How Much Water Storage Do I Need?
- Water Tank Solutions
- 4 hours ago
- 9 min read
How to calculate the
right rainwater storage
for your home
If your home relies primarily, or entirely, on roof-collected rainwater, choosing the right amount of water storage is one of the most important decisions you can make .
Too little storage can leave you relying on an alternative water supply during a prolonged dry period. Too much storage can mean spending more money and using more space than necessary.
So, how much water storage do you actually need?
The answer depends on four key things:
How much water your household uses + how much water your roof can collect + your local rainfall pattern + how much dry-period security you want.
How much water does a person use each day in New Zealand?
There isn't one single number that applies to every household.
Water use varies depending on the number of people in the home, showers and bathing habits, toilets, washing machines, appliances, leaks and outdoor water use.
One of the most useful sources of national data is BRANZ's Residential Water Use in New Zealand study. The study monitored water use across 66 New Zealand households and provides a useful benchmark for residential water consumption.
The study found:
Winter | Summer | |
Median household water use | 159 L/person/day | 231 L/person/day |
Mean household water use | 213 L/person/day | 292 L/person/day |
The median is particularly useful when thinking about a typical household because a relatively small number of very high-use households can pull the average upwards.
Summer demand is also noticeably higher than winter demand.
What about the Waikato?
Smart Water reports that Waipā's residential water use was approximately 180 litres per person per day in 2021, down from 250 litres per person per day in 2010. Hamilton's figure was 214 L/person/day in 2021.
For practical storage planning, around 200 litres per person per day is a useful conservative starting point for a Waikato household.
It's important to understand that 200 L/person/day is a planning allowance, not an official NZ average. It sits between the national winter and summer median figures and provides a straightforward figure to use when working through storage requirements.
How much water does your household need?
Using 200 litres per person per day as a planning figure gives you:
Household | Approx. daily use | Approx. monthly use |
1 person | 200 L | 6,000 L |
2 people | 400 L | 12,000 L |
3 people | 600 L | 18,000 L |
4 people | 800 L | 24,000 L |
5 people | 1,000 L | 30,000 L |
6 people | 1,200 L | 36,000 L |
These figures are for household use only.
If the same rainwater supply is also used for garden irrigation, animals, outdoor cleaning or other purposes, your actual demand will be higher.
Think about storage in terms of days, not just litres
Tank capacity can be easier to understand when you translate it into days of household demand.
For example, a four-person household using 200 litres per person per day would use:
4 × 200 L = 800 litres per day
A 25,000-litre tank would therefore contain approximately:
25,000 ÷ 800 = 31 days
So, in very simple terms, 25,000 litres represents about 31 days of household water for four people at this planning rate.
But there is an important qualification.
This calculation assumes:
no rain enters the tank during the period
household use remains at 200 L/person/day
there is no garden or other outdoor demand
the tank is completely full at the beginning of the period.
In reality, your tank is continually filling and emptying.
The important question isn't simply "How much water do I use?"
It's:
"How much water do I need to have stored to get through periods when rainfall doesn't keep up with demand?"
Indicative storage for household use
Using 200 L/person/day, the following table shows how much water would be required to cover different periods of household demand.
Household | 30 days | 45 days | 60 days |
2 people | 12,000 L | 18,000 L | 24,000 L |
3 people | 18,000 L | 27,000 L | 36,000 L |
4 people | 24,000 L | 36,000 L | 48,000 L |
5 people | 30,000 L | 45,000 L | 60,000 L |
6 people | 36,000 L | 54,000 L | 72,000 L |
These are indicative calculations for household use only, not universal tank-sizing requirements.
The appropriate amount of storage depends on your property, roof catchment, rainfall and how much resilience you want from your water supply.
Your roof is your water source
For a home relying on roof-collected rainwater, your roof is effectively the water source feeding your tank.
The basic calculation is simple:
1 mm of rainfall × 1 m² of roof area ≈ 1 litre of water
So, for example:
200 m² roof × 10 mm rainfall = approximately 2,000 litres of potential collection
That's before allowing for real-world losses such as first-flush diversion, collection inefficiencies and overflow.
The New Zealand Government's Building Performance guidance also identifies annual rainfall and the intended use of the water as important factors when determining the appropriate tank size.
Roof size matters
A large roof can collect considerably more water from the same rainfall event than a small roof. For example:
100 m² roof + 10 mm rain = approximately 1,000 L
200 m² roof + 10 mm rain = approximately 2,000 L
250 m² roof + 10 mm rain = approximately 2,500 L
WTS's own tank information uses the same principle when demonstrating potential rainwater collection from a typical four-bedroom home's roof catchment.
Don't rely on annual rainfall alone
It can be tempting to look at the annual rainfall for an area and assume that a property receives "enough rain".
But the timing and distribution of rainfall matters just as much as the annual total.
A property might receive plenty of rain over an entire year while still experiencing several weeks with little useful rainfall.
This is particularly important for homes relying entirely on roof water.
Rainfall also varies considerably across the Waikato, so a single regional rainfall figure shouldn't be treated as a universal value for every property.
When sizing a rainwater system, your actual location, roof catchment and rainfall pattern should be considered.
How much storage should a four-person home have?
Let's look at a practical example.
Imagine a four-person household using approximately 200 L/person/day:
4 × 200 L = 800 L/day
That's approximately:
5,600 L per week
24,000 L per month
48,000 L over 60 days
A 25,000-litre tank therefore represents around one month of household demand at this planning rate.
A total storage capacity of 50,000 litres would represent approximately two months of household demand.
WTS's existing tank information uses a four-person home with an indicative 250 m² roof catchment and recommends two 25,000-litre tanks (50,000 litres total) as an example of storage for harvesting wetter-period rainfall and carrying water into drier periods. The same information uses 160–200 L/person/day for household consumption.
However, this should be viewed as an example rather than a universal recommendation.
Your ideal storage capacity may be different depending on:
household size
roof area
local rainfall
garden size
outdoor water use
existing storage
how dependent you are on rainwater
how much dry-period security you want.
Government guidance for homes using rainwater as their sole supply
New Zealand's Building Performance guidance specifically recommends considering annual rainfall, intended water use and access to mains water when choosing tank capacity. It also notes that larger tanks are required where rainwater is being used for indoor household supply, particularly in areas with dry summers.
For a property where rainwater is the sole source of water, Building Performance currently states that a minimum 30,000-litre rainwater tank is needed and recommends discussing the appropriate size with a local supplier based on climate and household requirements.
This should be understood as government consumer guidance rather than a universal engineering calculation.
A particular property may require more storage depending on its roof area, household demand and desired level of resilience.
Outdoor water use is also a factor
Household consumption isn't necessarily your biggest summer demand.
If your rainwater supply also provides water for:
lawns
gardens
vegetable gardens
fruit trees
outdoor cleaning
vehicles
animals or livestock
irrigation
then your total water requirement can increase significantly.
This is one reason why tank sizing shouldn't be based on the number of people in the house alone.
Efficient irrigation can also make a difference. A properly designed irrigation system can help deliver water where it is needed and avoid unnecessary water use.
If you're planning a new rainwater system, or your existing system regularly runs low, it's worth looking at both sides of the equation: how much water you're using and how much water your roof can collect.
So, how much water storage do you actually need?
There isn't one tank size that is right for every household.
A useful starting point is:
1. Work out your household demand
Allow around 200 L/person/day as a practical planning figure.
Remember that actual consumption can be lower or considerably higher.
2. Add your outdoor demand
If your tank supplies irrigation, gardens, animals or other outdoor uses, allow for this separately.
3. Look at your roof catchment
A larger roof can collect more water from every rainfall event.
1 mm of rain on 1 m² of roof ≈ 1 litre of potential collection.
4. Consider your local rainfall pattern
Don't rely solely on an annual rainfall figure. The timing and length of dry periods matter.
5. Decide how much resilience you want
Do you want enough storage to get through a few weeks without significant rain?
Or do you want a greater buffer against prolonged dry periods?
6. Consider the whole system
Your tank, roof catchment, filters, pump, treatment system and irrigation all need to work together.
What does El Niño mean for rainwater storage?
Earth Sciences New Zealand confirmed in July 2026 that El Niño conditions had been reached in the tropical Pacific and were expected to strongly influence New Zealand's weather into 2027.
Historically, El Niño has been associated with a greater likelihood of drier-than-usual conditions in northern and eastern parts of New Zealand, although New Zealand's weather is influenced by many different climate drivers and an El Niño event does not guarantee a dry season at any particular location.
Earth Sciences New Zealand's seasonal outlooks are the best source for keeping up with expected rainfall and soil moisture conditions as the season approaches.
For households relying on roof water, the message isn't that you should automatically buy the biggest tank available.
It's that dry-period resilience becomes more important when your water supply depends entirely on rainfall.
Read more about El Niño's potential effects for the Waikato HERE
What if you don't have room for a 25,000 or 30,000-litre tank?
Not every property has the space for large-scale storage.
For urban homes, smaller tanks can still provide useful additional water storage.
Depending on how they are connected, smaller tanks can be used for:
garden irrigation
toilet flushing
laundry
outdoor water use
rainwater harvesting
reducing demand on mains water
providing additional water during restrictions or dry periods.
WTS supplies slimline urban tanks intended for properties where space is limited.
You don't necessarily have to become completely independent of mains water for rainwater harvesting to be worthwhile.
Even relatively modest storage can capture water when it is available and reduce demand on other supplies.
Find out more about teh benefits of urban sized tanks HERE
Storage is only one part of the system
A rainwater tank is not a standalone solution.
A complete roof-water system can include:
Roof and gutters →
pre-tank filtration →
tank storage →
pump →
filtration/treatment →
household
The exact configuration depends on what you intend to use the water for.
New Zealand Government guidance notes that rainwater used for drinking and other potable household purposes needs appropriate treatment or purification, and recommends considering measures such as pre-tank filtration and UV treatment.
WTS supplies a full range of tank accessories including pre-tank filters, tank level indicators and level sensors and filtration systems to help manage storage, water quality and system performance.
Read about the benefits of pre-tank filters HERE
Need help working out how much storage you need?
At WTS, we don't believe in simply choosing a tank based on household size.
We look at the whole water system, including your household requirements, roof catchment, intended water use, storage, pumping and treatment.
We supply and install water storage systems for homes, lifestyle properties, farms and commercial applications throughout the Waikato and beyond.
If you're unsure whether your existing storage is sufficient, or you're planning a new rainwater supply, we can help you work through the requirements and identify a practical solution.
Contact WTS for help and advice on Water Storage
Plan for the dry period—not just the average year
Rainwater storage is ultimately about managing the gap between water coming in and water going out.
Your roof determines how much rain you can collect. Your household determines how much water you use. Your tank determines how much of that water you can carry forward.
With El Niño expected to influence New Zealand's weather into 2027, now is a sensible time to look at your water storage while your tank is full—not when the level is already getting low.
The right amount of storage isn't necessarily the biggest tank you can fit. It's the amount that gives your household the right balance between collection, consumption, cost and resilience.
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