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What Size Hot Water Cylinder Do I Need?

Trying to figure out hot water cylinder sizes is a confusing task. And that’s why we’ve created this 5-minute guide to help you through it. Here, we’ll explain the following:

  • Why you need to spec the right sized cylinder for your property
  • What size hot water cylinder you need
  • Whether you need a vented or unvented cylinder

Hot Water Cylinder Sizes

Establishing the correct hot water cylinder sizes for your property is essential to efficient hot water distribution. And there are many variables to factor in. If you miss one, you could end up with a cylinder that’s too large or too small.

What Happens if Your Cylinder Is Too Small, or Too Big?

If you don’t get the right size hot water cylinder for your property, you’ll have 1 of 2 problems:

  • It’s too small – you run out of hot water and have to wait for more to heat up
  • It’s too big – your heating bill will increase because you’re heating hot water that you don’t need. What’s more, bigger cylinders cost more money, so it’s a waste of cash in the first place.

How Much Hot Water Do You Need?

No 2 properties — and households — are the same. One 2-bedroom property might need a 150L cylinder, whilst 210L won’t suffice for another. And that’s simply because the hot water demand might differ wildly between them.

Here are some things to consider when establishing your home’s hot water demand:

  • The number of baths in the property
  • How many people are using hot water at the same time
  • Regular temperature settings
  • The time the boiler takes to heat more hot water
  • The size of the property
  • The number of sinks, basins and taps

Now that we’ve gone over the variables, let’s consider some data from a hot water consumption study performed by the Energy Savings Trust. The study profiled a number of households, and the findings show the following daily hot water consumption numbers:

Household Size  Hot Water (Litres Per Day)
1 ~80
2 ~100
3 ~130
4 ~150
5 ~180

How Much Hot Water Does The Average Person Use?

Household size isn’t the only factor in calculating hot water cylinder sizes. Another variable to consider is how much hot water each household member typically uses on a daily basis. Here’s a quick chart with some national averages:

Usage Hot Water (Litres Per Day)
Low 25
Medium 40
High 60

Now, these metrics alone aren’t very useful at calculating hot water cylinder sizes — one person might have a shower in the morning, the other at night. In this scenario, the heating system can replenish water lost from the cylinder throughout the day. And this is why hot water usage during peak demand is so essential.

How Much Water Do You Need to Fill a Bath?

When calculating hot water cylinder sizes, remember — not all baths are designed to the same dimensions; so, you’ll need to figure out how much water is needed to draw a full bath. Typically, you can expect 75-85L of water to fill an average bath.

Now here’s a vital thing to note — your 85L bath doesn’t need 85 litres of hot water. In fact, you’ll probably want your bath to be comfortably warm, at around 40°C. So, you mix the hot water from your cylinder with cold water from the mains when you draw your bath. This means that all-in-all, you’ll need about 60L of hot water from the cylinder to take a bath.

Does a Bath Use Less Water Than a Shower?

It depends on how long you’re in there. For the average 5-minute shower, expect to use approximately 50L of hot water. But if your ablutions take much longer than this, you’re consuming more water by showering than you would by drawing a bath.

What Size Hot Water Cylinder Do I Need?

We’ve gone over several different variables now, and we wouldn’t be surprised if you’re still confused. After all, all these statistics give you a great background, but fail to answer your primary question: “What Size Hot Water Cylinder Do I Need?”

So, to give you a more definitive answer, we’ve compiled a hot water cylinder sizes chart.

Hot Water Cylinder Sizes Chart

Use this chart as a general guideline when looking for a new cylinder installation:

Property Size Bathrooms Cylinder Size
1 Bedroom 1 120-150L
2 Bedrooms 1 150-180L
3 Bedrooms 1 or 2 180-210L
4 Bedrooms 2 210-300L
5 Bedrooms or more 2 300L or bigger

Please note that manufacturers’ cylinder size recommendations may vary, so be sure to check with whatever cylinder brand you’d like to install. Looking for Megaflow tank sizes? A Megaflow representative is the best person to ask for hot water cylinder sizes for your property.

If you’re not sure what brand you’re going with just yet, we’d recommend getting an inspection and quote from a qualified engineer.

Do I Need A Vented Or Unvented Cylinder?

If you are not converting a system, you’ll want whatever you already have.


If you have a boiler, hot water cylinder, and a storage tank (usually installed in loft spaces), you’ll need a vented cylinder.


A unvented system will use a boiler and cylinder. As it connects to mains water, there is no need for a water storage tank.

How to Size a Thermal Storage Tank?

Sizing a thermal storage tanks is a bit more complicated than figuring out hot water cylinder sizes. That’s because, unlike a hot water cylinder, a thermal storage tank holds central heating water, and is able to incorporate energy from several sources, such as a boiler, solar panels, etc. On top of storing central heating water, a thermal store can also be configured to supply domestic hot water.

And this is why there are far more factors and variables involved in sizing a thermal store. If you’d like to fit a thermal storage tank in your home, we’d urge you to get a quote from a qualified engineer.

What’s Next?

Thanks for reading how 5-minute hot water cylinder sizing guide.

Do you have more questions about cylinder sizes or installation? Leave a comment below and we’ll get back to you as soon as possible.

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