Hot Water Cylinder Size Calculator
Find out how many litres you need. Tell us who lives in your home, how many bathrooms you have and how you shower, and we'll show the right size for an unvented or vented cylinder, with a minimum and a more comfortable option.
What Size Hot Water Cylinder Do I Need?
Most UK homes need a cylinder between about 120 and 300 litres. A useful rule is roughly 35 to 55 litres of stored hot water per person, adjusted for how many bathrooms you have, whether you shower or bathe, and how powerful your showers are. A couple in a one-bathroom flat is often fine with 120 to 150 litres, a family of four with two bathrooms usually lands around 210 to 250 litres, and larger homes with several bathrooms go to 300 litres or more.
Cylinder Size Guide by Home Type
If you'd like a quick answer without the calculator, start here.
| Typical Home | Usual Unvented Size | Usual Vented Size |
|---|---|---|
| 1-2 bedrooms, 1 bathroom | 120 to 150 litres | 117 to 140 litres |
| 3 bedrooms, 1-2 bathrooms | 150 to 210 litres | 140 to 190 litres |
| 3-4 bedrooms, 2 bathrooms | 210 to 250 litres | 190 to 210 litres |
| 4+ bedrooms, 3+ bathrooms | 250 to 300 litres or more | Above typical vented sizes |
How the Calculator Works
We don't hide the maths. The calculator estimates the hot water you need stored at around 60 degrees Celsius, then rounds up to the next standard cylinder size.
| Factor | What We Assume |
|---|---|
| Per person (showers mostly) | 35 litres |
| Per person (showers and the odd bath) | 45 litres |
| Per person (regular baths) | 55 litres |
| Shower type | Standard 1.0x, power or pumped 1.2x, rainfall or multi-head 1.4x, electric or none 0.75x |
| Extra bathrooms | 8% more for each bathroom beyond the first, because more outlets run at once |
| Heat source | Boiler 1.0x, immersion only 1.15x (slower reheat), heat pump 1.1x |
The minimum size covers about 85% of the estimate and suits a household that's careful with hot water. The comfortable option is the next size up. It's an estimate based on typical UK use, so we confirm it at a survey.
Standard Cylinder Sizes
Unvented cylinders
Commonly about 120, 150, 180, 210, 250 and 300 litres, with larger sizes for big homes. Exact sizes differ by manufacturer. Learn more in our unvented cylinder guide.
Vented cylinders
Commonly about 117, 140, 162, 190 and 210 litres, usually copper. See our vented cylinder guide for when vented is the better choice.
Is a bigger cylinder better?
No. An oversized cylinder takes up more space, costs more to keep hot and loses more heat while it sits. The aim is the smallest size that comfortably covers your busiest morning. If you're short on height, some cylinders are made wider and shorter, so the right model can solve a space problem without going smaller on capacity.
Cylinder size and boiler size are separate decisions
The cylinder decides how much hot water you can store. The boiler and the cylinder coil decide how quickly it reheats. If you're also choosing a boiler, use our boiler size calculator, and read about system boiler installation to see how they pair.
Hot Water Cylinder Size: Common Questions
What size hot water cylinder do I need for a 3 bedroom house?
Most 3 bedroom houses with one or two bathrooms suit roughly 150 to 210 litres. If the household is large, or you use power showers and regular baths, 210 to 250 litres may be more comfortable.
How many litres of hot water do I need per person?
A useful planning figure is roughly 35 to 55 litres of stored hot water per person, depending on whether you mostly shower, mix showers and baths, or bathe often. Power and rainfall showers push this higher.
What size cylinder do I need for 2 bathrooms?
Two bathrooms in a family home typically suit around 210 to 250 litres. The right figure depends on how many people live there and how many showers or baths run in the same morning.
How many showers can I get from a 150 or 210 litre cylinder?
Assuming a standard mixer shower of about 7 minutes at 8 litres per minute, a shower needs roughly 34 litres of stored water at 60 degrees once it's mixed with cold. That's roughly 4 showers from 150 litres and 6 from 210 litres before it needs to reheat. Real figures vary with shower flow, length and cylinder temperature.
Is a bigger hot water cylinder better?
No. An oversized cylinder takes up more space, costs more to keep hot and loses more heat while it stands. Go up a size only if it suits how you actually live.
What are the standard hot water cylinder sizes?
Unvented cylinders commonly come in about 120, 150, 180, 210, 250 and 300 litres. Vented copper cylinders commonly come in about 117, 140, 162, 190 and 210 litres. Exact sizes vary by manufacturer.
What size cylinder do I need for a 4 bedroom house?
Usually around 210 to 300 litres, depending on how many bathrooms you have and how many people live there. Three or more bathrooms usually sits at the top of that range.
Does cylinder size depend on my boiler?
Partly. The boiler and the cylinder coil decide how fast the cylinder reheats. A bigger cylinder stores more, but a well-matched boiler and coil mean a smaller cylinder can recover quickly. We check both together at a survey.
Do I need a bigger cylinder with a heat pump?
Heat pump cylinders need a larger coil because heat pumps run at lower flow temperatures, and they're often specified a little larger. The final size is confirmed in a design check rather than a quick calculation.
Will a larger cylinder fit in my airing cupboard?
Measure the height, width and depth of the space, including room for pipework above and access for servicing. Some cylinders are wider and shorter, which can solve a height problem.
What temperature should a hot water cylinder be kept at?
Stored hot water is normally kept at around 60 degrees Celsius to limit Legionella risk. It's mixed with cold water at the tap, and a thermostatic mixing valve can limit outlet temperature to reduce scalding risk.
How accurate is this calculator?
It gives a sensible starting estimate based on typical UK household use. Real homes differ, so we confirm the size at a survey, checking your mains pressure, available space, heat source and how your household actually uses hot water.
Not Sure Which Size Is Right for You?
Send us your result and we'll check it against your mains pressure, your space and how your household really uses hot water, then give you a written quote.
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