What Size Rainwater Tank Do I Need?

When designing a to rainwater harvesting system one of the key considerations is determining the right size of the rainwater tank. To do this, you must first understand how much water you need and when. You then match that to your potential supply from the roof in order to find the right size water tank for your needs.

In this definitive guide, we discuss how to calculate your water usage, how much rainwater you can harvest based on the amount of rain you receive, how to manage risk of running low on water, and of course, how to calculate your rainwater storage requirement. We also touch on some key considerations to help you understand the range of rainwater tanks available and determine the best tank for you. 

Table of Contents

Understanding Rainwater Harvesting

What is Rainwater Harvesting?

Rainwater harvesting is the process of collecting and storing rainwater for later use. For most people, that means connection of roof guttering and downspouts to rainwater tanks or other storage systems. These systems often incorporate pumps to pressurise the system for your demand. Other considerations will be plumbing for scouring (or flushing) the tank, and disposal of any tank overflow. We have some great resources and articles that go into this in much greater detail. Have a look at our Rooftop Rainwater Harvesting page. There you will also see links to various relevant articles and our Rooftop Rainwater Harvesting Calculator – A useful tool that will help you design your system (more on that below).

Benefits of Rainwater Harvesting

Rainwater harvesting offers several significant benefits, all of which are aligned to our Every Drop Saved philosophy. These systems help to reduce your demand on the municipal (or utility) water supply, saving money on water bills and providing self-sufficiency. They are also an alternative water source during droughts or water restrictions, creating resilience in times of water scarcity.

How to Calculate the Best Tank Size

General Approach

To calculate the volume of water that must be stored, there are four main factors to consider:

1. Rainfall patterns. This varies considerably from place to place, and you’ll need to obtain data for your location. 
2. Catchment area. Typically, this will be your roof size in plan.
3. Water consumption. You can use a water usage calculator to assist with this. We can help with this!
4. Drought protection. How many days of drought protection do you need? That is, how many days of average daily demand do you need, to get you though no-rain periods, before you run out of water.

There is a fifth factor which is harder to quantify, but very important. This relates to risk. Are you prepared to run out of collected rainwater occasionally? If so you can get away with a smaller (cheaper) tank. You may need to stay connected to mains water. However, if you are fully self-sufficient and your family’s water is sourced entirely from rainwater harvesting, we recommend that your storage capacity is sized to provide buffer (or redundant) volume. This gives you extra drought protection days for comfort, which you might need in a dry year, or if you have a particularly high period of usage

There are three important calculations:

1. Calculate your household water usage, or the daily demand you intend to supply. This dictates how much rainwater you need to collect and store.
2. Calculate how much rainwater you can harvest from your roof (or other catchment). Of course, this must be greater than your demand and the degree to which it is greater is the buffer or the amount of surplus rain available to prevent running out of rainwater.
3. Calculate the size of the tank. Once you have determined you have sufficient rainwater harvesting potential to meet the demand, you can calculate the water tank size.

Calculation 1 – Your Water Usage, or Demand that is Used to Size Your Rainwater Harvesting Tank

It’s important to consider how much water you are going to draw from the tank. Calculate how much water you currently use and estimate how much water you’ll need for different purposes, for example household water per person per day, irrigation use etc.

The Household Water Usage Calculator is a handy tool to help you with this step. If you’d like, you can limit the numbers you enter to match the demand that will be put on your rainwater harvesting system.

Rainfall Patterns in Your Location

The amount of rainwater you can collect depends on the rainfall patterns in your location. Areas with high annual rainfall offer a greater potential for rainwater harvesting than those in dryer climates. Greater annual rainfall with short dry periods allows use of smaller tanks.

You can measure the rainfall at your location over a year. This requires a weather station with a tipping bucket. This will be the most accurate approach, but remember, your weather patterns may change year to year, and our climate is changing.

Alternatively, you can use official data provided by the relevant local authority. The following websites will assist:

• In Australia, go to the Bureau of Meteorology website.
• In the UK, go to the Met Office website.
• In the USA, go to the National Weather Service website.
• You may also find useful information on the World Bank Group Climate Change Knowledge Portal.

Look for the closest weather station to you and find the following two numbers:

1. The number of inches (or millimetres) of rain per year. Convert this to feet (or metres)
2. The longest dry period within the year (in days). This will be your drought protection requirement.

Roof Area (or Catchment Area)

The size of your roof or catchment area will determine how much rainwater you can capture. The larger the roof area that is diverted to your rainwater harvesting system, the more rainwater you can collect. You can always capture rainwater from sheds or other out-houses, as well as your main house roof, to maximise the size of your rainwater catchment.

You need to calculate the footprint of your roofed area on plan. Don’t worry about roof pitch as we will assume the rain falls straight down. For rectangular roofs, calculate this by multiplying the length by the width of the roof in metres. If your roof is not rectangular, you can sum the various roof sections.

Make sure you stick to either imperial (square feet) or metric (square metres). We need to keep units consistent for the calculations.

Calculation 2 – How Much Rainwater Can You Harvest

Multiply the amount of rain per year with the area of your roof to estimate the volume that you will capture in a year.

It is recommended that you reduce this number by around 2% to factor in losses due to evaporation.

When calculating, be sure to keep units consistent.

• If you are working in imperial, that will be feet; square feet; cubic feet. You can convert cubic feet to US Gallons by multiplying it by 7.48.
• If you are working in metric, that will be metres; square metres; cubic metres (which is equivalent to kilolitres).

Calculation 3 – The Size of the Tank.

From your water usage data, you can calculate a daily demand. If demand varies through the year (for example seasonal irrigation), calculate it based on the peak period of usage. 

You have already determined the number of days of drought protection you need. This is based on the duration of dry weather and your risk tolerance to running out of water.

Multiply daily usage by the number of drought protection days and you will have your volume. It is always sensible to round up to the nearest common tank size. You’ll need to check with your tank supplier to see what that is. 

If uncertain about the best tank volume, a larger tank is always preferable to a smaller one.

A few Considerations When Choosing the Right Tank

Available Space

The available space for your rainwater tank may also play a role in determining the size. Consider the size and dimensions of your space and choose a tank that fits. Tanks come in various shapes to make fitting into small areas easier. You can always choose a slim line tank, or a tall skinny tank if space is limited.

Storage Tank Options

There are various storage tank options available, including above-ground tanks, underground tanks, and bladder tanks. Consider the pros and cons of each option before deciding. The level of your outlet may be a consideration here, depending on how you intend to supply the water. You may need to consider position a pump downstream.

Large Tanks vs. Multiple Tanks

Depending on your water needs and available space, you may choose to have one large tank or multiple smaller tanks. Each has its advantages and disadvantages, so consider your specific requirements before deciding. Available space and maintenance will be considerations. If you go with multiple tanks, you’ll need to determine how they connect together and maintain turn-over of water (avoiding stagnant water).

A Final Thought

In conclusion, determining the right size rainwater tank for your needs requires consideration of rainfall patterns, roof area, water usage, and available space. There some basic calculations required which we have explained above. There are also some great tools available to help you calculate your requirements. The Household Water Usage Calculator and the Rooftop Rainwater Harvesting Calculator. There is also a Money Saver Calculator to help you determine how much money you could save by implementing rainwater harvesting!  With the right-sized tank, you can maximize your rainwater harvesting, save money, and reduce your reliance on the municipal water system.

FAQs

Q: What is the purpose of rainwater harvesting?

A: The purpose of rainwater harvesting is to collect and store rainwater for later use, reducing the reliance on mains water supply and conserving valuable water resources.

Q: How does a rainwater harvesting system work?

A: A rainwater harvesting system collects rainwater from the roof of a building and directs it into a storage tank. The water can then be used for various purposes such as irrigation, toilet flushing, and laundry.

Q: How can I determine how much rainwater I need to capture?

A: To determine how much rainwater you need to capture, you should consider your water usage requirements. Calculate the volume of water you typically use for different purposes such as gardening, flushing toilets, and washing clothes. This article has all of the information you need to calculate this.

Q: Can I use a rainwater harvesting system for drinking water?

A: It is generally recommended to use rainwater harvested from a rainwater tank for non-potable purposes such as irrigation and toilet flushing. However, if you’re able to collect clean rainwater, it may be used for drinking purposes as well. Treatment is also an option for this.

Q: How do I choose the right size rainwater tank?

A: Choosing the right size rainwater tank depends on factors such as the amount of rainfall in your area, your roof catchment area, your water usage needs, and the available space for the tank. This article has all of the information you need to calculate this.

Q: How much water can a rainwater tank hold?

A: The volume of water a rainwater tank can hold depends on its size. Tanks can range in size from a few hundred litres to millions (or mega) litres. It is important to consider your water needs and available space when selecting a tank.

Q: How much rainfall do I need to harvest enough water?

A: The amount of rainfall you need to harvest enough water depends on your water usage requirements. You can calculate the amount of rainfall needed by considering the volume of water you need and the efficiency of your rainwater harvesting system. The links provided in the article give a good idea of how much rainfall you can expect to receive in your area.

Q: Can I install one large rainwater tank instead of multiple smaller tanks?

A: Yes, you can choose to install one large rainwater tank instead of multiple smaller tanks. However, it is important to ensure that the tank size is sufficient to meet your water storage needs. Consider factors such as available space and future water usage requirements when making this decision.