The Big Mistakes People Make When Choosing a Water Tank Size

Choosing the best water tank for you should be a straightforward decision — but in reality, it’s one of the most common areas where homeowners, farmers and builders get it wrong.

Too small, and the tank runs dry just when you need it most.
Too large, and you’ve paid for capacity you’ll never fully use.
Poorly placed, and it becomes a daily frustration instead of a helpful asset.

At The Water Tank Factory, one of the most common conversations we have is with customers who say:
“I wish I’d gone bigger,” or “I didn’t realise how much water we’d actually use.”

The good news? Most sizing mistakes are easy to avoid once you know what to look for. In this guide, we’ll walk through the biggest water tank sizing mistakes people make — and how to choose a tank that truly suits your property, lifestyle and local conditions.

This is by far the most common issue. Many people size their tank based only on current usage, not future needs. But water use tends to increase over time, especially when people realise how convenient stored rainwater is.

Common water uses people forget to factor in include:

• Increased garden irrigation during hot summers
• Lawn watering
• Washing cars, boats or caravans
• Filling ponds or water features
• Extra guests or growing families
• Livestock or hobby animals
• Fire-fighting reserve storage

Once a tank is installed, people often become more comfortable using water — which means a tank that once seemed “big enough” suddenly isn’t.
How to avoid it:
Choose a tank that allows for growth, not just today’s needs. Where space allows, upsizing usually delivers better long-term value.

Your roof is your water source — and if it’s not factored in correctly, tank sizing can miss the mark entirely. Two homes in the same street can collect vastly different amounts of rainwater depending on:

• Roof size
• Roof pitch
• Roofing material
• Number of downpipes connected to the tank

A small roof feeding a huge tank may never fill it efficiently.
A large roof feeding a small tank may cause constant overflow and wasted water.

How to avoid it:
Match your tank size to your roof catchment area. Larger roofs benefit from larger or multiple tanks, while smaller roofs may be better paired with modest capacity but efficient storage.
This is where multiple linked tanks — a specialty of The Water Tank Factory — can provide flexibility without overcommitting upfront.

Rainfall is not evenly spread across Australia — and even within the same state, seasonal patterns can vary dramatically.
Some regions receive:

• Heavy summer downpours followed by long dry periods
• Steady year-round rainfall
• Long dry winters with short wet seasons

People often assume:
“It rains enough here — I don’t need a big tank.” But the timing of rainfall matters just as much as the total amount.
How to avoid it:
Consider not just how much rain falls annually, but when it falls. In regions with dry seasons, larger storage allows you to bridge the gap between rain events.

It’s tempting to choose the cheapest tank that meets your minimum size requirement — but this often leads to regret.
Smaller tanks may:

• Empty faster
• Require frequent top-ups
• Limit water usage flexibility
• Provide no buffer during dry spells

Additionally, cheap tanks often compromise on:

• Wall thickness
• UV stability
• Long-term durability

How to avoid it:
Think of your tank as a long-term asset, not a short-term purchase. A properly sized, Australian-made tank with superior shot weight and durability (like those from The Water Tank Factory) will deliver far greater value over time.

Tank size isn’t just about volume — it’s also about how the water will be used.
Common oversights include:

• Not allowing enough storage for internal household use
• Assuming gravity feed will be sufficient
• Not planning pump capacity properly
• Forgetting future plumbing connections (toilets, laundry, irrigation)

A tank that’s too small may struggle to maintain consistent pressure for multiple outlets.
How to avoid it:
Plan your plumbing layout at the same time as your tank size. If your tank will supply multiple fixtures, larger capacity helps maintain reliable pressure and pump performance.

Sometimes the mistake isn’t choosing a tank that’s too small — it’s choosing one that doesn’t suit the space.
This often happens when:

• Tanks block access paths
• Tanks dominate small yard
• Tanks are placed where future extensions are planned
• Space constraints limit usable capacity

How to avoid it:
Size isn’t just litres — it’s shape.
The Water Tank Factory offers:

• Slimline tanks for narrow spaces
• Underdeck tanks for raised homes
• Underground tanks for invisible storage
• Round tanks for maximum capacity where space allows

Often, choosing the right shape allows you to go bigger without losing space.

In many parts of Australia, water tanks play a vital role in emergency preparedness. Even households connected to town water can lose pressure during:

• Bushfires
• Power outages
• Storm events

A tank sized only for daily use may not provide enough reserve in emergencies.
How to avoid it:
If you live in a bushfire-prone or semi-rural area, consider allocating dedicated emergency storage — either through a larger tank or a separate reserve tank.

Gardens consume more water than people expect — especially during hot summers.
Common surprises include:

• Vegetable gardens needing daily watering
• Fruit trees requiring deep irrigation
• Raised garden beds drying out quickly
• New landscaping increasing water demand

Many homeowners install tanks after creating gardens, only to realise the tank is undersized.
How to avoid it:
Factor garden size and future plans into your tank choice. If food growing or landscaping is part of your lifestyle, larger capacity provides freedom and flexibility.

Technically, you can always add another tank later — but in practice, it’s often more expensive and inconvenient.
Challenges include:

• Limited space after landscaping
• Plumbing reconfiguration
• Additional delivery and installation costs
• Council or access constraints

How to avoid it:
Where possible, choose a size that meets both current and future needs from the start — or plan for modular expansion using linked tank systems.

Water tank sizing isn’t guesswork — it’s a combination of climate, roof area, usage patterns, space and future plans. Many mistakes happen simply because people don’t ask the right questions early.

How to avoid it:
Speak with specialists who understand Australian conditions, not generic sellers. The Water Tank Factory team works with homeowners, farmers and builders every day and can help you choose a size that actually works.

A well-sized tank:

• Reduces water bills
• Increases water independence
• Supports gardens and livestock
• Improves emergency resilience
• Delivers long-term value
• Makes daily life easier

And because tanks are long-term investments, getting it right the first time matters.

Choosing the right water tank size isn’t about buying the biggest tank possible — it’s about choosing the smartest size for your property, climate and lifestyle.

Most people don’t regret going bigger.
But many regret going too small.

With the largest range of tank sizes and shapes in Australia, Australian-made construction, the highest shot weight in the market and decades of real-world experience, The Water Tank Factory helps customers avoid costly mistakes and choose water tanks that truly work.

If you’re unsure about sizing, layout or future needs, call The Water Tank Factory on 1300 826 523 or jump online to get expert advice before you buy.