What Storage Tank Volume Do I Need: Calculation Based on Number of People and Bathrooms

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What tank volume do I need: calculation according to number of people and bathrooms

One of the most frequently asked questions when choosing a condensing boiler and hot water tank set is: "How many litres of tank do we actually need?" At first glance, the answer seems simple – the more people, the bigger the tank. In practice, however, it's not that straightforward. The tank volume actually depends on a combination of several factors: the number of people in the household, the number of bathrooms and shower cubicles, the boiler output, how quickly the tank can heat up water, and last but not least, the habits of the household itself. If you choose a tank that is too small, you risk running out of hot water in the middle of your morning shower. If you choose one that is too big, you're unnecessarily paying to heat water you'll never use, and it also takes up unnecessary space in the boiler room.

This article will give you concrete numbers, formulas, and practical scenarios so you can correctly estimate the required tank volume before you call an installer or choose a set in our e-shop.

Basic principle: a tank is not just a "bucket of hot water"

Before we get into concrete calculations, it's important to understand what a hot water tank in a set with a condensing boiler actually does and what we can realistically expect from it. The tank works as a thermal storage device – the boiler supplies heat to it, the water in it heats up to a set temperature (typically 55–60 °C) and from there travels to the points of use. The key is the combination of two parameters: tank volume (how many litres of water are stored in it) and boiler output resp. heat exchanger output (how quickly the tank can heat up a new batch of cold water).

That's why the same tank volume – say 120 litres – works completely differently with a boiler with an output of 12 kW and with a boiler with an output of 25 kW. A more powerful boiler heats the tank considerably faster, which means that even with a shorter break between the morning and evening draw, the household will have enough hot water. This is a crucial point that many customers forget when choosing.

The diagram shows the stratification of hot water in the tank – hot water naturally stays at the top, cold water enters from the bottom. The boiler's heat exchanger heats the water in the tank from the bottom up, which ensures a constant supply of hot water when drawn from the top.

Hot water consumption: what the real numbers say

Standards and manufacturers work with an average hot water consumption of 40–60 litres per person per day at a temperature of 45 °C. In practice, however, this depends on whether people shower or bathe, how long their morning hygiene routine takes, and how much hot water they consume in the kitchen. Let's look at realistic values:

Quick shower (5 minutes): 35–45 litres of hot water (with a mixing fitting with a flow rate of 9 l/min)
Standard shower (8–10 minutes): 60–80 litres of hot water
Bath in a tub: 120–180 litres of hot water (depending on the size of the tub)
Washing dishes, cooking, washing hands: 15–25 litres per person per day

This means that the actual daily hot water consumption per person ranges from about 50 litres (frugal household, showers only) to 150+ litres (a family with bathtubs and long showers). For practical calculations, Central Europe commonly uses a value of 50–60 litres per person per day at a tank temperature of 60 °C, assuming this water mixes with cold water to about 40–42 °C at the point of use.

Important note: the tank does not need to have a volume equal to the entire daily consumption. It depends on the intervals at which the boiler reheats the tank. If you have a powerful boiler, a 120-litre tank can cover even a 4-member family – the boiler simply reheats the tank in 20–30 minutes and you have a full tank of hot water again.

Reference table: tank volume by number of people

Below is a clear table that we use in practice as a first reference point. The values apply to a standard household with a shower cubicle (not a bathtub), with a condensing boiler with an output of at least 18–24 kW:

Number of people	Minimum tank volume	Recommended tank volume	Note
1–2 people	75 litres	100–120 litres	Apartment, shower only
2–3 people	100 litres	120–150 litres	Apartment or smaller house
3–4 people	120 litres	150–200 litres	Family house, 1 bathroom
4–5 people	150 litres	200–250 litres	Family house, 1–2 bathrooms
5–6 people	200 litres	250–300 litres	Larger house, 2 bathrooms
6+ people	250 litres	300+ litres	Large house, 2+ bathrooms

These values, however, are only a starting point. In the following sections we'll show when and why you need to go above these numbers – or, conversely, why less can be enough in some cases.

Impact of the number of bathrooms on tank selection

The number of bathrooms is just as important a parameter as the number of people – and in practice, it's a factor that people most often underestimate. Two bathrooms in a house doesn't automatically mean two people shower simultaneously, but it is a real possibility. And it is precisely simultaneous draw-off of hot water from multiple points that is the critical moment the tank must be sized for.

Imagine a typical morning scenario in a family house with 4 people and 2 bathrooms: at 7:00, two children shower in one bathroom, at 7:15 the parents step in. If the tank doesn't have sufficient volume, or the boiler can't keep up with reheating it, the parents will encounter lukewarm or cold water. This is not a hypothetical case – it's something we deal with repeatedly in practice with orders.

Practical recommendation by number of bathrooms:

1 bathroom (shower only): follow the table above, don't overestimate the volume
1 bathroom with a bathtub: add at least 50–80 litres on top of the reference value
2 bathrooms (showers): work with a value one level higher than shown in the table
2 bathrooms (1 bathtub + 1 shower): at least 200 litres for a 3-member family, 250 litres for a 4-member family
3 or more bathrooms: a tank of 300 litres or more, possibly consider two tanks or a larger industrial tank

How to correctly calculate the required tank volume: step-by-step procedure

Now let's look at a concrete calculation procedure that in practice gives more reliable results than just an "orientation table". The procedure is basically simple, but all the relevant parameters need to be incorporated into it.

Step 1: Determine the daily hot water consumption

Calculate your household's estimated daily hot water consumption. Base it on real habits, not on normative values, which can be far from reality.

Formula: Qd = n × q, where Qd is the daily hot water consumption (litres), n is the number of people, and q is the average consumption per person per day. For q, we recommend using these values:

Frugal household (only quick showers, minimal cooking): 40–45 l/person/day
Standard household (showers, cooking, cleaning): 55–65 l/person/day
Comfortable household (long showers, frequent baths): 80–120 l/person/day

Step 2: Identify the morning peak draw-off window

The tank doesn't need to have a volume equal to the entire daily consumption, but it must cover the so-called morning peak draw-off window – i.e. the time when the household consumes the most hot water in a short period. Typically this is in the morning between 6:30 and 8:00 (showers for the whole family) or in the evening between 19:00 and 21:00 (children's baths, evening showers for adults).

Estimate how many litres of hot water are consumed during your morning peak. Example for a 4-member family: 4 showers × 60 litres = 240 litres of hot water in about 60 minutes. This doesn't mean, of course, that you need a 240-litre tank – the boiler reheats the tank during those 60 minutes, so the actual required tank capacity is lower. For a typical boiler output of 20–24 kW, about 80–100 litres are reheated in 30 minutes, and about 150–200 litres in an hour.

Step 3: Factor in boiler output (tank heating time)

This is a technical parameter that most customers neglect when choosing. The actual capacity of the system depends on the combination of the tank and the boiler. Approximate time to heat the tank from 10 °C to 60 °C:

12 kW boiler + 120 l tank: heating approx. 50–60 minutes
18 kW boiler + 120 l tank: heating approx. 30–40 minutes
24 kW boiler + 150 l tank: heating approx. 35–45 minutes
24 kW boiler + 200 l tank: heating approx. 50–65 minutes

The shorter the heating time, the smaller a tank you can use. For most family houses the rule is: a boiler with an output of 20+ kW and a 150–200 litre tank is the ideal combination for a 4-member family.

Specific practical scenarios from everyday order practice

Theory is one thing, practice is another. In the following examples I'll show real situations we encounter with orders.

Scenario A: 3-room apartment, 2 adults + 1 child, 1 bathroom with a shower cubicle

This is probably the most common case in city apartments. Daily hot water consumption: 3 people × 55 litres = about 165 litres. Morning peak: usually 2–3 showers in a row in the morning. The boiler in the apartment will typically have an output of 18–24 kW. In this scenario, a tank of 75–120 litres is quite sufficient. A 120-litre tank gives you a comfortable buffer for two showers, and the third person will wait a maximum of 20 minutes for the boiler to reheat the rest. For this type of apartment, for example, the Vaillant VU 246/5-3 ecoTEC pro + VIH Q75 B set is suitable – a compact boiler with a 75-litre tank, ideal where boiler room space is limited and consumption is not extreme.

Scenario B: Family house, 4 people, 1 bathroom with a bathtub + 1 shower cubicle

Here the situation gets more complicated. A bathtub uses 120–180 litres of hot water in one go, which is already a bigger draw from the tank than a regular shower. If children shower in the morning and mum prefers a bath in the evening, the tank must have enough capacity for both of these draw-off scenarios. We recommend a tank of at least 150 litres, ideally 200 litres. For this scenario, a set such as the Vaillant VU 246/5-3 ecoTEC pro + VIH R 150 B is suitable, where the 150-litre tank in combination with a powerful condensing boiler easily covers a combination of a bath and a shower during one evening.

Scenario C: Larger family house, 5 people, 2 full bathrooms, both with bathtubs

This is a demanding scenario where a 120-litre tank is absolutely insufficient, and customers who bought one without expert advice then call back dissatisfied. With two bathtubs and 5 people, we recommend a tank of at least 200–250 litres, or more in the case of partly simultaneous draw-off. Here, not only the tank capacity is important, but also the boiler output – a boiler under 20 kW simply cannot reheat the tank fast enough. It's worth considering the Protherm Gepard Condens 25 MKO + FE 120 BM dolná + Thermolink P set as a base and adding a tank with greater capacity or a combination of two tanks.

Scenario D: Renovation of an older boiler room, transition from a combi boiler to a separate tank

During renovations, we often encounter people who just want to "replace the old boiler with a new one." When we also recommend changing the tank, the first reaction is resistance. But it's the right thing to do – the old tank (usually 80–100 litres combined with the boiler) was sized for a different type of source and a different level of comfort. Today, when a condensing boiler is efficient and cheap to run, people usually add a bathroom, buy a bigger bathtub, or extend the time spent in the shower. The tank needs to be upsized. Moreover, the old tank may be heavily oxidized or scaled with limescale, which dramatically worsens its efficiency – the heat exchanger transfers heat through a thick layer of scale, and the tank heats up twice as slowly.

When a 120-litre tank is enough and when it's not: clear boundary conditions

From practice, we know there are two common mistakes: people either buy a tank that is too small (because they wanted to save money or followed outdated rules) or one that is too big (because someone sold it to them "with a margin"). Both situations are problematic.

A 120-litre tank IS ENOUGH if:

There are a maximum of 3 people in the household
Only one shower cubicle is available (no bathtub)
Showers don't happen simultaneously (the family has a staggered morning routine)
The boiler has an output of at least 18 kW and reheats the tank in 30–40 minutes

A 120-litre tank IS NOT ENOUGH if:

There are 4 or more people in the household
There is at least one bathtub
There are two or more bathrooms with the possibility of simultaneous draw-off
The boiler has an output of only 12 kW (too long a heating time)
The house has underfloor heating and the boiler has to prioritize heating over reheating the tank

For a more detailed comparison of specific sets and their suitability for different types of households, see the article How to choose a condensing boiler and hot water tank set in our Knowledge Center.

Special situations: holiday properties, guesthouses, and properties with seasonal operation

Besides family houses and apartments, we also encounter tank selection for holiday cottages, guesthouses, and similar properties with a specific operating regime. Here, somewhat different rules apply.

Holiday property with irregular operation: if the property is empty most of the week and 6–8 people arrive at once on the weekend, the tank must cover the sudden demand. In this case, a tank of 200–300 litres is recommended, possibly combining a condensing boiler and tank with solar pre-heating. The tank is charged before guests arrive (timer or remote control) and fully covers the first wave of draw-offs.

A guesthouse with 4–6 rooms: here we're talking about a tank of 400–600 litres or a cascade connection, or possibly a powerful tank with fast continuous heating. This is, however, a category outside standard residential installation and requires an individual calculation from a designer.

Standing vs. wall-mounted tank: how it affects volume selection

The type of tank (standing or wall-mounted design) is directly related to the available volumes. Wall-mounted tanks are typically manufactured up to a volume of 120–150 litres, because a larger volume would be too heavy when hung, and the wall structure may not be able to support it. A standing tank has no such limitation – it is commonly made from 120 to 500+ litres.

So if you calculate a required volume of 200 litres, this automatically means a standing tank. For a 3-member household with one bathroom with a shower, where a 100–120 litre tank is enough, you can use a wall-mounted design, which is easier to install and doesn't require as much space in the boiler room. We cover this topic in more detail in the article Standing vs. wall-mounted tank with a condensing boiler: differences and advantages.

Available sets from Protherm with a lower boiler output and a 120-litre tank – for example the Protherm Gepard Condens 12 MKO + FE 120 BM dolná + Thermolink P – are designed precisely for smaller households (2–3 people, an apartment or smaller house), where a standing 120-litre tank in the form of a "lower" unit perfectly complements the compact boiler. The hydraulic connection via Thermolink P ensures that the tank always gets priority over heating – i.e. hot water for the shower will always come before heat in the radiators. How Thermolink P works and the whole logic of the hydraulic connection is explained in detail in the article Thermolink P and hydraulic connection of the boiler and tank: how it works.

Tank heat losses: why a bigger tank doesn't always mean better economics

One thing that is rarely discussed when choosing a tank is the heat losses of the tank in standby mode. Every tank has insulation, and heat escapes through every insulation. The bigger the tank, the bigger the surface area, the bigger the heat losses. Modern tanks have a loss of 0.5–1.5 kWh/24 hours depending on size and insulation quality, but old tanks can have losses of 3–5 kWh/day – that's a significant amount on the gas bill with year-round operation.

That's why the practical rule applies: don't buy a bigger tank than you need. A large tank gives a comfortable reserve, but if you never fill it (or the water turns over more slowly), it causes unnecessary heat losses, and during longer periods of standing water (e.g. a holiday) it can also be a problem from a hygiene perspective – water sits for a long time in the tank at a temperature favorable for the growth of legionella (40–50 °C).

The solution for larger tanks (200+ litres) is to set a periodic anti-legionella heating to 65–70 °C once a week, which the boiler will do automatically if you set it. More about proper operation and settings in the article Maintenance and service of a boiler and tank set: what and how often to check.

Tank and solar energy: how it affects the required volume

If you plan to add solar collectors for hot water pre-heating in the future, this needs to be taken into account when choosing the tank. A solar tank usually has a larger volume (180–300 litres) and different internal equipment (two heat exchangers – one for the solar circuit, one for the boiler). If you buy a standard tank today and decide on solar in 3 years, you'll have to replace the tank. In our order practice, we recommend that customers who are at least considering solar buy a bivalent tank right from the start – the extra cost is relatively small compared to the cost of a later replacement.

Tank and water pressure: an overlooked connection

A larger tank means a larger water column and different hydraulic conditions. At low pressures in the public water supply (below 2 bar), a larger tank can cause slow filling of a bathtub or uncomfortable pressure in the shower. Conversely, at high pressure (above 5 bar), a pressure reducing valve is necessary so the tank doesn't get overpressurized. This is a technical matter for the installer, but as a customer you should be aware of it – ask your installer about the water pressure in the property before choosing a tank.

Frequently Asked Questions (FAQ)

Is an 80-litre tank enough for a family of four (2 adults + 2 children)?

Basically, no. For a 4-member family, 80 litres is really the minimum, which will only work if showers never happen simultaneously and mornings are staggered. We recommend at least 120–150 litres. An 80-litre tank is suitable at most for a 2-member household without a bathtub, where showers are taken one after another with a break for the tank to reheat.

We have both a bathtub and a shower cubicle – is a 150-litre tank enough for 4 people?

It depends on how you use the bathtub and shower. If the bathtub is only for the children once or twice a week and the adults just shower, 150 litres will suffice. If the bathtub is filled daily and adults shower in the morning at the same time, 150 litres may be on the edge – in that case we recommend a 200-litre tank and a boiler with an output of at least 20 kW for sufficiently fast heating. An alternative is the Vaillant VU 246/5-3 ecoTEC pro + VIH R 120 B set, where the 120-litre tank works great for a 3-member family with standard demands.

Is it better to have a bigger tank with a smaller boiler, or a smaller tank with a bigger boiler?

Generally, a more powerful boiler with an appropriately sized tank is a more efficient solution than a large tank with a weak boiler. A powerful boiler reheats the tank quickly, so you don't need a huge reserve. A large tank with a slow boiler causes long waiting times after the reserve is exhausted and unnecessary heat losses. The ideal is a balanced combination – a boiler of 20–24 kW with a tank of 120–200 litres covers most family houses.

Can I replace a 120-litre tank later with a bigger one without replacing the boiler?

Yes, the tank and boiler are separate components in the set. The boiler stays, the tank is replaced. However, you need to check whether the hydraulic connection (Thermolink or direct circuit) and the hot/cold water pipes allow for a larger tank – a larger tank is physically bigger and heavier, so the boiler room must have enough space and the floor must be able to handle the weight of the full tank (a 200-litre tank full of water weighs about 220–240 kg).

How does the tank behave during a longer holiday (2–3 weeks)?

The tank should not remain in standby heating mode without use for longer than 5–7 days, because standing water at 45–55 °C can support the growth of legionella bacteria. Before going on holiday, we recommend turning off the tank (a "holiday" mode on the control) or setting the boiler to anti-legionella heating at 70 °C and then turning it off. After returning, let the tank reheat to full temperature before first use. A larger tank capacity does not improve this situation in any way – on the contrary, more water stands longer.

What tank do you recommend for a new-build 4-room family house with two bathrooms?

For a new-build 4-room house with 4–5 people and two bathrooms (one with a bathtub), we recommend a 200-litre tank combined with a condensing boiler with an output of 20–25 kW. This combination reliably covers morning peaks as well as evening baths, while not being excessive in terms of heat losses. If you want a reserve for the future (the possibility of solar collectors), invest from the start in a bivalent tank with two heat exchangers.

Summary: how to proceed when choosing the tank volume

Choosing the right hot water tank volume isn't rocket science, but it does require a systematic approach. To conclude, let's recap the key principles:

Size the tank according to the number of people and the number of bathrooms – both factors are equally important
Factor in the boiler output – a more powerful boiler allows for a smaller tank, a less powerful boiler requires a larger tank
The peak hot water draw-off (morning showers, evening baths) is decisive, not the average daily consumption
Don't buy a bigger tank than you need – a bigger tank = higher heat losses
If you're planning solar or a bathtub in the future, take that into account in today's choice
For most family houses (4 people, 1–2 bathrooms), the ideal is a tank of 150–200 litres with a boiler of 20–24 kW

If you're not sure, take a look at the specific boiler and tank sets in our category – each set is designed as a functional whole, where the tank volume and boiler output are balanced with each other. For a more detailed look at choosing the right set, also read the related article Boiler with tank or instantaneous water heater: which is more worthwhile, which compares both basic approaches to hot water heating from both an economic and comfort perspective.

Do you have a question about this topic?

Can't decide or are you dealing with a specific situation in your household? Write to us - we'll be happy to advise.