What tank volume do I need for my home

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What tank volume do I need for my house – a complete guide to selection

Selecting the correct pressure tank volume is one of those decisions where mistakes only become apparent after some time – and when they do, they usually do so in a very unpleasant way. A pump that starts every twenty seconds, hums and jumps in pressure surges, slowly destroys the entire household plumbing system. On the other hand, an oversized tank unnecessarily takes up space and costs money you never needed to spend. The goal of this article is to give you a concrete guide – not vague recommendations, but actual numbers, formulas and real-life scenarios, so you can know exactly what you need.

If you are just starting to make your decision and want to first understand what a pressure tank actually does and why you need it, I recommend reading the article How to choose a pressure tank for a household water system and Pressure tank with bladder or without bladder – what is the difference. Here we will focus purely on the volume – that is, how many liters you actually need.

Why volume matters – basic physics in short

The pressure tank plays one key role in the household water system: it stores water under pressure, which you can draw off without the pump having to start immediately. This storage is called in technical terminology the usable (effective) volume – and note, this is always significantly less than the total nominal volume of the tank.

Why? Because the tank is divided by a membrane or bladder (or in older galvanized tanks without a bladder by a layer of air) into two parts: the air chamber and the water chamber. The air is compressed to a preset pressure (called pre-charge pressure), and when the pump fills the tank with water to the maximum pressure (shut-off pressure), the air has even less space. When you draw water, the air expands and pushes it out until the pressure drops to the pump start pressure. Then the pump starts again.

This is the key number you must always keep in mind: the usable volume of a pressure tank is typically 25 to 38 percent of its nominal volume, depending on the pressure difference (hysteresis) set between the pump start and stop pressure. With a classic setting of 2.0–4.0 bar (start/stop), it comes out to roughly 30–35 percent of the nominal volume.

What is the minimum number of pump starts and why it is controlled by the tank volume

Every electric motor of a pump has a certain maximum allowed number of starts per hour. For standard single-phase household pumps (0.5–1.5 kW), it is usually 20 to 30 starts per hour, for more powerful three-phase pumps it can be even less – typically 15–20 starts per hour. The manufacturer states this in the pump's technical specifications and you should take this number seriously: each start causes thermal and mechanical shock to the windings and bearings.

A properly designed pressure tank ensures that the pump will start at most as many times as the manufacturer recommends – even in the worst-case scenario of continuous small water withdrawals (e.g., a constantly running toilet flusher, a slightly dripping tap). Precisely these small, frequent withdrawals are the most dangerous for the pump.

Basic formula for calculating the required pressure tank volume

There are several versions of calculation formulas, but in practice, this approach has proven most effective, based on the pump's performance and the required number of starts:

Minimum usable volume (in liters) = (Q × 60) / (4 × n)

where:

Q = pump flow rate in liters per minute (l/min)
n = maximum allowed number of starts per hour
the constant 4 ensures an optimal cycle for the worst-case scenario

Concrete example: you have a pump with a performance of 50 l/min (3 000 l/hour) and the manufacturer allows 20 starts per hour.

Usable volume = (50 × 60) / (4 × 20) = 3 000 / 80 = 37.5 liters

Since the usable volume is about 30–35 % of the tank's nominal volume, you need a tank with a nominal volume of at least: 37.5 / 0.33 = approximately 114 liters nominal. In practice, you would go for a 150-liter or even a 200-liter tank, as it is always reasonable to have a reserve.

From the graph, it is clear why a 24-liter tank is insufficient for most households with a more powerful pump – the pump would start almost 50 times per hour, which is literally destroying its lifespan. It is suitable only for very low flow rates or for garden irrigation with minimal requirements.

Real-life scenarios – how many liters for specific situations

Garden cottage or summer house for one person

Typical situation: well, small surface pump (e.g., 30–40 l/min), one bathroom, kitchen. Water consumption is irregular, mostly on weekends. In such a case, a tank with a nominal volume of 24 to 50 liters is sufficient. For garden irrigation or a shower, the Pressure tank AQUA OLA 24l horizontal is a practical option – the horizontal design allows flexible placement directly next to the pump and takes up minimal space. It is, however, important to realize that 24 liters nominal provides only 7–8 liters of usable volume, which is not enough for simultaneous use at two points.

Family house with permanent occupancy, 3–4 people

This is the most common scenario we encounter. A pump with a capacity of 50–80 l/min, 2–3 bathrooms, kitchen, washing machine, garden. The critical moment is the morning peak when two people are showering simultaneously and the washing machine is running on the first cycle. In this case, you need to plan for a tank with a nominal volume of 100 to 200 liters. Specific recommendation: for a house with three people, the Galvanized pressure tank HVP 200L with 6 bar certification is a very popular choice – the vertical design saves floor space and the water gauge provides an immediate overview of the current supply.

Larger family house or small farm, 5–8 people

We are now entering the area where we need a nominal volume of 200 to 500 liters. Larger irrigation areas are added, often also feeding livestock, a swimming pool or an outdoor shower. The pump is usually more powerful – 80 to 150 l/min, sometimes even more. For such needs, the Galvanized pressure tank HVP 300L or stainless steel alternatives come into consideration.

Garden with automatic irrigation

An irrigation system with solenoid valves is a specific application. Each opening and closing of a section = one pressure cycle. With 6–8 sections alternating every 20 minutes, the pump can start the system 18–24 times per hour. You need a sufficiently large reserve so that the pump does not have to run with every transition between sections. Recommended nominal volume: at least 100–150 liters, ideally 200 liters.

Pressure tank without vacuum – different calculation rules

Older, and still commonly used in Slovakia, galvanized tanks without a membrane or vacuum operate on a different principle. Air and water are in direct contact – air is gradually absorbed into the water and the tank "wets out" (loses its air cushion). They need to be regularly topped up with air, otherwise they operate without any reserve and the pump runs almost continuously.

For tanks without a vacuum, the required nominal volume is significantly higher – it is recommended to calculate with 3 to 4 times the volume compared to a tank with a vacuum for the same usable volume. Therefore, these tanks are mostly manufactured in larger volumes (100, 200, 300, 500 liters and more). In practice, 200 liters is the absolute minimum for a typical family house, 300 liters is the standard for comfort. The article Pressure tank with vacuum or without vacuum – what is the difference covers this topic in depth.

Stainless steel tanks – when are they a solution and in what volume

Stainless steel pressure tanks offer a significant advantage when used for drinking water in terms of hygiene and resistance to corrosive media. For volumes up to 100 liters, a stainless steel construction is economically feasible and very well accepted in practice. The Stainless steel pressure tank CH 90 L covers the needs of a smaller family house (2–3 people) with a pump up to 50 l/min. If you have a larger house, a larger family or a more powerful pump, or garden irrigation, the logical choice is the Stainless steel pressure tank CH 300 L, which provides a full reserve even for demanding operations.

Stainless steel tanks are usually equipped with a vacuum (membrane), so the usable volume is higher in relation to the nominal volume compared to galvanized tanks without a vacuum. Their advantage is also long-term maintenance-free operation – they are not prone to corrosion of internal surfaces and do not require regular anode replacement or protective coatings.

More about material differences can be found in the article Zinc-coated vs. stainless steel pressure tank – what is better.

Table of recommended volumes according to house size and number of people

Type of building / number of people	Pump flow rate	Tank with bladder (min.)	Tank without bladder (min.)
Garden cabin, 1–2 people	20–35 l/min	24 – 50 L	100 L
Family house, 2–3 people, 1 bathroom	35–60 l/min	60 – 100 L	150 – 200 L
Family house, 3–5 people, 2 bathrooms	50–80 l/min	100 – 150 L	200 – 300 L
Larger house + garden, 4–6 people	60–100 l/min	150 – 200 L	300 L
Farm / small business	Over 100 l/min	300 – 500 L	500+ L

Mistakes customers make when choosing the volume – practical experience

Mistake No. 1: Confusing nominal and usable volume. A customer buys a 24-liter tank and is surprised that the pump runs all the time. In fact, only 6–8 liters of the 24 liters are actually usable. A single shower with a flow rate of 10 l/min would drain the supply in less than a minute and the pump would have to start again. The tank is not sufficient.

Mistake No. 2: Underestimating garden irrigation. Garden irrigation can consume 1 000–3 000 liters per hour for a larger garden. If a customer does not include this in the calculation when choosing the tank, they end up with a tank that is insufficient and the pump is overloaded exactly during the summer months when the garden is most important.

Mistake No. 3: Buying a too small tank "for testing". A small tank is cheap, that is true. But if it turns out to be insufficient, you will have to buy a larger one – and the small one will then stand in a corner or you will sell it at a loss. You end up paying twice. In our practice, we see this scenario almost every season. The first time is always cheaper to buy a larger tank.

Mistake No. 4: Ignoring pressure losses in the piping. If your house is large or has multiple floors, pressure losses in the piping are significant. The pressure tank must operate at a higher cut-in pressure, which reduces the effective hysteresis range and thus the usable volume. For two-story houses, add at least 0.3–0.5 bar when setting the hysteresis difference, which effectively reduces the usable volume of the tank.

Mistake No. 5: Insufficient reserve for power or pump failure. Some customers expect that a large pressure tank will serve as a water reserve during a power outage. Partially yes – after a power outage, you have the usable volume available, which is typically a few liters. But if you need a real reserve for hours, a pressure tank alone does not solve it. Storage tanks are used for that. You should clarify the difference between pressure and storage tanks before purchasing.

How to consider pressure and pressure switch settings in the calculation

You cannot calculate the volume without knowing the pressure at which you will set the system. The standard setting for a family house is:

Switch-on pressure (Pon): 1.5 – 2.5 bar (typically 2.0 bar)
Switch-off pressure (Poff): 3.0 – 4.5 bar (typically 3.5 – 4.0 bar)
Air pressure in the tank (Pv): Pon minus 0.2 bar (typically 1.8 bar)

The greater the pressure difference (hysteresis) between switching on and off, the greater the usable volume from the same tank. Specifically: with a hysteresis of 1.5 bar (e.g., 2.0/3.5 bar), you get about 28–32 % of the nominal volume; with a hysteresis of 2.5 bar (e.g., 2.0/4.5 bar), it can be 35–40 %. For houses with higher pressure requirements (e.g., a house height over 10 meters, long pipe runs), set a higher Pon – but keep in mind that Poff also increases, and the pump must be dimensioned for that pressure.

This topic is discussed in detail in the article What pressure to set in the pressure tank and how to check it – I recommend reading it in parallel with calculating the volume, as both are closely related.

Vertical or horizontal tank – impact on volume selection

The orientation of the tank (vertical standing vs. horizontal lying) has no direct impact on hydraulic function or volume calculation – they are physically equivalent. The choice of orientation mainly affects the spatial solution of the installation and accessibility. Smaller tanks (up to 50 liters) are available in both versions, while larger ones (over 100 liters) are almost exclusively vertical, as they would occupy too much space in a horizontal position. The article Standing, lying, or vertical pressure tank – which type to choose discusses this issue in more detail.

Standby capacity and accumulation – when a larger tank makes sense

In some cases, it is advantageous to choose a larger tank than the minimum volume calculated. Specific situations where this makes sense:

Unstable water source: If your well has low yield (less than 500 l/hour), a larger pressure tank acts as a buffer that smooths out peaks. The pump can continue pumping even when direct consumption is zero, storing the supply.
Frequent power outages: A larger supply in the tank provides water for the first necessary time after a power outage – for flushing, handwashing, etc.
Automatic irrigation with solenoids: Each section opening requires a pressure pulse. A larger tank provides this pulse without immediate pump start-up.
Presence of small children or elderly people in the household: A larger comfort supply means that pressure does not drop even during a long bath or simultaneous use of multiple outlets.

Practical step-by-step procedure for selection

Based on all the information above, you can proceed as follows:

Determine the flow rate of your pump in l/min or m³/h (from the technical sheet or pump label).
Determine the maximum allowed number of starts per hour (also from the pump technical sheet).
Calculate the minimum usable volume using the formula: (Q × 60) / (4 × n).
Choose the assumed efficiency of the tank: with a bladder = 33–38 %, without a bladder = 20–28 %.
Calculate the required nominal volume: usable volume / efficiency coefficient.
Add a reserve of 20–30 % for peaks, garden irrigation, and comfort.
Round up to the nearest available tank size in the catalog.

Example of the entire process for a typical family house: pump 60 l/min, 20 starts/hour, tank with bladder (35 %), garden irrigation for 400 m²:

Minimum usable volume = (60 × 60) / (4 × 20) = 45 liters
Minimum nominal volume = 45 / 0.35 = 128 liters
With 25 % reserve = 128 × 1.25 = 160 liters
Result: 200-liter tank is the correct choice

Frequently asked questions (FAQ)

Can I connect two pressure tanks in parallel if one is not enough?

Yes, connecting two tanks in parallel is technically problem-free and is used in practice. Both tanks must be connected to the same location in the system, must have the same air pressure and the same connection dimensions. The resulting usable volume is the sum of both tanks. This is a reasonable solution when your original tank is later insufficient and you do not want to change the entire installation – simply add a second tank next to the first one. More on installation can be found in the article Mounting a pressure tank to a home water system step by step.

What happens if I install a tank that is too large?

Nothing bad happens in terms of functionality – the pump will start even less often, and you will have a larger supply. The disadvantages are economic (higher tank cost) and spatial (tank takes up more space). In some cases, with very infrequent water consumption, water in the tank may stagnate for longer, which is unsuitable for drinking water without disinfection – but this is not a problem with normal daily use.

How can I find out if my current pressure tank is too small?

The most reliable sign is the number of pump starts per hour – count them during normal morning use (showers, cooking, laundry). If the pump starts every minute or even more frequently, the tank is insufficient. Another sign is rapid pressure fluctuations when using two outlets simultaneously (e.g., shower + flushing). The article Common pressure tank problems and how to fix them contains further diagnostic tips.

Does the volume calculation apply to submersible pumps as well as surface pumps?

The basic formula applies to both types, as it is based on flow and allowed number of starts – these are pump parameters regardless of its location. Submersible pumps may have a higher allowed number of starts (some models up to 30–40/hour due to water-cooled motors), which can slightly reduce the calculated minimum volume. Always check the exact number in the technical sheet of your pump.

Can I use a pressure tank for heating or only for water supply?

No – pressure tanks for home water supply (water tanks) are designed for water at room temperature and are not approved for heating. For heating systems, there are special expansion tanks that are dimensioned for different temperatures and pressures and have different certification. These products are a different category with different standards.

How long does a pressure tank last and when should the bladder (membrane) be replaced?

The lifespan of a quality tank is 10–20 years, but the bladder (membrane) can wear out sooner – typically after 5–10 years of intensive use. Signs of a damaged bladder are: water dripping from the air valve during pressure check, or sudden loss of storage function (the pump starts as fast as without the tank). Bladders are replaceable on most tank models, so you do not need to buy a whole new tank. Details can be found in the article Maintenance and service of pressure tanks – what and when to check.

Conclusion – investing in the right size always pays off

Selecting the correct pressure tank volume is not rocket science, but it does require a few minutes with a calculator and a few technical parameters of your pump. Equipped with these numbers, you can choose exactly the tank your house really needs – not the one that costs the least, and not the one that is unnecessarily large without reason.

If you have a typical family house with 3–5 people and a more powerful pump, you will be in the range of 150 to 300 liters of nominal volume. For a cottage or garden water supply, a smaller tank is sufficient, but even there, 24 liters is at the limit of usability for any more than a minimalist application.

If you are unsure, it is always wiser to go for a higher capacity – the cost of a larger tank is negligible compared to the cost of repairing an overloaded pump. And if you are hesitating between materials, bladder types, or tank orientation, other articles in this knowledge section will help you decide on these issues as well.

Do you have a question on this topic?

Having trouble making a decision or dealing with a specific situation in your household? Write to us – we'll be happy to help.