How much rainwater can a roof harvest for irrigation?
One inch of rain falling on one square foot of roof is 0.623 gallon of water. Multiply that by the roof's footprint and the month's rainfall and you have the rain that falls on the roof. Not all of it reaches the tank: some wets the surface, splashes or evaporates. The runoff coefficient is the share that does. This is the supply equation in Texas A&M AgriLife Extension's rainwater harvesting guide:
Monthly harvest
Gallons = Rainfall (in) × 0.623 × Roof area (sq ft) × Runoff coefficient
Liters = Rainfall (mm) × Roof area (m²) × Runoff coefficient
Before losses, 1 inch of rain on a 1,000 sq ft roof is 623 gallons; at a runoff coefficient of 0.85, about 530 gallons reach the tank. In metric, 10 mm on 100 m² is 1,000 liters, or 850 liters at 0.85.
Runoff coefficients
Texas A&M AgriLife gives roofs of metal, gravel or asphalt shingle a runoff coefficient range of 0.75 to 0.95, and notes that the best catchments have hard, smooth surfaces such as metal. Take Care of Texas, a Texas Commission on Environmental Quality program, likewise describes rooftops as typically 75–95% efficient and notes that 0.85 is a common choice in examples.
| Surface | High | Low |
|---|---|---|
| Roof: metal, gravel, asphalt shingle | 0.95 | 0.75 |
| Paving: concrete, asphalt | 0.95 | 0.70 |
| Paving: brick | 0.85 | 0.70 |
| Gravel | 0.70 | 0.25 |
Why monthly timing matters
A year's total harvest can look like plenty, but rain rarely falls when gardens need it most. In the AgriLife guide's San Antonio example, winter harvest exceeds garden demand because evapotranspiration is low, while summer demand runs well ahead of the rain. A tank bridges that gap by storing the surplus months for the deficit months. That's why this calculator works month by month rather than from an annual total.
Check local rules. Rules on collecting and using rainwater vary by state and city. If you connect a rainwater system to a municipal or private water supply for top-up, AgriLife notes you must use an air gap or another approved backflow prevention device, and follow local plumbing and building codes.
How this rainwater harvesting calculator works
The calculator follows the three steps in Texas A&M AgriLife Extension's guide: monthly supply, monthly demand, and a month-by-month "checkbook" of stored water. It then searches for the tank size that makes the most of the harvest.
1. Monthly supply
Harvest
US: Harvest (gal) = Rainfall (in) × 0.623 × Roof area (sq ft) × Runoff coefficient
Metric: Harvest (L) = Rainfall (mm) × Roof area (m²) × Runoff coefficient
The calculator uses 144 ÷ 231 = 0.6234 gallon per square foot per inch. AgriLife's worksheets round it to 0.623, so their figures can differ by about 0.1%.
2. Monthly demand
Garden demand
US: Demand (gal) = ETo (in) × Plant water use coefficient × 0.623 × Garden area (sq ft)
Metric: Demand (L) = ETo (mm) × Plant water use coefficient × Garden area (m²)
As in the AgriLife method, rain falling directly on the garden isn't subtracted, which makes demand a conservative, dry-weather estimate.
3. Monthly water balance
Each month
Water available = Stored water + Harvest
Water used = the smaller of Water available and Demand
Stored water next month = the smaller of (Water available − Water used) and Tank size
The tank starts empty in January. The calculation repeats the year until the pattern settles, then reports that typical year. Any shortfall is supplemental water from another source.
4. Tank size and share of demand covered
Coverage
Share of demand covered = Water used in a year ÷ Garden demand in a year × 100
Most possible = the smaller of annual harvest and annual demand
The recommended tank is the smallest one that reaches the most possible water use. A larger tank adds nothing, because it would never fill further or empty lower.
Rounding
Volumes are rounded to about three significant figures, such as the nearest 10 gallons for a typical tank, with "about", because they rest on average rainfall and estimated demand. Shares of demand are shown to the nearest whole percent.
When this calculator isn't the right tool
It balances a roof's rainfall against a planted area's needs. The garden and tree calculators handle related questions:
- Household greywater as the supply instead of rain. Use the greywater irrigation calculator.
- A weekly watering amount for a bed rather than a yearly supply plan. See the garden watering calculator.
- How much of the rain on your garden itself reaches the roots. The effective rainfall calculator estimates that.
- A whole-property water budget including lawn and other supplies. Use the landscape water budget calculator.
Rainwater harvesting for irrigation example
This follows the San Antonio, Texas example in Texas A&M AgriLife's guide: a 1,625 sq ft asphalt shingle roof with a runoff coefficient of 0.9, feeding 1,200 sq ft of high-water-use trees, shrubs and flowers (coefficient 0.75). San Antonio's average rainfall totals 32.92 inches a year and its ETo 62.78 inches, using the monthly figures in the guide. The Fill in the San Antonio, TX example button above loads these months.
- Annual harvest: 32.92 × 0.6234 × 1,625 × 0.9 ≈ 30,013 gallons, shown as about 30,000 gallons. January alone gives 1.66 × 0.6234 × 1,625 × 0.9 ≈ 1,513 gallons.
- Annual demand: 62.78 × 0.75 × 0.6234 × 1,200 ≈ 35,222 gallons, shown as about 35,200 gallons. July alone needs 7.96 × 0.75 × 0.6234 × 1,200 ≈ 4,466 gallons.
- Most possible coverage: harvest is less than demand, so at best the roof covers 30,013 ÷ 35,222 ≈ 85% of the garden's needs.
- Tank size: the monthly balance finds that a tank of about 2,307 gallons, shown as about 2,310 gallons, carries enough winter and spring surplus to use all of the harvest.
- Without a tank: only rain used in the month it falls counts, which covers about 79% of demand. A 1,000-gallon tank raises that to about 82%.
- Supplemental water: 35,222 − 30,013 ≈ 5,210 gallons a year must come from another source.
A metric example
A hypothetical garden in a climate with wet winters and dry summers has an 80 m² roof with a runoff coefficient of 0.75, so each millimeter of rain gives 80 × 0.75 = 60 L. Monthly rainfall from January to December is 80, 70, 60, 40, 30, 10, 5, 5, 20, 60, 90 and 100 mm (570 mm a year). The 40 m² garden has medium water-use plants (0.50), so each millimeter of ETo means 40 × 0.5 = 20 L of demand. Monthly ETo is 30, 40, 70, 100, 130, 160, 180, 160, 110, 70, 40 and 30 mm.
- Annual harvest: 570 × 60 = 34,200 L.
- Annual demand: 1,120 mm × 20 = 22,400 L. Harvest exceeds demand, so full coverage is possible.
- Summer shortfall: from May to September demand exceeds harvest by 800 + 2,600 + 3,300 + 2,900 + 1,000 = 10,600 L.
- Tank size: storing that much by the end of April covers 100% of demand with a tank of 10,600 L. The wet months supply far more than that, so it fills every year.
- Smaller tanks: with no tank, only 11,800 L is used as it falls, about 53% of demand. A 2,000 L tank raises that to 13,800 L, about 62%.
How to plan a rainwater irrigation system
Get monthly rainfall and ETo
Use long-term monthly averages for a weather station near you. In the United States, NOAA's U.S. Climate Normals publish average monthly precipitation for thousands of stations. Monthly ETo comes from agricultural weather networks, extension services and some water utilities; AgriLife's guide lists monthly rainfall and ETo for several Texas cities. If you have weather data but no published ETo, the evapotranspiration calculator estimates it.
Measure the roof correctly
AgriLife's guide measures a sloped roof by the area it covers, usually the building's length times its width, eave to eave. For an L-shaped house, split the roof into rectangles and add them. Count only the sections whose gutters and downspouts will feed the tank.
Choose storage and connections
AgriLife suggests elevating tanks to use gravity, placing them near the plants and downspouts, and connecting several smaller tanks where one large tank won't fit. Make the overflow outlet the same size as the inlet so heavy storms can spill safely. Drip systems fed from a tank need filtering; AgriLife recommends 200-mesh screens, cleaned regularly.
Keep the water clean
A first-flush diverter sends the dirtiest early runoff away from the tank. AgriLife describes a roof-washing standpipe that fills with the first 10 gallons for every 1,000 sq ft of roof before water flows to the cistern. Leaf screens over gutters also help.
Common mistakes
- Sizing from the annual total. A roof that harvests more than the garden needs in a year can still run dry every summer without enough storage.
- Using the sloped roof area. Rain falls on the footprint, not the tilted surface.
- Counting every downspout. Only gutters that drain to the tank add to the catchment.
- Planning on an average year only. Dry years will fall short, so keep a backup water source.
What changes how much of your garden rainwater can cover
| Factor | Effect | What to adjust |
|---|---|---|
| Year-to-year rainfall | AgriLife notes that supply fluctuates from year to year with the weather. | Treat the result as an average year and plan a backup supply. |
| Roof area and surface | A larger or smoother roof harvests more. | Connect more roof sections or use a higher coefficient only if the roof supports it. |
| Garden area and plants | Demand scales with planted area and the plant coefficient. | AgriLife suggests reducing planted area or density, or choosing lower-water-use plants. |
| Mulch | AgriLife lists mulch as a way to reduce evaporation and demand. | Mulch beds; the real demand may be below the calculated one. |
| New plantings | AgriLife notes new plantings need more water during establishment, which can take 1 to 3 years. | Expect higher demand while plants establish. |
| Rain on the garden | Rain on the garden itself reduces irrigation need but isn't subtracted here. | In wet months, actual use from the tank will be lower. |
| Storage size | More storage carries surplus into dry months, up to the recommended size. | Compare your tank's coverage with the recommended tank's. |
Frequently asked questions
How much rainwater can I collect from my roof?
Multiply the rainfall in inches by 0.623, by the roof's footprint in square feet, and by a runoff coefficient for losses. One inch of rain on a 1,000 square foot roof is 623 gallons before losses, or about 530 gallons at a runoff coefficient of 0.85. In metric, each millimeter of rain on each square meter of roof is 1 liter before losses.
What size rain tank do I need to irrigate my garden?
It depends on how rainfall and garden demand line up month by month. The tank needs to hold the surplus from wet months until the dry months use it. In Texas A&M AgriLife's San Antonio example, a 1,625 square foot roof and a 1,200 square foot high-water-use garden need a tank of about 2,310 gallons to use all of the harvest, which covers about 85% of the garden's demand.
What runoff coefficient should I use for my roof?
Texas A&M AgriLife gives roofs of metal, gravel or asphalt shingle a range of 0.75 to 0.95, with hard, smooth surfaces such as metal at the high end. Take Care of Texas notes that 0.85 is a common choice in examples. If you're unsure, 0.85 is a reasonable middle value; a lower figure gives a more cautious estimate.
Can rainwater supply all of my garden's irrigation?
Only if your roof harvests at least as much as the garden needs over a year and your tank is big enough to carry the surplus into the dry months. If harvest is less than demand, you can increase supply with more roof area or reduce demand with a smaller planted area, lower-water-use plants or mulch, as Texas A&M AgriLife suggests.
Why doesn't a bigger tank always help?
A tank only helps when some months harvest more than the garden uses. Once the tank is big enough to carry every surplus into the dry months, extra capacity never fills. And if the roof harvests less than the garden needs in a year, no tank can cover more than harvest divided by demand.
How do I measure my roof for rainwater harvesting?
Measure the footprint, the area the roof covers as seen from above, usually the building's length times its width from eave to eave. Don't measure along the slope. For an L-shaped or complex roof, split it into rectangles and add them, counting only sections whose gutters drain to the tank.
Where can I find monthly rainfall and ETo for my area?
For monthly rainfall in the United States, NOAA's U.S. Climate Normals give long-term averages for thousands of weather stations. Monthly reference evapotranspiration (ETo) comes from agricultural weather networks, extension services and some water utilities. Texas A&M AgriLife's rainwater guide lists both for several Texas cities.
If you have weather data but no ETo, use the evapotranspiration calculator.
Do I need a first-flush diverter?
It's a simple way to keep the dirtiest runoff out of the tank. Texas A&M AgriLife describes a standpipe that captures the first 10 gallons for every 1,000 square feet of roof before water flows to the cistern. Cleaner water also means less clogging in drip filters, which AgriLife recommends be 200-mesh.
Who built and checked this calculator
Licensed Professional Engineer and Irrigation Association Certified Irrigation Designer with more than 20 years in irrigation design, system auditing and landscape water management. Full profile
How this page was checked
- Supply, demand and monthly balance checked against the Texas A&M AgriLife San Antonio example.
- Tank sizing and both worked examples verified by hand.
- Runoff and plant coefficients checked against the AgriLife guide's appendices.
Sources
- Persyn, R.A., Porter, D.O. and Silvy, V.A. Rainwater Harvesting (B-6153). Texas Cooperative Extension, Texas A&M University System, 2004.
- Take Care of Texas (Texas Commission on Environmental Quality). Calculating Your Potential for Rainwater Harvest.
- NOAA National Centers for Environmental Information. U.S. Climate Normals. [VERIFY URL]
- Allen, R.G., Pereira, L.S., Raes, D. and Smith, M. FAO Irrigation and Drainage Paper 56. Crop evapotranspiration: Guidelines for computing crop water requirements, Chapter 1. Food and Agriculture Organization of the United Nations, 1998.