Rainwater harvesting is a smart, sustainable way to secure water for your home, garden, or livestock. In Tasmania, with its diverse climate, the amount of water you can collect from your roof depends on three key factors:
-
Your roof area (m²)
-
Your local average annual rainfall (mm)
-
Collection efficiency (how much of the rainfall you can actually capture)
The Rainwater Collection Formula
You can estimate your potential rainwater yield with a simple calculation:
Roof area (m²) × Annual rainfall (mm) × Runoff coefficient = Litres per year
-
Roof area: Use the footprint of your roof (not the slope length).
-
Annual rainfall: Check Bureau of Meteorology (BOM) data for your area.
-
Runoff coefficient: Typically 0.9 (90% efficiency) for a clean, pitched roof.
Example:
200 m² roof in Launceston (≈820 mm annual rainfall):
200 × 820 × 0.9 = 147,600 L per year
Average Rainfall Across Tasmania
Tasmania’s rainfall varies significantly depending on location:
| Location | Avg. Annual Rainfall (mm) | Example Yield from 200 m² Roof (L/year) |
|---|---|---|
| Hobart | 615 | 110,700 |
| Launceston | 820 | 147,600 |
| Devonport | 975 | 175,500 |
| Burnie | 1,070 | 192,600 |
| Queenstown | 2,400 | 432,000 |
| Smithton | 1,050 | 189,000 |
| Ulverstone | 980 | 176,400 |
| St Helens | 750 | 135,000 |
| Swansea | 600 | 108,000 |
| New Norfolk | 540 | 97,200 |
(calculations assume a 200 m² roof and 0.9 efficiency)
This variation shows why tank sizing should always consider your local rainfall and household demand.
How Much Storage Do You Need?
It’s one thing to know how much water you can collect — but how much do you need to store? That depends on your usage and whether you’re relying solely on tank water or supplementing mains.
Household Use (Family of 4, Solely on Tank Water)
Average household water use in Australia is around 150–200 L per person per day.
For a family of 4 relying solely on tank water:
-
Daily use: ~600–800 L
-
Weekly use: ~4,200–5,600 L
-
Monthly use: ~18,000–24,000 L
Storage examples using Tankworld Tasmania’s tanks:
-
590 L – 2,000 L tanks: Suitable only as header tanks or for small, short-term needs.
-
10,000 L tank: ~2–3 weeks’ supply for a household of 4.
-
14,100 L tank: ~3 weeks’ supply.
-
24,000 L tank: ~1 month’s supply.
-
48,000 L (2 × 24,000 L tanks): ~2 months’ supply.
-
72,000 L (3 × 24,000 L tanks): ~3 months’ supply.
Garden Use Only
Garden water needs are highly seasonal. A typical vegetable garden might need 25 mm per week during summer.
-
Small garden (20 m²): ~500 L per week → a 590 L tank is enough for topping up.
-
Medium garden (50 m²): ~1,250 L per week → a 2,000 L tank provides a 1–2 week buffer.
-
Large garden (100 m²): ~2,500 L per week → a 10,000 L tank offers a month of buffer.
For gardens, a 10,000 L to 14,100 L tank is ideal for most properties, with 24,000 L or multiple tanks suited to larger orchards or livestock watering.
Daily, Weekly, and Monthly Breakdown
| Usage Scenario | Daily Use | Weekly Use | Monthly Use | Suitable Tank Sizes (Tankworld) |
|---|---|---|---|---|
| Household of 4 (sole use) | 600–800 L | 4,200–5,600 L | 18,000–24,000 L | 14,100 L (short-term), 24,000 L (monthly), multiples for 2–3 months |
| Small garden (20 m²) | ~70 L | 500 L | 2,000 L | 590 L (short-term), 2,000 L |
| Medium garden (50 m²) | ~180 L | 1,250 L | 5,000 L | 2,000 L (short-term), 10,000 L |
| Large garden (100 m²) | ~350 L | 2,500 L | 10,000 L | 10,000 L, 14,100 L, 24,000 L |
Putting It Together
-
Step 1: Work out your potential collection using roof size × rainfall.
-
Step 2: Estimate your usage (household or garden).
-
Step 3: Choose a storage size that gives you a buffer for dry months.
For most Tasmanian households, a 24,000 L poly water tank is a strong starting point. From there, multiples can be added:
-
2 × 24,000 L = 48,000 L (~2 months for a family of 4)
-
3 × 24,000 L = 72,000 L (~3 months for a family of 4)
Talk to the Local Experts
At Tankworld Tasmania, we manufacture and supply tanks from 590 L right up to 24,000 L, with the option of combining multiple tanks for larger capacity. With over 30 years of experience in Tasmanian conditions, we can help you choose the right size and setup for your property.
Contact us today to size the perfect rainwater tank system for your home, garden, or farm.