What Size Water Tank Do I Need? How UK Businesses Calculate Storage Capacity by Sector and Application
Choosing the wrong water tank size is one of the most avoidable and costly mistakes in any water storage project. Too small and you risk running out of water at the worst possible moment, interrupting operations, failing a compliance requirement, or leaving a site without supply during peak demand. Too large and you pay for capacity you never use, risk water stagnation inside the tank, and create unnecessary installation costs.
The right size water tank is not about choosing the biggest option available. It is about calculating the correct capacity for your specific application, your daily water usage, your peak demand periods, and the sector you operate in. This guide explains how to work out what size water tank you need, what factors affect the calculation, and how Butek Tanks' range of corrugated steel water storage tanks covers every capacity from 2m³ to 5,000m³ across every UK sector.
What Affects Water Tank Size?
Before arriving at a tank size, it is important to understand the factors that determine how much storage capacity any site actually needs. There is no single standard size that suits every application. The right size water tank depends on several variables working together.
Daily water demand is the starting point for any tank sizing calculation. This means the total volume of water consumed across the site in a typical 24-hour period, accounting for all uses: drinking water, process water, irrigation, cleaning, livestock, fire suppression, or any other application the tank will serve. Calculating daily water demand accurately is the foundation of choosing the right tank.
Peak demand is equally important and is frequently underestimated. Peak demand refers to the highest rate at which water is drawn from the tank at any one time, typically during the busiest period of the site's operating day. A tank that meets average daily demand but cannot supply water fast enough during peak usage will cause the same problems as a tank that is simply too small.
Storage period determines how many days of supply the tank needs to hold. Some applications require only one day of buffer storage. Others, particularly agricultural sites under SSAFO regulations or remote sites with infrequent deliveries, require four to six months of storage capacity. The storage period multiplied by the daily demand gives the minimum tank volume required.
Water source and replenishment rate affects the sizing calculation because a tank that is continuously topped up from a mains supply needs less capacity than a tank that is filled periodically by a tanker or relies on rainwater collection. If the replenishment rate is slower than the consumption rate, the tank must be large enough to bridge the gap.
Number of occupants or users is relevant for potable water and welfare applications. A common starting point is 150 litres per person per day for general water usage across drinking, washing, sanitation, and cooking. For a site with 50 occupants requiring one day of buffer storage, the minimum tank size would be 7,500 litres or 7.5m³.
Application type is the final major variable. Different applications have entirely different sizing requirements. A cold water storage tank in a commercial building is sized differently to an irrigation water tank on a farm, which is sized differently again to an effluent storage tank on an industrial site or a fire suppression tank on a construction site.
How to Calculate the Right Water Tank Size
The basic calculation for any application follows the same logic:
Daily water demand (litres) multiplied by the number of days of storage required equals the minimum tank capacity needed.
For example: a food processing site that uses 20,000 litres of process water per day and requires two days of buffer storage needs a minimum tank capacity of 40,000 litres, or 40m³.
For rainwater harvesting applications, the calculation also incorporates the catchment area and local rainfall data. The standard formula is: roof area in square metres multiplied by annual rainfall in metres multiplied by a runoff coefficient of 0.85, which gives the annual harvest in litres. Dividing this by 365 gives the average daily yield, which can then be matched against daily demand to establish the right tank size for the application.
For irrigation applications, the calculation is based on crop type, field area, irrigation frequency, and the number of days between irrigations. A storage solution that holds enough water for one full irrigation cycle is the typical starting point, with additional capacity added to account for dry periods and low rainfall.
For fire suppression and sprinkler systems, tank sizing is determined by the flow rate required by the system in litres per minute, multiplied by the duration of the design fire. This calculation must comply with the relevant British Standard and is typically carried out by a fire engineer. Sizing errors in fire suppression tanks carry serious life-safety implications and should always involve a qualified installer and specialist guidance.
Water Tank Sizes by Application: UK Guide
The following guide covers the most common applications for water storage tanks across UK sectors, with indicative capacity ranges. All sizes refer to Butek Tanks' corrugated steel water storage tanks, which are available from 2m³ to 5,000m³ and can be connected in series for larger volumes.
Potable Water Storage for Commercial Buildings
Cold water storage tanks in commercial buildings, offices, hotels, schools, and healthcare premises typically serve as buffer storage between the mains supply and the building's internal distribution system. A common rule of thumb for cold water storage tank sizing in commercial buildings is one day's demand as a minimum, with many facilities management specifications requiring between one and two days.
For a medium-sized office building with 100 occupants, daily cold water demand at 150 litres per person gives 15,000 litres. A cold water storage tank of 15m³ to 20m³ would be appropriate, accounting for peak demand periods. High-quality water tanks for potable applications must be WRAS-approved and fitted with an appropriate liner, with strength and durability to match the installation environment.
Potable Water Storage for Construction Sites
Construction site welfare facilities require a reliable water supply for drinking, welfare facilities, and sanitation. HSE guidance recommends a minimum of 45 litres of potable water per person per day for construction welfare use. A site with 80 workers requires a minimum of 3,600 litres per day. For a site supplied by weekly tanker delivery, the water tank you need would be a minimum of 25,200 litres or approximately 25m³, with most specifiers adding 20 to 30 percent contingency to avoid running out of water between deliveries.
Agricultural Water Storage
Farm water storage requirements vary significantly by enterprise type. Livestock water demand ranges from approximately 5 litres per day for sheep to 70 litres per day for a dairy cow. A dairy farm with 200 cows requires a minimum of 14,000 litres per day for livestock alone, before accounting for parlour washing, yard cleaning, and other farm water uses.
Our slurry and digestate storage tanks are separately sized under SSAFO regulations, which require a minimum of four months of storage capacity. For a 200-cow dairy producing 40,000 litres of slurry per week, four months of storage requires approximately 700,000 litres or 700m³.
Rainwater Harvesting Storage
For commercial rainwater harvesting, the right size water tank is determined by balancing the annual catchment yield against demand. A large warehouse roof of 5,000m² in the Midlands, with an average annual rainfall of 650mm and a runoff coefficient of 0.85, yields approximately 2,762,500 litres per year or 7,568 litres per day. If the site uses harvested rainwater for toilet flushing and vehicle washing at a demand of 3,000 litres per day, a storage tank of 15m³ to 20m³ provides between five and seven days of buffer, which is the standard specification for commercial rainwater harvesting systems.
Aquaculture and Fish Farming
Tank sizing in aquaculture is determined by stocking density, species requirements, and system design. In a recirculating aquaculture system, the holding tank volume must support the biomass of fish at the required stocking density while maintaining water quality within species-specific parameters. For Atlantic salmon production, a common stocking density is 50kg per m³. A production unit targeting 10,000kg of fish at harvest therefore requires a minimum holding volume of 200m³, with additional volume for settlement, treatment, and buffer tanks within the RAS loop.
Industrial and Process Water Storage
Industrial water storage requirements are highly site-specific and depend on process type, flow rate, and the consequences of supply interruption. A manufacturing site that uses 100,000 litres of process water per day and requires 48 hours of buffer storage to manage supply risk needs a storage capacity of 200m³. Industrial tanks must be sized not only for volume but for the chemical compatibility of the stored liquid with the liner material, making liner kit selection an integral part of the sizing decision.
GRP Tanks vs Corrugated Steel Tanks: Size and Capacity Considerations
When comparing different water tanks for large-scale commercial and industrial applications, the size range available is a critical factor. GRP tanks and plastic tanks are available up to around 200,000 litres in standard configurations. Corrugated steel tanks offer a significantly larger range of sizes, from 2m³ to 5,000m³ in a single tank, with the ability to connect multiple units for even greater capacity. For applications requiring larger tank volumes, the sectional and modular nature of corrugated steel tanks makes them the most practical storage solution across UK commercial, agricultural, and industrial sectors.
Old tank replacement is also a common driver for new tank specification. When replacing an old tank, the replacement sizing calculation should reassess current demand rather than simply matching the old tank size, as water usage and site requirements may have changed significantly since the original installation.
Choosing the Right Liner for Your Tank Size and Application
Tank size and liner choice are closely connected decisions. Our liner kits are manufactured in-house across a full range of materials, each suited to different applications and liquid types:
Butyl rubber: the premier choice for potable water storage, WRAS-approved, non-leaching, and suitable for the full range of tank sizes from 2m³ to 5,000m³.
EPDM rubber: a cost-effective WRAS-approved alternative to Butyl for potable and clean water storage applications.
Landflex ES: the standard specification for effluent, slurry, digestate, and chemically aggressive liquid storage, with outstanding chemical resistance across the full range of industrial and agricultural applications.
HDPE: the industry standard for aquaculture tanks, non-toxic and easy to clean between production cycles.
PVC: suited to trade effluent and general commercial liquid storage where chemical demands are lower.
The liner material affects not only the suitability of the tank for its application but also the long-term lifespan and maintenance requirements of the installation. Choosing the right liner at the point of sizing avoids costly relining work later.
Protecting Your Tank Investment with the Right Roof Kit
Once the right tank size and liner have been selected, covering the tank correctly is the final step in a complete storage specification. An uncovered tank of any size is vulnerable to contamination, evaporation, Legionella risk, and in agricultural applications, SSAFO compliance failure. Our roof kits are available across the full range of tank sizes and include heavy-duty steel roofs, Aqua-Float floating covers that prevent up to 98% of evaporation, and anti-algae covers for potable water applications.
Frequently Asked Questions About Water Tank Sizing
What size water tank do I need for a commercial site?
The right size water tank for a commercial site depends on daily water demand, peak demand periods, the number of occupants or users, and the required storage period. A common starting point is one day's demand at 150 litres per person per day, with additional capacity added for peak demand and supply buffer. Butek Tanks offers a free site survey to calculate the correct tank size for your specific application.
How do I calculate the right water tank size?
Multiply your daily water demand in litres by the number of days of storage you need. For example, 10,000 litres per day multiplied by three days of storage gives a minimum tank capacity of 30,000 litres or 30m³. For rainwater harvesting, multiply your roof area by annual rainfall and a 0.85 runoff coefficient to calculate your annual yield, then size the tank to hold your required buffer volume.
What is the largest water tank size available from Butek Tanks?
Butek Tanks' corrugated steel water storage tanks are available as single units up to 5,000m³. For larger volumes, multiple tanks can be connected in series. This range of sizes makes Butek Tanks the most flexible storage solution available for UK commercial, agricultural, and industrial applications.
Are steel tanks available in the same sizes as GRP or plastic tanks?
Yes, and in significantly larger sizes. Corrugated steel tanks from Butek are available from 2m³ to 5,000m³, covering the full range of capacities available in GRP and plastic tanks and extending well beyond them for high-volume industrial, agricultural, and utility applications.
How does liner choice affect tank sizing?
The liner material determines which liquids the tank can safely store and affects long-term durability. Choosing the correct liner at the point of sizing avoids relining costs and ensures the tank remains compliant and operational across its full service life. Butek Tanks' team specifies the correct liner as part of the tank sizing and design process.
Can I add capacity to an existing tank?
Yes. Butek Tanks' corrugated steel tanks can be expanded by connecting additional tank units in series, allowing storage capacity to grow with your operation without replacing the existing installation. This sectional design is one of the key advantages of corrugated steel over fixed concrete or GRP tanks.
Conclusion
Getting the right water tank size is not guesswork. It is a calculation based on your daily demand, peak usage, storage period, application type, and the liquid being stored. Whether you need a small potable water buffer for a construction site welfare unit or a large multi-thousand cubic metre process water or slurry storage system, the correct sizing process is the same: establish your demand, calculate your required capacity, choose the right liner, and cover the tank correctly.
At Butek Tanks, we have been manufacturing corrugated steel water storage tanks since 1965. Our range covers 2m³ to 5,000m³ across every sector, with liner kits, roof kits, and accessories specified as part of a complete tank solution. Contact our team today to arrange a free site survey and get an accurate tank size recommendation for your site and application.