Guidance on use of rainwater tanks

B. Determining the required size of tank to be installed

Page last updated: March 2011

Maximum volume

The maximum amount of rainwater that can be collected can be calculated using the formula:

run-off (litres) = A x (rainfall – B) x roof area

‘A’ is the efficiency of collection and values of 0.80-0.85 (that is, 80-85% efficiency) have been used (Martin 1980).

‘B’ is the loss associated with absorption and wetting of surfaces and a value of 2 mm per month (24 mm per year) has been used (Martin 1980).
‘Rainfall’ should be expressed in mm and ‘roof area’ in square metres (m2).

For example, the run off from a 200m2 roof which receives 750 mm of rain per year, with an efficiency of collection of 80% (A = 0.80) and loss of 2 mm per month (B = 24) would be:

Run-off = 0.8 x (750-24) x 200 = 116,160 Litres (or 116.2 kL)


The maximum volumes of rainwater that can be collected from roofs of various areas and at a range of average annual rainfalls are shown in Table B1. If the maximum volumes are less than the annual water demand, then either the catchment area will need to be increased or water demand will need to be reduced.

Table B.1: Maximum volumes of water that can be collected depending on roof size and annual rainfall
Maximum volumes of rainwater per year (kl)*
Annual rainfall (A)
(mm)
Roof area (m2) (B)
100
150
200
250
300
400
500
150
10
15
20
25
30
40
50
200
13
21
27
35
42
53
70
250
18
27
36
45
54
72
90
300
22
33
44
55
66
88
110
400
30
45
60
75
90
120
150
500
38
57
76
95
114
152
191
600
46
69
92
115
138
184
230
800
62
93
124
155
186
248
310
1000
78
117
156
195
234
312
390
1200
94
141
188
235
282
377
470

*These volumes were calculated using a value of 0.8 for A and 24 mm for B.


Security of supply

Where a tank is the sole source of supply, determining the maximum volume of water that can be collected is only the first step to determining whether the available tank capacity provides adequate security of supply. The next step is to calculate the size of the tank needed to ensure the volume of water that is collected and stored will be sufficient to meet demand throughout the year, including during the drier months or through periods of low or no rainfall.

There are several mathematical models available for determining the size of tank needed to provide a defined level of security of supply. In some cases, state and territory government departments have used computer-based models to prepare tables of calculated required tank size (see Section 7).

The simplest way of checking the tank size that is estimated to provide sufficient water throughout an average year is to use monthly rainfall data and assume that at the start of the wetter months the tank is empty. The following formula should then be used for each month:

Vt = Vt-1 + (Run-off – Demand)
Vt = theoretical volume of water remaining in the tank at the end of the month.
Vt-1 = volume of water left in the tank from the previous month. Run-off should be calculated as discussed above (A = 0.8, B = 2 mm).

Starting with the tank empty then Vt-1 = 0. If, after any month, Vt exceeds the volume of the tank, then water will be lost to overflow. If Vt is ever a negative figure then this indicates that demand will exceed the available water. Providing the calculated annual run-off exceeds the annual water demand, Vt will only be negative if periodic overflows reduce the amount of water collected so it is less than the demand.

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Tank size is not necessarily based on collecting total roof run-off. For example, from Table B1 the maximum water that can be collected from a roof area of 200 m2, with an annual rainfall of 1000 mm, is about 156 kL. If the water demand is less than this, some overflow may occur while demand is still met. If water demand is to be met throughout the year, the tank should be large enough so that Vt is never negative.

Calculations should be repeated using various tank sizes until Vt is ≥0 at the end of every month. The greater the values of Vt over the whole year, the greater the security of meeting water demand when rainfalls are below average or when dry periods are longer than normal. The greater the security, the larger the size and cost of the tank.

For example, if the theoretical volume left in the tank from the previous month is 5000 L (5 kL), the annual run-off 116.2 kL and the annual demand is 98 kL, then the theoretical volume in the tank would be:

Vt = 5 + (116.2 – 98) = 23.2 kL

Table B.2: Tank sizes to provide 99% security of supply
Tank size (kilolitres)*
Volume required (L/day)
Annual rainfall (mm)
Roof area (m2)
100
150
200
300
400
500
600
60
150
200
300
400
500
600
900
14
8
6
5
20
8
6
5
4
12
7
5
4
3
24
10
43
100
200
250
300
400
500
600
900
1200
9
11
10
13
12
9
8
15
11
10
8
7
33
20
12
9
8
22
17


40
200
300
350
400
500
600
900
1200
34
29
23
36
23
19
28
26
18
16
40
30
24
22
16
14
29
26
22
20
14
47
26
24
20
18
400
500
600
700
900
1200
47
50
39
49
44
34
51
47
44
39
31

* The tank sizes shown were determined from summarised data provided by the South Australian Water Corporation and the Department of Water, Land and Biodiversity Conservation (SA). The original data were estimated using a computer simulation based on averaged rainfalls and rainfall patterns and using a value of 0.8 for A and 2 mm per month for B.

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Table B.3: Tank sizes to provide 90% security of supply
Tank size (kilolitres)*
Volume required (L/day)
Annual rainfall (mm)
Roof area (m2)
100
150
200
300
400
500
600
60
150
200
300
400
500
600
14
6
4
3
6
3

2
15
4
3
20
10

14
100
150
200
300
400
500
600
900
11
8
6
10
6
5
4
18
6
5
4
33
10
6
4
3
19
8

34
17


27



200
250
300
350
400
500
600
900
1200
26
18
25
13
10
20
15
10
8
26
19
12
10
7
6
29
17
14
10
8
26
20
13
11
8
7
21
17
12
10
400
350
500
600
700
900
1200
34
39
27
21
42
30
27
19
16
30
22
21
16
13
44
24
19
18
13
12
600
500
600
700
800
900
1200
50
37
50
43
28
47
40
34
24

* The tank sizes shown were determined from summarised data provided by the South Australian Water Corporation and the Department of Water, Land and Biodiversity Conservation (SA). The original data were estimated using a computer simulation based on averaged rainfalls and rainfall patterns and using a value of 0.8 for A and 2 mm per month for B.

Table B.4: Water demands per day, month or year
Water demands (litres)
Per day
Per month (30.5 days)
Per year (365 days)
60
1830
21900
100
3050
36500
150
4575
54800
200
6100
73000
300
9150
109500
400
12200
146000
500
15250
182500
600
18300
219000
800
24400
292000