What Size Hot Water Cylinder Do I Need? Sizing by Household
Most homes need a 120 to 300 litre hot water cylinder. How to size one by the number of people and bathrooms, why reheat time matters as much as litres, and the mistakes that leave you short of hot water.
As a starting point: a 1 to 2 person flat needs about 120 to 150 litres, a 3 to 4 person house with one bathroom about 150 to 210 litres, a 4 to 5 person house with two bathrooms about 210 to 300 litres, and homes with three or more bathrooms 300 litres or more. Peak demand matters more than the number of bedrooms: if two showers or a bath and a shower run at the same time, size up. How fast the cylinder reheats matters as much as its size.
Key Takeaways
- Size by people and simultaneous use, not by bedrooms.
- Rough guide: 120-150 L for 1-2 people, 150-210 L for 3-4, 210-300 L for 4-5 with two bathrooms, 300 L+ for three or more bathrooms.
- Stored water is kept at 60°C or more and mixed with cold at the tap, so a cylinder gives more usable hot water than its label suggests.
- A boiler-heated coil reheats much faster than an immersion heater; a slow reheat is often the real reason hot water runs out.
- Heat pumps store water at lower temperatures and reheat more slowly, so they usually need a larger cylinder.
For most homes the right hot water cylinder is somewhere between 120 and 300 litres. As a starting point, a 1 to 2 person flat needs about 120 to 150 litres, a 3 to 4 person house with one bathroom about 150 to 210 litres, a 4 to 5 person house with two bathrooms about 210 to 300 litres, and anything with three or more bathrooms 300 litres or more. The number that matters most is how much hot water you use at the busiest time of day, and how quickly the cylinder can heat up again afterwards.
If your hot water already runs out, check our guide to hot water running out quickly first. A faulty thermostat, immersion or coil can make a correctly sized cylinder feel too small.
Hot water cylinder size chart
| Household | Bathrooms | Typical cylinder size |
|---|---|---|
| 1 to 2 people, flat or small house | 1 | 120 to 150 litres |
| 3 to 4 people | 1 | 150 to 210 litres |
| 4 to 5 people | 2 | 210 to 300 litres |
| Larger household | 3 or more | 300 litres or more, often a twin-coil cylinder |
These ranges assume a cylinder heated by a gas boiler with water stored at around 60°C. Move up a size if the household takes long showers, has a large bath, or regularly has two showers running at the same time. Move down if you are out most of the day and rarely use more than one outlet at once.
How to work out the right size for your home
- Picture the busiest hour. For most families that is the weekday morning. Count how many showers and baths happen back to back, and whether any overlap.
- Estimate the water used. A mains-pressure shower commonly uses around 10 litres a minute or more of mixed water, so an eight-minute shower can use 80 litres or more. A full bath uses a similar amount or more, all at once.
- Remember the mixing. The cylinder stores water at 60°C or more, but you shower at around 40°C, so cold water is mixed in at the tap. Each litre stored gives more than a litre of shower water. That is why a 150-litre cylinder can cover more than 150 litres of showering.
- Allow for reheating during the peak. If the boiler coil is heating the cylinder while showers are running, it tops up as you go. A fast-reheat cylinder can do more with fewer litres.
- Check the space. Measure the height and width of the airing cupboard, including room for pipework, valves and access. Larger cylinders get taller and wider, and an unvented cylinder needs room for its safety valves and expansion vessel.
Why reheat time matters as much as litres
Two cylinders of the same size can feel very different. A modern cylinder with a high-output coil heated by the boiler can recover quickly, often well within an hour. An old cylinder with a small coil, or one heated only by an immersion heater, can take a couple of hours or more to heat a full tank from cold.
That is why replacing an old cylinder with a modern one of the same size often fixes a hot water shortage. It is also why a household on an immersion-only system, or on a cheap overnight electricity tariff, usually needs a bigger cylinder than a household with a gas boiler: the water heated overnight has to last all day. If your immersion is not heating at all, see immersion heater not working.
Too big vs too small
Too small is the more obvious problem. The hot water runs out halfway through the second shower, and the household ends up staggering showers or boosting the immersion.
Too big is easier to miss, but it has costs of its own:
- A bigger cylinder costs more to buy and install, and takes up more cupboard space.
- It takes longer to heat from cold and loses more heat through the day, even with good insulation.
- If most of the water sits unused, it still has to be heated to 60°C regularly to keep Legionella under control, which wastes energy.
The right answer is usually the smallest cylinder that covers your busiest hour, with a fast reheat.
Other things that affect the choice
- Vented or unvented. An unvented cylinder gives mains-pressure showers, which usually means higher flow and more water used per minute. That can push the size up. Our vented vs unvented cylinder guide explains the difference and which suits your mains supply.
- Mains flow. With an unvented cylinder, the mains has to supply every outlet running at once. A big cylinder will not fix weak mains flow; that needs testing first. If pressure is already poor, see unvented cylinder low hot water pressure.
- Heat pumps and solar. Heat pumps store water at lower temperatures and reheat more slowly, so they normally need a larger cylinder with a bigger coil. Solar thermal needs a twin-coil cylinder with extra capacity.
- Direct or indirect. An indirect cylinder is heated by a coil from the boiler, with an immersion as backup. A direct cylinder is heated only by electric immersion elements and is common in flats without gas.
- Combi instead. If you have one bathroom and good mains flow, a combi boiler with no cylinder may be simpler. Our combi vs system boiler comparison covers when that works.
Running out of hot water?
Tell us how many people live in the home, how many bathrooms you have and when the hot water runs out. We'll check whether the cylinder is the wrong size or just not working properly, and recommend the right fix.
📞 Call 020 7870 3200Or read about our hot water cylinder repair and replacement and unvented cylinder service.
Frequently Asked Questions
What size hot water cylinder do I need for a family of four?
A family of four with one bathroom usually needs around 150 to 210 litres. If there are two bathrooms and showers are often used at the same time, around 210 litres or more is a safer choice, especially with teenagers or long morning showers.
What size cylinder do I need for two bathrooms?
Most homes with two bathrooms need about 210 to 300 litres, depending on how many people live there and whether both showers run at once. With an unvented cylinder, check that the mains flow can supply two showers at the same time as well.
Is a bigger hot water cylinder always better?
No. An oversized cylinder costs more, takes up more space, takes longer to heat and loses more heat through the day. It is better to match the size to real use and choose a fast-reheat model than to fit the biggest cylinder that will fit.
How long does a hot water cylinder take to heat up?
It depends on the cylinder and the heat source. A modern cylinder heated by a boiler coil can recover in well under an hour, often much less. An immersion heater on its own is far slower and can take a couple of hours or more to heat a full cylinder from cold.
Do I need a bigger cylinder with a heat pump?
Usually, yes. Heat pumps heat water to a lower temperature and more slowly than a gas boiler, so the cylinder needs to hold more to give the same amount of usable hot water. Heat pump cylinders also have a larger coil designed for that lower flow temperature.
Sources and further reading
Source links checked 27 September 2026. Scheme rules and manufacturer instructions can change; use the current official guidance for your account or exact appliance.