Rainfall harvesting calculator.
One millimeter of rain on one square metre of roof equals one litre
To calculate the approximate amount of rainwater you are likely to harvest, use the following formula:
Water volume (Litres) = average annual rainfall (millimetre)
x coefficient of runoff x roof area (metres2)
A coefficient of runoff of 0.9 can be used to obtain a rough estimate.
eg: 100 m2 x 550 mm rainfall x 0.9 = 49,500 litres. View Gable roof area calculation.
Refer to the anual rainfall chart for some Australian cities below to help make your calculations.
Australian cities, average rainfall shown in mm. Averaged over period of 1908–2004.
|
CITY
|
JAN
|
FEB
|
MAR
|
APR
|
MAY
|
JUN
|
JUL
|
AUG
|
SEP
|
OCT
|
NOV
|
DEC
|
TOTAL
|
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
|
Melbourne
|
48.2
|
47.0
|
50.6
|
58.2
|
56.6
|
49.8
|
47.9
|
50.3
|
58.7
|
67.4
|
59.3
|
59.1
|
653.2
|
|
Romsey
|
42.2
|
44.7
|
42.1
|
51.5
|
58.1
|
66.1
|
66.1
|
76.7
|
75.0
|
67.2
|
68.8
|
49.3
|
708.8
|
|
Albury
|
52.8
|
34.6
|
40.8
|
47.5
|
62.9
|
71.9
|
86.7
|
88.5
|
71.4
|
72.5
|
53.2
|
53.9
|
736.7
|
|
Bendigo
|
33.0
|
32.2
|
36.0
|
41.4
|
54.8
|
61.0
|
56.4
|
58.6
|
54.2
|
52.4
|
37.4
|
33.1
|
550.5
|
|
Canberra
|
59.8
|
51.2
|
55.6
|
49.3
|
47.5
|
37.9
|
52.4
|
47.6
|
65.2
|
61.9
|
58.7
|
46.0
|
633.1
|
|
Sydney
|
103.3
|
117.4
|
131.2
|
127.2
|
123.3
|
128.1
|
98.1
|
81.5
|
68.7
|
76.9
|
83.1
|
78.1
|
1217.0
|
|
Newcastle
|
91.4
|
105.6
|
121.9
|
115.9
|
118.6
|
117.8
|
97.2
|
76.2
|
73.7
|
74.3
|
69.5
|
82.4
|
1144.6
|
|
Wollongong
|
106.8
|
108.7
|
118.5
|
131.2
|
115.8
|
109.0
|
91.0
|
61.4
|
66.7
|
68.1
|
71.9
|
86.5
|
1135.6
|
|
Brisbane
|
159.6
|
158.3
|
140.7
|
92.5
|
73.7
|
67.8
|
56.5
|
45.9
|
45.7
|
75.4
|
97.0
|
133.3
|
1146.4
|
|
Cairns
|
386.6
|
453.0
|
420.3
|
197.7
|
93.2
|
46.5
|
28.7
|
27.9
|
34.6
|
39.6
|
94.4
|
180.4
|
2002.8
|
|
Townsville
|
262.8
|
297.5
|
187.8
|
67.6
|
34.6
|
19.6
|
13.0
|
16.5
|
10.1
|
25.6
|
59.3
|
127.0
|
1121.5
|
|
Gold Coast
|
175.3
|
190.0
|
202.0
|
135.8
|
131.5
|
93.0
|
74.6
|
55.8
|
57.9
|
86.7
|
103.8
|
132.1
|
1438.6
|
|
Adelaide
|
19.2
|
13.7
|
26.2
|
38.7
|
62.6
|
83.1
|
77.8
|
68.1
|
63.6
|
48.5
|
29.6
|
26.8
|
558.1
|
|
Mount Gambier
|
26.5
|
25.6
|
35.8
|
54.7
|
71.8
|
84.7
|
99.6
|
93.5
|
72.8
|
62.6
|
46.5
|
37.4
|
711.5
|
|
Port Lincoln
|
13.2
|
14.8
|
20.0
|
36.4
|
57.4
|
74.6
|
78.9
|
69.0
|
49.7
|
35.9
|
21.9
|
19.1
|
490.9
|
|
Perth
|
8.6
|
13.3
|
19.3
|
45.5
|
122.7
|
182.4
|
172.9
|
134.6
|
79.9
|
54.5
|
21.7
|
13.9
|
869.4
|
|
Albany
|
23.7
|
23.1
|
38.3
|
67.9
|
119.6
|
132.5
|
144.8
|
127.4
|
102.0
|
80.4
|
43.4
|
29.6
|
932.6
|
|
Kalgoolie
|
22.8
|
31.5
|
23.6
|
22.1
|
27.8
|
29.6
|
25.4
|
21.7
|
14.5
|
15.1
|
18.2
|
16.1
|
268.4
|
|
Broome
|
173.0
|
185.0
|
99.6
|
26.6
|
28.7
|
18.2
|
6.3
|
1.7
|
1.5
|
1.4
|
8.1
|
49.1
|
599.2
|
|
Darwin
|
423.3
|
361.1
|
319.3
|
98.9
|
26.5
|
2.0
|
1.4
|
5.7
|
15.4
|
70.7
|
141.8
|
247.9
|
1713.9
|
|
Alice Springs
|
38.0
|
44.1
|
32.3
|
17.3
|
20.0
|
14.5
|
14.0
|
9.8
|
8.6
|
21.6
|
27.5
|
38.1
|
285.7
|
|
Katherine
|
271.6
|
256.1
|
188.7
|
37.7
|
6.1
|
0.3
|
1.4
|
1.0
|
7.7
|
38.0
|
87.0
|
203.6
|
1099.2
|
|
Hobart
|
48.0
|
39.8
|
45.7
|
51.8
|
47.0
|
54.6
|
52.8
|
53.0
|
52.4
|
62.6
|
54.5
|
57.2
|
619.5
|
|
Devonport
|
44.8
|
37.8
|
48.1
|
62.5
|
77.8
|
82.9
|
99.9
|
90.9
|
76.3
|
67.0
|
57.0
|
52.5
|
797.4
|
|
Launceston
|
51.8
|
28.9
|
34.7
|
53.5
|
67.9
|
67.3
|
79.4
|
83.0
|
66.4
|
55.7
|
49.3
|
45.5
|
683.3
|
Rainfall information supplied by the Bureau of meteorology www.bom.gov.au
How much rain water can I collect from my roof ?
To calculate your possible rain water catchment from your roof, firstly measure up
your house as shown in the example below. Don’t forget the verandah, carport, garage
or any other area of roof that you may be able to use for collection.
For a gable roof, the area will be slightly more, due to the pitch, so multiply the area
by 1.3 for a steep pitched roof, 1.1 for an average pitched roof.
Area Measurements
2.4 x 2.4 = 5.76
7 x 4 = 28
14 x 4 = 56
roof area 89.76
x 1.1(roof pitch) =
total catchment area = 98.74 sq. m
View Rainwater Tank Size Calculator
