Free calculator · New Zealand
What size hot water cylinder do I need?
Answer in litres, in about twenty seconds, with the working shown.
The short answer: most New Zealand homes need between 180 and 300 litres. Allow 50 litres per person per day, then add 50 litres for every bathroom with a bath, 20 for every shower-only bathroom, 10 per laundry and 5 per separate WC — and round up to the next size made.
So a four-person house with two bathrooms, one of them with a bath, a separate WC and a laundry comes to 285 litres a day, which means a 300 litre cylinder. That is the most common answer we give. The calculator below does the same arithmetic for your house and shows you every litre of it.
The calculator
Enter your household details
Cylinders get undersized far more often than oversized, and you only find out on the third shower of the morning. Nothing is sent anywhere — this runs in your browser.
People living here
Everyone who showers here regularly
Bathrooms — shower only
No bath in the room
Bathrooms — shower and bath
A full bath is the biggest single draw in the house
Separate WCs
Toilet and basin, no shower
Laundries
Tub and washing machine hot feed
The eight sizes
What each cylinder size suits
Cylinders are made in eight sizes from 180 to 500 litres. Bigger tank, taller and wider — which matters when it has to fit in the cupboard it is replacing.
Drawn to scale against each other. Above 500 litres we fit two cylinders rather than one very large tank — it recovers faster, it fits through a doorway, and if one fails you still have hot water.
The quick answer
Cylinder size by household
If you would rather not touch the calculator, find the row that looks most like your house. Every figure here comes out of the same arithmetic the calculator uses.
| Household | Bathrooms | Daily demand | Cylinder |
|---|---|---|---|
| 1 person | 1 bathroom (shower only) | 85 L a day | 180 L |
| 2 people | 1 bathroom with a bath | 165 L a day | 180 L |
| 2 people | 2 bathrooms, 1 with a bath | 185 L a day | 200 L |
| 3 people | 2 bathrooms, 1 with a bath | 235 L a day | 250 L |
| 4 people | 2 bathrooms, 1 with a bath | 285 L a day | 300 L |
| 5 people | 2 bathrooms, 1 with a bath | 335 L a day | 350 L |
| 5 people | 3 bathrooms, 1 with a bath | 355 L a day | 400 L |
| 6 people | 3 bathrooms, 1 with a bath | 405 L a day | 450 L |
| 7 people | 3 bathrooms, 2 with baths | 485 L a day | 500 L |
The working
How the calculation is done
Add up a day's hot water at storage temperature, then round up to the next size made.
| What | Litres a day | Why that number |
|---|---|---|
| Each person | 50 L | The New Zealand design allowance is roughly 45 to 60 litres per person per day at storage temperature. We use the middle of it. |
| Bathroom, shower only | 20 L | The room adds draw-off on top of the people — a second shower running at the same time is the case that catches households out. |
| Bathroom with a bath | 50 L | A filled bath is 80 to 110 litres and most of that is hot. It is the single biggest draw in the house. |
| Separate WC | 5 L | Hand basin only, so it barely registers — but it is not nothing. |
| Laundry | 10 L | The tub, and the hot feed on the washing machine if it has one. |
Then round up: 180, 200, 250, 300, 350, 400, 450, 500 litres. There is no 275 litre cylinder, so a house that works out at 260 gets a 300 — and the spare 40 litres is the cheapest insurance in the system.
The other half
Recovery rate
Recovery, not just storage
What matters on a busy morning is how fast the cylinder puts heat back in while you are using it. A 250 litre cylinder that recovers quickly can beat a 300 that does not. We size the store and the heat source together.
What is heating it
A cylinder fed by a hot water heat pump wants a large indirect coil and a slow, steady charge. One fed by a gas boiler recovers much faster and can be smaller for the same household. They are not interchangeable.
Cylinder location
Distance to the furthest tap decides how much hot water is wasted waiting. A cylinder in the garage feeding an ensuite at the far end of the house needs a recirculation loop or a different location.
Whether it fits
A 500 litre cylinder is a large object. Height under the ceiling, width through the doorway, and floor loading in an upstairs cupboard all decide what is actually installable — not just what the numbers say.
Questions
Common questions about cylinder size
What size hot water cylinder do I need for a family of 4?
Usually 300 litres. Four people at 50 litres each is 200 litres a day, and a typical four-person house has two bathrooms — one with a bath, one shower-only — plus a WC and a laundry, which adds another 85. That is 285 litres, so the next size made is 300. If your house has three bathrooms or two baths, step up to 350.
Is a 180 litre hot water cylinder big enough for 2 people?
For two people with one bathroom, yes, comfortably — that works out at about 165 litres a day even with a bath in the room. Add a second bathroom and you are at 185, which pushes you to 200. 180 litres is the smallest cylinder we fit and it is the usual choice for apartments, sleepouts and couples.
What size hot water cylinder do I need for 5 people?
350 to 400 litres. Five people is 250 litres a day before any fixtures. With two bathrooms, one of them with a bath, you land on 335 and take a 350. With three bathrooms, or if all the showers run at once on a school morning, go to 400.
How many showers can you get out of a 300 litre cylinder?
Roughly six to eight back to back, on an eight-minute shower at a normal flow rate, before the temperature starts dropping. It depends on your shower heads more than your cylinder — a rain head can use twice the water of a water-efficient one, which halves that number. It also depends on how fast the cylinder is reheating while you use it.
Is it better to get a bigger hot water cylinder than you need?
Slightly bigger, yes; much bigger, no. Oversizing by one step costs very little extra to run, because a well-insulated cylinder loses only a small amount of standing heat. Oversizing by three steps means paying for storage you do not use, finding room for it, and on a heat pump system, a longer charge time. Undersizing is the more common mistake.
Does the answer change if it is a heat pump rather than an element?
The litres do not, but the thinking does. An element reheats fast, so a smaller cylinder can be topped up through the day. A hot water heat pump heats gently and relies on what is already stored, so if you are running one, we would take the calculator's answer and not go below it. Matching the coil in the cylinder to the heat source matters as much as the size.
What size cylinder do I need if I have a bath?
Add 50 litres a day for each bathroom with a bath in it. A filled bath is 80 to 110 litres and most of that is hot, so it is the single biggest draw in the house — more than two and a half times a shower-only bathroom. A household that runs a bath every evening should size for it rather than hope.
How do I tell if my current cylinder is too small?
The last shower of the morning runs cool, or someone has to wait between showers. Filling a bath empties it. The element or heat pump is still recovering at midday. If so, the cylinder is undersized for how the house is used now — often because more people live there than it was designed for.
What size hot water cylinder is standard in New Zealand?
180 litres is the most common size in older housing stock, and it is why so many New Zealand houses run out of hot water — it was sized for a smaller household and a single bathroom. In new builds 300 litres is now the usual specification. Cylinders are made from 180 up to 500 litres; above that we would fit two rather than one very large tank.
Can I just replace my cylinder with the same size?
You can, but a replacement is the time to correct the size if needed. If the house has gained a bathroom or an occupant since the old cylinder went in, a like-for-like swap keeps the same shortfall. Check the size with the calculator first.
Auckland-wide
Have the size checked for your house
We size cylinders properly — off your fixtures, your heat source and where the tank has to live — as part of the quote. It costs nothing and it is a lot cheaper than finding out in the shower.