24 July 2025

What size water tank do I need?

Your Complete 2025 Guide to Choosing the Right Tank for Your Home

If you're thinking about installing a water tank at your home in New Zealand, Australia, or the US, you're not alone. As climate patterns shift and more regions face increasing water restrictions and dry spells, having a rainwater harvesting system has become a smart, sustainable, and often necessary solution.

Whether you're living off-grid, supplementing mains supply in the suburbs, or building a new home, one of the most common questions is: what size water tank do I need? In this guide, we’ll walk you through everything you need to consider, tailored for residential homeowners, so you can make an informed decision.

Why Install a Water Tank in 2025 and beyond?

The benefits of installing a water tank go far beyond saving on your water bill:

  • Environmental sustainability: Reduce reliance on municipal water supplies and contribute to water conservation.
  • Future-proofing your home: With NIWA forecasting a doubling of drought days across most New Zealand regions in the next 20 years, water tanks are quickly becoming essential. In Australia, the New South Wales government also predicts a growth in the number of drought days across the country. In the US, you can keep a track on current drought conditions on the USGS website.
  • Versatile use: Rainwater can be used for gardening, toilet flushing, laundry, and even drinking (when treated correctly).
  • Cost savings: Rainwater use can significantly reduce metered water charges, especially in Auckland, where water rates continue to rise.

According to Auckland Council, the average household can reduce its mains water use by up to 40% by using rainwater for non-potable purposes.

If you are still not sure if a water tank is for you, check out our recent post, Is it worth getting a water tank?  Where you will find lots of good advice about the benefits of installing a water tank as well as a look at some of the different options available.

Who Should Install a Water Tank?

Water tanks aren't just for rural or off-grid properties. Residential tanks are now common across suburban and semi-urban homes. You should consider installing a tank if:

  • You're off-grid or your property is far from a mains supply.
  • You're in an urban area but want to reduce water costs or future-proof against shortages.
  • You’re planning a new build, and want to meet council expectations or Green Star ratings.
  • You're committed to sustainable living.

In short, almost every residential property can benefit from a water tank, regardless of location.

What Size Water Tank Do You Need?

This depends on several key factors:

1. Your Household Water Usage

The average water usage per person in Auckland is 140–170 litres per day (Watercare). In Sydney, that number is estimated to be closer to 200 litres per day (Sydney Water) and in New York, the daily water consumption per person is estimated to be closer to 227-260 litres (New York City Environmental Protection)

Household Size Annual Water Usage (Approx.)
1 person 51,100 – 62,050 litres
2 people 102,200 – 124,100 litres
4 people 204,400 – 248,200 litres

If you're using rainwater as your sole supply, your tank needs to store enough to meet these requirements.

2. Roof Catchment Area

Rainwater is typically harvested from your roof. You can estimate your catchment area by calculating the roof footprint:

Roof Area (m²) = Length x Width

If you’re unsure, you can check property data on Homes.co.nz in New Zealand or your building plans.

3. Annual Rainfall in Your Area

Rainfall varies widely by region. In New Zealand, visit NIWA’s climate data to check your location.

For example:

  • Auckland: ~1,114mm/year
  • Wellington: ~1,200mm/year
  • Christchurch: ~650mm/year

4. Tank Volume Calculation

Formula:

Harvestable Water = Roof Area (m²) x Annual Rainfall (mm)

Example:

  • Roof size = 172m²
  • Rainfall (Auckland) = 1,114mm
  • 172 x 1,114 = 191,608 litres/year harvestable

5. Intended Water Use

Use Case Recommended Minimum Tank Size
Garden only 500L – 2,000L
Garden + Laundry/Toilets 5,000L – 10,000L
Full household use 20,000L+

Larger tanks offer a better buffer against dry spells. Opt for bigger where space allows, especially if you're off-grid.

Legal and Compliance Considerations

Before installing a water tank, check your local council’s regulations:

  • Consent requirements: Larger or elevated tanks may require building or resource consent.
  • Potable water use: If you plan to drink harvested rainwater, it must be filtered and treated to council standards.
  • Stormwater systems: Integration with existing infrastructure may also need approval.

Check Auckland Council’s water tank guidelines for more information.

Space and Aesthetic Considerations

One of the biggest barriers for homeowners is space:

  • Slimline tanks (e.g., 1,000L–3,000L) are ideal for urban homes and can be installed alongside walls or fences.
  • Underground tanks save space but involve higher installation costs.
  • Above-ground round tanks offer higher capacity but require room and can be visually intrusive.

If aesthetics matter, look into architecturally designed tanks or those that can be painted to match your home. You can read more about water tank screening in a recent post.

Beyond Size: Smart Water Monitoring

Once you install a tank, tracking your water levels and usage is crucial, especially in dry periods. That’s where Smart Water sensors come in.

Our award-winning tank monitors provide:

  • Real-time water level readings
  • App-based monitoring and usage trends
  • Leak detection and early alerts
  • Usage history for smarter planning

Read our guide on how to measure your tank water levels accurately.

Customer Testimonials

If you are considering investing in a Smart Water tank level indicator, here are some customer testimonials:

“I just wanted to let you know that your Smart Water solution has been a game changer for my family. We knew that it would be useful, but with the drought over both winter and summer last year, it enabled us to manage our water volumes very effectively. It was the first time in eight years that we didn't need to order in a tanker or two of additional water.” – David Hallet, Hamilton, New Zealand

“I have been a customer of Smart Water for 5 years and in this time I have been most impressed with their product / customer service. I live in a rural location and rely on this technology, should I require the Smart Water team we can always get hold of them, on the very rare occasion this has extended over the weekend. Smart Water is a very professional company to deal with, they fully understand the importance of true customer service. I have always recommended Smart Water Systems in the past and will continue to do so.” – Charlie Clark, Tauranga, New Zealand

What Makes Smart Water Different?

While many guides stop at "bigger is better," here's what truly sets a future-ready water tank system apart:

  • Modular systems: These allow you to expand capacity as your needs grow.
  • Integration with greywater systems: Reusing household wastewater for irrigation.
  • Solar-powered pump systems: Ideal for off-grid properties or those seeking energy independence.
  • Smartphone-connected ecosystems: Control and track everything from your phone.

Summary: Choosing the Right Tank Size in 2025 and Beyond

Think long-term. With changing climate patterns, rising costs, and increasing sustainability awareness, investing in the right-sized tank is more important than ever.

To recap:

  • Use your roof size and rainfall data to calculate harvestable water.
  • Estimate household needs based on occupancy and intended use.
  • Check space constraints and council regulations.
  • Choose smart tech to monitor and manage your water effectively.

Need Help?

Our team at Smart Water can help you size and manage your tank with precision. Whether you’re off-grid, retrofitting, or building new, we’ve got the tools and expertise to help you save water and stay informed.

FAQs

How long will a 10,000-litre tank last?

Depends on usage. For a 2-person household using rainwater for non-potable needs, it could last 1–2 months in dry periods.

Can I use rainwater for drinking?

Yes, but it must be filtered and treated according to council guidelines.

Do I need council consent for a water tank?

Consent may be required based on size, location, and connection to household plumbing. Always check with your local authority.

What’s the best tank material?

Polyethylene is common, affordable, and durable. Concrete and stainless steel offer longer lifespans and better thermal stability.

Glossary

  • Catchment area: The surface from which rainwater is collected, usually a roof.
  • Potable water: Water that is safe to drink.
  • Greywater: Gently used water from laundry, showers, and sinks.
  • Smart monitoring: Technology used to track and report water levels and usage.

For tailored advice and smart water monitoring systems, contact Smart Water today.