Non-potable use, permits and safety
- Non-potable only. This calculator sizes tanks for untreated rainwater used for toilet flushing, laundry where it is allowed, and garden watering. Don't drink untreated rainwater or use it for cooking. Drinking-water systems need treatment and disinfection, and often an approved design.
- Permits. The U.S. Department of Energy's FEMP guidance notes that rainwater harvesting systems may require a permit from local or state government, so plan for it early. Rules vary a lot by place, especially for plumbing rainwater into a house.
- Keep it clean and safe. FEMP recommends a first-flush diverter to keep debris out and screening to stop mosquitoes breeding. Fit a secure lid, route the overflow away from foundations, and put the tank on a level, solid base, because water weighs about 8.34 lb per gallon (1 kg per litre).
How to measure the catchment
Use the roof's plan-view area (its footprint, overhangs included) for the part of the roof whose gutters drain to the tank. Texas A&M AgriLife: “For a sloped roof, measure the area covered by the entire roof, usually the length times the width of the building.” The slope doesn't matter, because rain falls over the footprint. If only one side of a gable roof drains to the tank's downspout, use half the footprint. Add a garage or shed roof with “+ Another roof”. The roofing calculator passes its section footprints to this page.
Where the rainfall figures come from
The calculator starts with Austin, Texas averages. To use your own area, click Fill in rainfall for my area. It uses the approximate location of your internet connection (city level, worked out by Cloudflare, our host, from your IP address), so there is no permission prompt. Use my precise location asks your browser for your position instead, and you can also type a latitude and longitude. Nothing is fetched until you click.
The 12 months come from NASA POWER, a free NASA service that publishes long-term climate averages for any point on Earth. We use its monthly precipitation climatology for January 2001 to December 2020 (the “corrected precipitation” parameter, PRECTOTCORR, from the MERRA-2 reanalysis). It is given in mm per day, and the calculator multiplies by the days in each month. The point is rounded to 0.1° (about 11 km) first. The values fill the months under “Edit monthly rainfall”, where you can change any of them.
We picked NASA POWER because it covers the whole world, needs no key, is US government data with no fee or licence restriction on use, and answers in a fraction of a second with ready-made monthly averages. NOAA's Climate Normals are better for the US (they come from real rain gauges) but cover only the US. Open-Meteo's ERA5 history is global too, but it would mean downloading 30 years of daily values per lookup, and its free service is only for non-commercial sites.
Gridded averages are smoothed over an area about 50 km wide, so they can differ from a nearby rain gauge, most of all near coasts and hills. We compared them with official averages:
| Place | NASA POWER, 2001–2020 | Official average | Source of the official average | Difference |
|---|---|---|---|---|
| Austin, Texas | 839 mm (33.0 in) | 921 mm (36.3 in) | NOAA NCEI, Austin Camp Mabry, 1991–2020 | -9% |
| London, UK | 709 mm (27.9 in) | 616 mm (24.3 in) | Met Office, Heathrow, 1991–2020 (from the station’s monthly data) | +15% |
| Sydney, Australia | 888 mm (35.0 in) | 1,213 mm (47.8 in) | Bureau of Meteorology, Observatory Hill, 1858–2020 | -27% |
| Ho Chi Minh City, Vietnam | 1,373 mm (54.1 in) | 1,653 mm (65.1 in) | Viet Nam Meteorological and Hydrological Administration via WMO, 1991–2020 | -17% |
So for Sydney the grid is about a quarter too dry, which makes the tank look bigger than it needs to be. If your national weather service publishes monthly averages for a station near you, type those in instead.
Monthly rainfall and evapotranspiration
To type your own monthly averages, open “Edit monthly rainfall”. In the US, NOAA's Climate Normals list monthly averages for thousands of stations. In other countries, use your national weather service's climate averages. You can paste a row of 12 numbers into January and they fill the whole year. Average years overstate what you can count on. The Texas Water Development Board's manual gives Dallas as an example: about 35.0 in average annual rainfall, but only 29.3 in when the monthly medians are added up. For a more cautious design, enter median or dry-year months.
The garden demand uses monthly reference evapotranspiration (ET), the water a well-watered reference grass loses in that month. It is in the collapsed “Garden water need” box. The defaults are Austin, Texas averages from Texas A&M AgriLife:
| Month | Rainfall | Reference ET |
|---|---|---|
| Jan | 2.21 in5.6 cm | 2 in5.1 cm |
| Feb | 2.02 in5.1 cm | 2.66 in6.8 cm |
| Mar | 2.36 in6 cm | 4.3 in10.9 cm |
| Apr | 2.63 in6.7 cm | 5.27 in13.4 cm |
| May | 5.12 in13 cm | 7.55 in19.2 cm |
| Jun | 3.42 in8.7 cm | 8.28 in21 cm |
| Jul | 2.03 in5.2 cm | 8.12 in20.6 cm |
| Aug | 2.51 in6.4 cm | 8.2 in20.8 cm |
| Sep | 2.88 in7.3 cm | 6.22 in15.8 cm |
| Oct | 3.99 in10.1 cm | 4.93 in12.5 cm |
| Nov | 3.02 in7.7 cm | 3.08 in7.8 cm |
| Dec | 2.53 in6.4 cm | 2.08 in5.3 cm |
How the tank size is worked out
The calculator follows Texas A&M AgriLife's monthly “checkbook” method, which FEMP's rainwater guidance also describes:
- Supply = rainfall × 0.623 × catchment area × collection efficiency. One inch of rain on 1 ft² is 0.623 gal. In metric, 1 mm on 1 m² is 1 L.
- Demand = indoor use (people × litres or gallons per person per day × days in the month) + garden use (ET × plant water use coefficient × 0.623 × garden area).
- Balance: each month, the tank gains the supply and loses the demand. It can't go below empty (other water makes up the shortfall) or above full (the rest overflows).
- Storage from the balance: if the roof collects at least what you use in a year, this is the largest running deficit, the water you must have saved before the dry months. If it collects less, it is the most the tank would ever hold, which AgriLife calls the maximum cumulative storage. Both are worked out over two repeated years.
- Recommended storage = the larger of that and the dry-spell buffer, rounded up to standard tanks.
Collection efficiency covers first-flush losses, splash-over and evaporation. FEMP gives 0.75–0.9 as typical. AgriLife's runoff coefficients:
| Surface | Low | High |
|---|---|---|
| Roof: metal, gravel, asphalt shingle | 0.75 | 0.95 |
| Paving: concrete, asphalt | 0.70 | 0.95 |
| Paving: brick | 0.70 | 0.85 |
| Gravel | 0.25 | 0.70 |
Plant water use coefficients (AgriLife): low 0.2 (blue grama, desert willow), medium 0.5 (buffalograss, bermudagrass), high 0.75 (St. Augustine, fescue). For indoor use, the Water Research Foundation's Residential End Uses of Water study (2016) found toilets average 14.2 gal53.8 L and clothes washers 9.6 gal36.3 L per person per day in the US homes it measured. Those figures are the presets.
The dry-spell buffer
Average months hide dry spells. The buffer keeps enough water for a set number of days at the daily use of your highest-demand month, and the recommendation is the larger of the buffer and the monthly-balance storage. The Texas Water Development Board describes a simple rule popular with installers: size storage for about three months without rain, which is annual demand ÷ 4. To follow it, set the buffer to 90 days. That gives more security but costs more. If your roof collects less than you use, the buffer is capped at the most the tank could fill in an average year.
Standard tank and barrel sizes
The calculator rounds up to the tank sizes below, using one tank or up to four of one size, whichever gives the least total capacity. Rain barrels and water butts are the small end. Linking several barrels works for small roofs but gets unwieldy past a few hundred gallons.
| Size | Type | Diameter × height (example) |
|---|---|---|
| 55 gal | Rain barrel (drum) | varies |
| 165 gal | Poly tank | 31 × 56 in |
| 305 gal | Poly tank | 46 × 50 in |
| 550 gal | Poly tank | 67 × 44 in |
| 1,100 gal | Poly tank | 87 × 51 in |
| 1,550 gal | Poly tank | 87 × 67 in |
| 2,500 gal | Poly tank | 95 × 91 in |
| 3,000 gal | Poly tank | 95 × 107 in |
| 5,000 gal | Poly tank | 102 × 152 in |
| 10,000 gal | Poly tank | 141 × 160 in |
| Size | Type | Diameter × height (example) |
|---|---|---|
| 210 L | Water butt | varies by maker |
| 1,000 L | Round poly tank | varies by maker |
| 2,000 L | Round poly tank | varies by maker |
| 3,000 L | Round poly tank | varies by maker |
| 5,000 L | Round poly tank | varies by maker |
| 10,000 L | Round poly tank | varies by maker |
| 22,500 L | Round poly tank | varies by maker |
A full tank is heavy: 1,100 gal of water weighs about 9,200 lb, and 5,000 L weighs 5 tonnes, plus the tank. Check what the base and ground can carry.
Worked examples
Austin house, toilets for 2 people and a 500 ft² gardenAustin house, toilets for 2 people and a 46.5 m² garden
Roof catchment area 1,500 ft² · Collection efficiency 85 % · People in the household 2 · Indoor rainwater use per person per day 14.2 gal · Garden area watered from the tank 500 ft² · Plant water use Medium (0.5)Roof catchment area 139.35 m² · Collection efficiency 85 % · People in the household 2 · Indoor rainwater use per person per day 53.8 L · Garden area watered from the tank 46.45 m² · Plant water use Medium (0.5)
- Recommended storage: 2,142 galRecommended storage: 8,110 L
- Standard tank to buy: 2 × 1,100 gal tanksStandard tank to buy: 3 × 3,000 L tanks
- Storage from the monthly balance: 696 galStorage from the monthly balance: 2,634 L
- Rainwater collected per year: 27,596 galRainwater collected per year: 104,461 L
- Water demand per year: 20,143 galWater demand per year: 76,249 L
Rainfall and ET: Austin averages.
Same house, toilets and laundry
Roof catchment area 1,500 ft² · Collection efficiency 85 % · People in the household 2 · Indoor rainwater use per person per day 23.8 gal · Garden area watered from the tank 500 ft² · Plant water use Medium (0.5)Roof catchment area 139.35 m² · Collection efficiency 85 % · People in the household 2 · Indoor rainwater use per person per day 90.1 L · Garden area watered from the tank 46.45 m² · Plant water use Medium (0.5)
- Recommended storage: 2,718 galRecommended storage: 10,290 L
- Standard tank to buy: 1 × 3,000 gal tankStandard tank to buy: 4 × 3,000 L tanks
- Demand met by rainwater (with 1 × 3,000 gal tank): 100%Demand met by rainwater (with 4 × 3,000 L tanks): 100%
- Other water needed per year: 0 galOther water needed per year: 0 L
Rainfall and ET: Austin averages.
Garden only: 1,000 ft² roof, 800 ft² of high-water-use plantsGarden only: 92.9 m² roof, 74.3 m² of high-water-use plants
Roof catchment area 1,000 ft² · Collection efficiency 85 % · People in the household 0 · Indoor rainwater use per person per day 14.2 gal · Garden area watered from the tank 800 ft² · Plant water use High (lawn, vegetables: 0.75)Roof catchment area 92.9 m² · Collection efficiency 85 % · People in the household 0 · Indoor rainwater use per person per day 53.8 L · Garden area watered from the tank 74.32 m² · Plant water use High (lawn, vegetables: 0.75)
- Recommended storage: 1,779 galRecommended storage: 6,736 L
- Standard tank to buy: 2 × 1,100 gal tanksStandard tank to buy: 4 × 2,000 L tanks
- Rainwater collected per year: 18,397 galRainwater collected per year: 69,641 L
- Water demand per year: 23,448 galWater demand per year: 88,759 L
- Other water needed per year: 5,051 galOther water needed per year: 19,119 L
Rainfall and ET: Austin averages. The roof collects less than the garden uses, so the tank is sized to hold the surplus from the wet months.
Testing a 550 gal tank for the first houseTesting a 2,082 L tank for the first house
Roof catchment area 1,500 ft² · Collection efficiency 85 % · People in the household 2 · Indoor rainwater use per person per day 14.2 gal · Garden area watered from the tank 500 ft² · Plant water use Medium (0.5) · Tank size to test 550 galRoof catchment area 139.35 m² · Collection efficiency 85 % · People in the household 2 · Indoor rainwater use per person per day 53.8 L · Garden area watered from the tank 46.45 m² · Plant water use Medium (0.5) · Tank size to test 2,082 L
- Demand met by rainwater (with a 550 gal tank): 99%Demand met by rainwater (with a 2,082 L tank): 99%
- Other water needed per year: 146 galOther water needed per year: 552 L
- Overflow per year: 7,598 galOverflow per year: 28,763 L
Rainfall and ET: Austin averages. In an average year the smaller tank still covers almost everything. The larger recommendation comes from the 30-day dry-spell buffer, which protects you in drier-than-average spells.
Common mistakes
- Using the sloped roof area. Rain falls over the footprint, so use the plan-view area.
- Counting roof that doesn't drain to the tank. Only the gutters and downspouts connected to it count.
- Designing for average rainfall only. Dry years happen. Use median months or a longer buffer if you rely on the tank.
- Forgetting the overflow. A full tank still receives water. Send the overflow somewhere safe, away from foundations.
- Plumbing rainwater indoors without checking the rules. Indoor non-potable systems often need permits, backflow protection and labelled pipes.
Checked against
Each case below is re-run against the calculator by our automated test suite.
| Case | Our result | Checked against | Result |
|---|---|---|---|
| 1,625 ft² roof, San Antonio monthly rainfall, runoff coefficient 0.9: rainwater per year | 30,013 gal | Texas A&M AgriLife: Rainwater Harvesting (supply worksheet example) | Match (AgriLife rounds the factor to 0.623; we use 0.6234) |
| 1,000 ft² roof, 1 in of rain every month, 100% efficiency: rainwater per year | 7,481 gal | Hand calculation | Match |
| 1,000 ft² roof, 2 in a month except 3 dry months, garden-only demand of 1,000 ft² at 1 in ET and coefficient 0.5: storage from the monthly balance | 935 gal | Hand calculation | Match |
| Case | Our result | Checked against | Result |
|---|---|---|---|
| 151 m² roof, San Antonio monthly rainfall, runoff coefficient 0.9: rainwater per year | 113,611 L | Texas A&M AgriLife: Rainwater Harvesting (supply worksheet example) | Match (AgriLife rounds the factor to 0.623; we use 0.6234) |
| 92.9 m² roof, 2.5 cm of rain every month, 100% efficiency: rainwater per year | 28,317 L | Hand calculation | Match |
| 92.9 m² roof, 5.1 cm a month except 3 dry months, garden-only demand of 92.9 m² at 2.5 cm ET and coefficient 0.5: storage from the monthly balance | 3,540 L | Hand calculation | Match |
Frequently asked questions
How much water can I collect from my roof?
About 0.62 gal per ft² of roof footprint per inch of rain (roughly 600 gal per inch on 1,000 ft²), times a collection efficiency of about 0.75–0.9. In metric, 1 mm on 1 m² is 1 L before losses.
What size rainwater tank do I need?
It depends on when it rains and when you use water. Fill in the monthly rainfall for your area (or type it) and your water use: the calculator runs a month-by-month balance and adds a dry-spell buffer.
Where does the rainfall data come from?
From NASA POWER: monthly averages for 2001 to 2020 at your point, from satellite and model data on a grid about 50 km wide. Your location is used only to fetch these figures and is not stored. A local rain gauge can differ, so you can edit any month.
Is rainwater safe to drink?
Not untreated. This calculator is for non-potable uses such as toilet flushing, laundry where allowed, and garden watering.
Do I need a permit for a rainwater tank?
Possibly. FEMP notes that rainwater harvesting systems may require a permit from local or state government. Ask your local building or water authority, especially before plumbing it indoors.
How many rain barrels do I need?
Divide the recommended storage by the barrel size, 55 gal (or 210 L for a water butt). The calculator shows the count, but past a few barrels one tank is usually simpler.
Sources & assumptions
The results are planning estimates, and the cost figures only use prices you enter. Product yields, densities and coverage rates vary, so check the label or ask your supplier, and follow local codes. Figures marked "approximate" or "rule of thumb" are general practice, not exact standards.
- Texas A&M AgriLife Extension: Rainwater Harvesting (supply, demand and storage worksheets)
- Texas A&M AgriLife: rainwater harvesting calculators
- Texas Water Development Board: The Texas Manual on Rainwater Harvesting (3rd edition)
- U.S. Department of Energy FEMP: Rainwater Harvesting Systems Technology Review
- U.S. Department of Energy FEMP: Rainwater Harvesting Tool Help Guide
- U.S. Department of Energy FEMP: Best Management Practice #14, Alternative Water Sources
- Water Research Foundation: Residential End Uses of Water, Version 2 (executive report)
- NOAA NCEI: U.S. Climate Normals
- NASA POWER: Prediction Of Worldwide Energy Resources (data access)
- NASA POWER: climatology API documentation
- Met Office: Heathrow historic station data
- Bureau of Meteorology: climate statistics, Sydney (Observatory Hill)
- WMO World Weather Information Service: Ho Chi Minh City climate
- BARR Plastics: rainwater storage tank sizes
- Polymaster: round rainwater tank range
See how we calculate and test our formulas and the changelog. Last updated 2026-10-08.
Reviewed by the MaterialWhiz Editorial Team. This estimate uses the assumptions shown in the calculator. Review the result against your supplier's product coverage and site conditions. Last reviewed: October 10, 2026. See how we calculate · Report a correction.