confirm your local regulations before installing a large system, especially if you plan to use the water indoors.
General Estimating Notice: This calculator estimates rainwater harvest volume and tank size from your roof area, rainfall, roof type, and collection efficiency, using the standard 0.623 gallons per square foot per inch and typical efficiency figures. Actual yield varies with rainfall patterns, roof material, debris, first-flush losses, and evaporation. Harvested rainwater is generally for non-potable use unless properly treated, and collection may be regulated in your area — confirm local rules and water-quality requirements before installing a system. For planning purposes only.