How Long Will Collected Rainwater Last for Garden Watering

how long will collected rainwater last for garden watering? For the worked example, an 80 square metre metal roof in an 800 millimetre rainfall area collects about 54,700 litres a year, which covers 109 weeks at 500 litres a week if you could store it all. Weeks covered is a volume figure, not a freshness figure.

How Long Will Collected Rainwater Last for Your Roof

Your answer changes with roof size, rainfall, efficiency and weekly use. The answer is a simple division. Take the litres your roof can collect in a year and divide that total by the litres your garden uses in a week. The result is weeks covered. Double the weekly use and weeks covered are cut in half.

The calculator on this site is the Rainwater Harvesting Calculator, and it asks for roof size, roof surface type, annual rainfall, efficiency and garden weekly water use. It then gives water collected per year, water collected per storm and weeks of garden watering covered.

What the Calculator Counts

The formula on the page is collectable water equals roof area multiplied by rainfall multiplied by runoff coefficient multiplied by efficiency. In metric units, 1 millimetre of rain on 1 square metre of roof is 1 litre.

Runoff coefficient accounts for water that splashes, evaporates or soaks into the roof material. Efficiency accounts for gutter losses and first flush losses, and the page gives about 90 percent for efficiency. The page does not state any runoff coefficient value. The value 0.95 for a metal roof is derived from the worked example arithmetic, not a page stated figure. 54,700 divided by 80, 800 and 0.9 gives 0.9497, which rounds to 0.95. The two loss factors together are 0.95 multiplied by 0.9, which is 0.855.

The Worked Example in Full

The worked example on the page is an 80 square metre metal roof in an 800 millimetre rainfall area, at 90 percent efficiency. The page states about 54,700 litres a year, about 1,710 litres from a single 25 millimetre storm, and 109 weeks of watering at 500 litres a week.

Recompute the yearly total. Roof area 80 multiplied by rainfall 800 gives 64,000. Multiply by the derived metal coefficient 0.95 to get 60,800, then multiply by efficiency 0.9 to get 54,720 litres, stated on the page as about 54,700 litres. For one storm, 80 multiplied by 25 gives 2,000, multiplied by 0.95 gives 1,900, multiplied by 0.9 gives 1,710 litres. For weeks, 54,700 divided by 500 gives 109.4 weeks, stated as 109 weeks rounded down to whole weeks.

Yearly Totals by Roof Size and Rainfall

The table below gives yearly collectable litres for three roof sizes and three rainfall levels, using the formula and the derived metal coefficient 0.95 with 90 percent efficiency. Every total is roof area multiplied by rainfall multiplied by 0.95 multiplied by 0.9.

Roof size	Rainfall 500 mm	Rainfall 800 mm	Rainfall 1200 mm
50 square metres	21375 litres	34200 litres	51300 litres
80 square metres	34200 litres	54720 litres	82080 litres
120 square metres	51300 litres	82080 litres	123120 litres

The totals scale in direct proportion to each input. A 50 square metre roof at 1,200 millimetres collects 51,300 litres, and a 120 square metre roof at 500 millimetres also collects 51,300 litres, because the product of area and rainfall is the same. Find the cell closest to your own roof and rainfall, then carry that yearly total into the weeks table below.

Weeks Covered at Three Weekly Uses

The table below turns the yearly totals above into weeks covered, at weekly uses of 250, 500 and 1,000 litres a week. Each cell is the yearly total divided by the weekly use, rounded down to whole weeks.

Roof size and rainfall	250 litres a week	500 litres a week	1000 litres a week
50 square metres, 500 mm	85 weeks	42 weeks	21 weeks
50 square metres, 800 mm	136 weeks	68 weeks	34 weeks
50 square metres, 1200 mm	205 weeks	102 weeks	51 weeks
80 square metres, 500 mm	136 weeks	68 weeks	34 weeks
80 square metres, 800 mm	218 weeks	109 weeks	54 weeks
80 square metres, 1200 mm	328 weeks	164 weeks	82 weeks
120 square metres, 500 mm	205 weeks	102 weeks	51 weeks
120 square metres, 800 mm	328 weeks	164 weeks	82 weeks
120 square metres, 1200 mm	492 weeks	246 weeks	123 weeks

A small roof in a dry area is the tightest case. At 50 square metres and 500 millimetres, the yearly total of 21,375 litres covers 85 weeks at 250 litres a week, 42 weeks at 500 litres a week, and only 21 weeks at 1,000 litres a week.

A large roof in a wet area is the most generous case. At 120 square metres and 1,200 millimetres, the yearly total covers 492 weeks at 250 litres a week, 246 weeks at 500 litres a week, and 123 weeks at 1,000 litres a week. Read those as a yearly total divided by weekly use, not as water stored for 492 weeks. Plan weekly use with a garden watering schedule calculator and use a real figure, not a guess.

Imperial Rounded Equivalents

Metric and imperial examples are kept separate in this guide, and the imperial figures here are rounded equivalents, not exact conversions. The page gives the imperial factor as roughly 0.62 gallons per square foot per inch, minus losses. The table below applies that factor with the derived metal coefficient 0.95 and 90 percent efficiency.

Roof size	Rainfall 20 inches	Rainfall 32 inches	Rainfall 48 inches
1000 square feet	10602 gallons	16963 gallons	25445 gallons
1500 square feet	15903 gallons	25445 gallons	38167 gallons
2000 square feet	21204 gallons	33926 gallons	50890 gallons

An 80 square metre roof is about 861 square feet, and 800 millimetres is about 31.5 inches. Using the imperial method for those figures gives about 14,377 gallons, while the metric worked example of 54,720 litres is about 14,456 gallons. The gap is rounding. Keep one set of units through a whole calculation, and use the other set only as a rounded check.

One Storm and a Few Weeks

The storm is fixed at 25 millimetres, which is 1 inch. The table below gives the collectable water from that one fixed storm, with the derived metal coefficient 0.95 and 90 percent efficiency, and the weeks it covers at 500 litres a week.

Roof size	Water from one 25 mm storm	Weeks at 500 litres a week
50 square metres	1069 litres	2 weeks
80 square metres	1710 litres	3 weeks
120 square metres	2565 litres	5 weeks

One storm on an 80 square metre roof gives 1,710 litres, about 3.4 weeks at 500 litres a week, rounded down to 3 weeks. On a 120 square metre roof it gives 2,565 litres, about 5 weeks, and on a 50 square metre roof it gives 1,069 litres, about 2 weeks. This is also the scale the page uses for tank sizing: a tank should hold enough for a few weeks of garden use, or enough to capture one typical storm. The page gives no specific tank size in litres, so this guide states no tank size either. See the rain barrel calculator on this site for the barrel side of the same question.

The Storage Catch

Rain falls unevenly through the year, and storage size decides how much you actually use. A yearly total of 54,700 litres divided by 500 litres a week gives 109 weeks, but that division assumes you can store all the water that falls and use it evenly.

That is why 109 weeks is a yearly total divided by weekly use, not water sitting in a tank for 109 weeks. The page suggests a tank for a few weeks of garden use, or to capture one typical storm. Use the weeks table as a ceiling, not as a stored amount.

What Changes the Answer

Four factors change the answer: roof size, rainfall, efficiency and weekly use.

Roof size is the first factor. The calculator asks for roof footprint, the ground area under the roof, not the sloping surface area. Double the footprint and the yearly total doubles.

Rainfall is the second factor. Double the rainfall at the same roof size and the yearly total doubles. Use a rainfall figure for your area, not a figure from a wetter place.

Efficiency is the third factor, covering gutter loss and first flush loss, given as about 90 percent.

Weekly use is the fourth factor, and it is the one you control most directly. At 80 square metres and 800 millimetres, 250 litres a week gives 218 weeks, 500 litres a week gives 109 weeks, and 1,000 litres a week gives 54 weeks if you could store it all. An irrigation scheduling calculator can help set a watering plan.

Common Mistakes

The most common mistake is reading weeks covered as weeks stored. It is a yearly total divided by weekly use.

The second mistake is using the sloping roof surface instead of the footprint, the ground area under the roof.

The third mistake is forgetting losses. In the worked example the loss factors take the total from 64,000 litres to 54,720 litres, so leaving them out overstates the answer.

The fourth mistake is treating the derived coefficient 0.95 as a page stated figure for every roof. It is derived for a metal roof only.

The fifth mistake is mixing metric and imperial figures in one calculation. Pick one set of units and keep it through the whole calculation.

The sixth mistake is using a variable storm depth as if the calculator gave it. The per storm figure uses a fixed 25 millimetre storm.

How to Work Out Your Own Number

Start with your roof footprint in square metres and your annual rainfall in millimetres. Multiply the two together, then multiply by the runoff coefficient for your roof and by efficiency. For a metal roof, use the derived coefficient 0.95 and efficiency 0.9, and label 0.95 as derived, not page stated.

Next, divide that yearly total by your weekly garden water use in litres a week. The result is weeks covered, rounded down to whole weeks. If you do not know your weekly use, work it out from your watering plan first.

Then check the answer against the storage catch. Ask how many weeks of water your storage can actually hold at one time. Finally, test the answer by changing one input at a time. Try a lower rainfall year, a higher weekly use, and a lower efficiency, and see how far weeks covered fall.

Frequently Asked Questions

Q: How long will collected rainwater last at 500 litres a week?
A: It depends on roof size and rainfall. For the worked example, an 80 square metre metal roof in an 800 millimetre rainfall area collects about 54,700 litres a year, covering 109 weeks at 500 litres a week if you could store it all.

Q: How many weeks does one storm cover?
A: One fixed 25 millimetre storm on an 80 square metre metal roof collects 1,710 litres, about 3.4 weeks at 500 litres a week.

Q: Does 109 weeks mean water stays in a tank for 109 weeks?
A: No. It is a yearly total divided by weekly use. Rain falls unevenly, and storage size decides how much you actually use.

Q: What roof and rainfall numbers do I need?
A: You need roof footprint, annual rainfall, and weekly garden water use.

Q: Can I use the same maths in gallons?
A: Yes, with the page factor of roughly 0.62 gallons per square foot per inch, minus losses.

Q: Why does efficiency change the answer so much?
A: Efficiency applies to the whole yearly total. In the worked example the two factors together are 0.95 multiplied by 0.9, which is 0.855.