03 July 2025

How much rainwater can you harvest from your roof?

Your Complete Guide to Rainwater Collection and Storage

Thinking about installing a water tank to harvest rainwater? Whether you’re a homeowner, business owner, or manage a golf course, understanding how much rainwater you can collect—and how to make the most of it—can help you save money, boost water security, and support sustainability goals.

Use our handy rainwater harvesting calculator below to work out how much rainwater you could potentially harvest each year. Whether you are thinking of installing a rainwater tank at home and need to know what size you will need, or you simply want to work out how much money you could save each year, our calculator will work out how many litres of water you could collect each year based on where you live in New Zealand and the size of your roof

Rainwater Harvesting Calculator

Why Rainwater Harvesting Matters

Rainwater harvesting is the process of collecting and storing rainwater from surfaces like roofs for later use. It’s an effective way to supplement your water supply, reduce reliance on mains water, and lower your environmental impact. With water scarcity and climate variability on the rise, capturing rainwater is becoming increasingly important for water security and resilience.

How Much Rainwater Can You Collect?

The amount of rainwater you can harvest depends on three main factors:

  • Roof Area: The larger your roof, the more rain you can collect.
  • Annual Rainfall: Local rainfall patterns determine your collection potential.
  • Collection Efficiency: Most systems collect about 90% of the rain that falls on your roof.

Rainwater Harvesting Formula

Harvested Rainwater (litres)=Roof Area (m2)×Annual Rainfall (mm)×0.9Harvested Rainwater (litres)=Roof Area (m2)×Annual Rainfall (mm)×0.9

For example, a 150 m² roof in a region with 1,000 mm of annual rainfall could yield up to 135,000 litres per year.

Tip: Use our calculator above to get a personalised estimate for your property!

Why Collect Rainwater in Winter?

Most rainfall occurs during the winter months, but water demand peaks in summer—especially for gardens, landscapes, and golf courses. By collecting and storing rainwater in winter, you ensure you have a reserve for the dry months, when restrictions and shortages are most likely.

  • Storage bridges the gap: Tanks and underground cisterns let you save winter rain for summer use.
  • Space-efficient alternative: For large properties or golf courses, modular tanks or underground systems are a practical alternative to building ponds or lakes, which can be costly and prone to evaporation.

Who Benefits from Rainwater Harvesting?

Audience Key Benefits & Uses
Homeowners Garden irrigation, car washing, toilet flushing, laundry, backup supply
Businesses Landscape irrigation, cooling towers, cleaning, toilet flushing, cost savings
Golf Courses Irrigation, green space maintenance, reduced need for large ponds or lakes

Beyond the Basics: Related Concepts

Explore more about water management and sustainability:

Practical Tips for Getting Started

  • Check local regulations: Some areas have rules about tank size, placement, and use.
  • Size your system: Use our calculator to find the right tank size for your needs.
  • Maintain your system: Clean gutters and filters regularly to ensure water quality.
  • Plan for overflow: Ensure excess water is safely directed away from buildings.

Real-World Examples

“After installing a 10,000-litre tank, we kept our garden green all summer without using mains water—even during restrictions.”

—Auckland homeowner

“Switching to rainwater for irrigation cut our water bills by 40% and kept the course playable through drought.”

—Golf course manager

What size water tank do I need?

Once you know how much water you can potentially harvest from your roof each year, you will have a much better idea of the size of the tank you need. Of course, a lot of this will also depend on consumption. How many people do you have at home? Do you need to water the garden in the summer months? Do you have more than one car at home that needs washing on a regular basis? All of these questions will help you to get a better idea of the size of the tank you will need around the home.

For more information, make sure you check out our handy guide – What size water tank do I need?

Measuring the water in your tank

If you already have a water tank installed at your home or property, then you may want to have a better idea of how much water you have stored at any given time. This can help you to plan and manage your water consumption at home, especially if your water tank is your only supply of water to your property.

At Smart Water, we set out to help Kiwis to monitor their water consumption more accurately when using tank water. Our latest Smart Water SW9000 series system is a culmination of over 10 years of intensive research and innovative design.

Our award-winning products provide state-of-the-art monitoring systems that not only provide you with an accurate reading of the water level in your tank but also work intuitively for the everyday user, providing you with all the information you need about your water consumption on an easy-to-read display or through our app.

You can read a lot more about measuring the water in your tank in a recent blog post.

Contact the team

If you want to accurately measure the level of water in your water tank, talk to the team today. With accurate water level readings, average water consumption data and accurate predictions about future usage, the Smart Water system will help you to manage your water tank levels all year round.

Ready to see how much rainwater you can harvest?

Scroll up and use our calculator now to start planning your sustainable water future!

Rainwater harvesting is a smart, sustainable choice for homes, businesses, and landscapes of all sizes. With the right tools and information, you can maximise your water savings and resilience year-round.