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Calculate cylindrical and rectangular water tank capacity in litres, cubic metres (mΒ³), UK gallons, and US gallons. Professional plumbing engineering tool with storage sizing guides, water demand analysis, and BS EN 806 compliance references for domestic, commercial, and industrial water storage systems.
Select tank shape, enter internal dimensions, choose units, and optionally specify water depth for partial-fill calculations. Results update instantly for cylindrical and rectangular storage tanks.
The fundamental formula for calculating the volume of a cylindrical water tank is derived from the geometry of a right circular cylinder. This applies to both vertical cylindrical tanks (found in loft spaces and on rooftops) and horizontal cylindrical tanks (common in underground installations and industrial storage).
Where:
For partially filled horizontal cylindrical tanks, the calculation is more complex and uses the following engineering formula:
Where d is the water depth measured from the bottom of the tank. This formula accounts for the circular segment area and is essential for accurate hydraulic storage calculations in plumbing systems, break tanks, and booster pump reservoirs.
Rectangular water storage tanks are the most common type used in domestic cold water storage, sectional tank assemblies, and commercial break tank installations. The volume calculation is straightforward:
Where all dimensions are internal measurements in consistent units. For partial fill, substitute the water depth for the height:
This formula underpins rectangular storage tank sizing estimates for apartment buildings, hotels, offices, and industrial facilities. When specifying sectional GRP tanks, the effective internal volume must account for internal flange projections and access hatches.
Water tank volume refers to the total internal storage capacity of a water tank, measured in litres (L), cubic metres (mΒ³), or gallons (UK or US). It represents the maximum quantity of water the tank can hold when filled to its designed operating level. In plumbing engineering and hydraulic systems design, tank volume is a critical parameter that determines:
In potable water storage systems, tank volume directly impacts water quality. Oversized tanks risk water stagnation and bacterial growth; undersized tanks risk supply failure during peak demand periods. The balance is achieved through careful water demand and tank capacity analysis.
Correctly sizing a water storage tank is one of the most important decisions in plumbing system design. Incorrect sizing leads to serious operational problems:
In residential plumbing systems, water storage tanks serve as cold water storage cisterns, typically located in loft spaces, airing cupboards, or dedicated tank rooms. Common domestic applications include:
Typical domestic storage requirements range from 200 to 500 litres for a family home, depending on occupancy and whether the property has a direct or indirect water supply system. For flats and apartments, individual domestic water tank sizing calculations usually allocate 100β150 litres per occupant.
Commercial buildings require significantly larger and more robust water storage systems. The design approach differs from domestic installations due to higher occupancy densities, extended operating hours, and regulatory requirements for emergency reserves.
| Building Type | Storage per Occupant (L/day) | Typical Total Storage | Retention Guidance |
|---|---|---|---|
| Office buildings | 90β120 | 5,000β50,000 L | 24-hour turnover |
| Hotels | 150β200 | 20,000β200,000 L | 12β24 hour turnover |
| Schools | 60β90 | 2,000β20,000 L | Daily turnover |
| Hospitals | 250β350 | 50,000β500,000 L | 12-hour minimum |
| Warehouses | 45β60 | 1,000β10,000 L | 24-hour turnover |
| Industrial facilities | 60β100 | 10,000β100,000 L | Process-dependent |
Values based on BS EN 806 and CIBSE Guide G guidelines. Always verify with local water regulations.
Underground tanks are commonly used for rainwater harvesting, fire reserve storage, and large-scale commercial water storage where above-ground space is limited. Key considerations include structural loading from surrounding soil, groundwater buoyancy (flotation prevention), access for maintenance, and protection against contamination from surface water ingress. Underground water tank calculations must account for excavation dimensions and structural wall thickness.
Overhead tanks utilise gravity to provide static pressure to the plumbing system below. Common in high-rise residential and hotel buildings, these tanks are typically constructed from GRP, steel, or concrete. The overhead water tank sizing formula must balance storage volume against structural loading on the building frame. Each metre of elevation provides approximately 0.1 bar (10 kPa) of static pressure at the outlet.
A break tank is a critical component in modern booster pump systems, creating a physical air gap between the mains water supply and the pumped distribution system. This prevents backflow contamination and provides a reservoir from which booster pumps can draw water without directly connecting to the mains β a requirement under UK Water Supply Regulations for Category 5 fluid risk applications.
Break tank volume calculations must consider:
Accurate water demand calculations form the foundation of proper tank sizing. Plumbing engineers use occupancy data, fixture unit analysis, and peak demand profiling to determine required storage volumes. The key formula for initial estimation is:
Where the storage duration factor typically ranges from 1.0 (24-hour storage) to 2.0 (48-hour storage for critical applications). Peak demand multipliers of 1.5Γ to 3.0Γ average demand are applied for systems with high simultaneity of use.
For apartment building water storage sizing, engineers calculate based on the total number of bedrooms plus an allowance for common areas. Hotel water tank volume calculations must include kitchen, laundry, and leisure facility demands in addition to guest room usage.
Modern sustainable plumbing systems increasingly integrate multiple water sources and storage solutions:
Sustainable storage design prioritises water efficiency, reduced energy consumption, and maximised use of on-site water resources while maintaining strict potable water hygiene standards.
Water storage tank design in the UK must comply with several key standards and regulations:
All potable water storage tanks must be constructed from WRAS-approved materials, incorporate screened vents and insect-proof overflow arrangements, and be accessible for inspection and cleaning.
A vertical cylindrical tank has an internal diameter of 1.5 metres and an internal height of 2.2 metres. Calculate the volume in litres.
Solution: r = 1.5 Γ· 2 = 0.75 m. V = Ο Γ 0.75Β² Γ 2.2 = 3.14159 Γ 0.5625 Γ 2.2 = 3.888 mΒ³ = 3,888 litres.
A rectangular GRP sectional tank measures 3.0 m Γ 2.0 m Γ 1.8 m internally. Calculate capacity in litres and UK gallons.
Solution: V = 3.0 Γ 2.0 Γ 1.8 = 10.8 mΒ³ = 10,800 litres. In UK gallons: 10.8 Γ 219.969 = 2,376 UK gallons.
A 120-bedroom hotel requires potable water storage. Using 180 L/person/day and a storage duration factor of 1.5 (36 hours), calculate the required tank volume.
Solution: Daily demand = 120 Γ 180 = 21,600 L. Storage volume = 21,600 Γ 1.5 = 32,400 litres (32.4 mΒ³).
A booster set delivers 300 L/min. Mains replenishment is 80 L/min. Minimum pump run time is 3 minutes. Emergency reserve is 20 minutes at peak. Calculate break tank volume.
Solution: Effective volume = (300Γ3) β (80Γ3) + (300Γ20) = 900 β 240 + 6,000 = 6,660 litres. Specify a 7,000-litre break tank.
A 50-unit apartment building with an average occupancy of 2.5 persons per unit requires cold water storage. Using 120 L/person/day with 24-hour storage, calculate total tank capacity.
Solution: Total occupants = 50 Γ 2.5 = 125. Daily demand = 125 Γ 120 = 15,000 L. Required storage = 15,000 litres (15 mΒ³).
| Diameter (m) | Height (m) | Volume (mΒ³) | Volume (Litres) | UK Gallons |
|---|---|---|---|---|
| 0.6 | 1.0 | 0.283 | 283 | 62 |
| 1.0 | 1.5 | 1.178 | 1,178 | 259 |
| 1.2 | 2.0 | 2.262 | 2,262 | 498 |
| 1.5 | 2.2 | 3.888 | 3,888 | 855 |
| 2.0 | 3.0 | 9.425 | 9,425 | 2,073 |
| 2.5 | 4.0 | 19.635 | 19,635 | 4,319 |
| Length (m) | Width (m) | Height (m) | Volume (mΒ³) | Volume (Litres) |
|---|---|---|---|---|
| 1.0 | 1.0 | 1.0 | 1.000 | 1,000 |
| 2.0 | 1.5 | 1.5 | 4.500 | 4,500 |
| 3.0 | 2.0 | 1.8 | 10.800 | 10,800 |
| 4.0 | 2.5 | 2.0 | 20.000 | 20,000 |
| 5.0 | 3.0 | 2.5 | 37.500 | 37,500 |
| From | To | Multiply By |
|---|---|---|
| Cubic metres (mΒ³) | Litres (L) | 1,000 |
| Litres (L) | Cubic metres (mΒ³) | 0.001 |
| Cubic metres (mΒ³) | UK Gallons | 219.969 |
| Cubic metres (mΒ³) | US Gallons | 264.172 |
| UK Gallons | Litres (L) | 4.54609 |
| US Gallons | Litres (L) | 3.78541 |
Measure the internal dimensions of your tank. For cylindrical tanks, use V = Ο Γ rΒ² Γ h (where r is radius and h is height). For rectangular tanks, use V = Length Γ Width Γ Height. Convert cubic metres to litres by multiplying by 1,000. Our calculator above performs all conversions automatically.
Cylindrical tank: V = ΟrΒ²h. Rectangular tank: V = L Γ W Γ H. For horizontal cylindrical tanks partially filled: V = L[rΒ²Β·arccos((rβd)/r) β (rβd)β(2rdβdΒ²)] where d is water depth.
Calculate the volume in cubic metres by multiplying internal dimensions (in metres), then multiply by 1,000 to get litres. A tank measuring 1m Γ 1m Γ 1m holds 1,000 litres. Use our calculator above for precise results.
Plumbing engineers calculate tank size based on occupancy, daily water demand per person (90β350 L depending on building type), storage duration (typically 24 hours), peak demand factors, and emergency reserves. BS EN 806 and CIBSE Guide G provide detailed methodologies.
A typical UK family home requires 200β500 litres of cold water storage. For a 4-person household at 120 L/person/day with 24-hour storage: 480 litres. Loft tanks commonly range from 227L (50 gallons) to 450L (100 gallons).
Cylindrical tanks distribute pressure evenly around their circumference, making them structurally efficient for pressurised applications. Rectangular tanks maximise space utilisation in plant rooms and are easier to manufacture as sectional GRP panels. Volume-for-volume, cylindrical tanks use less material.
Potable water storage refers to tanks holding water intended for human consumption β drinking, cooking, and personal hygiene. These tanks must be WRAS-approved, constructed from food-grade materials, and designed to prevent contamination per Water Supply Regulations.
A break tank incorporates a physical air gap (typically 20mm or twice the inlet diameter) between the mains water inlet and the tank's maximum water level. This prevents back-siphonage contamination. Booster pumps draw from the tank rather than directly from the mains.
Key regulations include the Water Supply (Water Fittings) Regulations 1999, BS EN 806, WRAS material approval requirements, BS 8558 for domestic water services, and HSG 274 Part 2 for Legionella control in water systems.
Underground tanks store water below ground level, protected from temperature extremes. They require structural design for soil and groundwater loads, waterproof construction, accessible manways for inspection, and often submersible pumps to deliver water to the building.
Tank turnover is the rate at which stored water is used and replenished. A 24-hour turnover means the entire tank volume is consumed and refilled daily. Low turnover increases stagnation risk and Legionella proliferation potential.
Booster pumps draw water from a break tank or storage cistern and increase pressure for distribution. The tank provides a reservoir so pumps don't connect directly to the mains. Pump controls monitor tank level to prevent dry-running.
Hotels calculate storage based on number of bedrooms, occupancy rate, star rating (affecting water usage), kitchen and laundry demands, and leisure facilities. Typical allowance is 150β200 L per guest per day with 24β36 hour storage duration.
For structural efficiency, cylindrical tanks are superior due to uniform hoop stress distribution. For space efficiency in rectangular plant rooms, rectangular sectional tanks are preferred. The choice depends on available space, budget, and application.
Engineers follow a systematic process: assess occupancy and water demand, determine storage duration, calculate required volume, select tank type and material, design inlet/outlet arrangements, integrate with pump systems, and ensure regulatory compliance.
A UK (imperial) gallon equals 4.54609 litres. A US gallon equals 3.78541 litres. One cubic metre equals 219.969 UK gallons or 264.172 US gallons. Always verify which gallon unit is being referenced in specifications.
Multiply cubic metres by 1,000. For example, 2.5 mΒ³ = 2,500 litres. This is because 1 cubic metre = 1,000 litres exactly.
A sectional tank is assembled from prefabricated panels (typically GRP or steel) bolted together on-site. This allows tanks to be built in spaces with limited access and enables large capacities. Common in commercial and industrial applications.
Calculate roof catchment area (mΒ²) Γ annual rainfall (m) Γ runoff coefficient (typically 0.8β0.9) Γ collection efficiency. Then size storage for the drier months based on monthly demand. BS 8515 provides detailed guidance.
Static pressure is the pressure exerted by a column of water at rest. Each metre of water height produces approximately 0.098 bar (9.8 kPa). A tank 10 metres above an outlet provides roughly 1 bar of static pressure.
A cold water storage cistern is a tank that holds potable water for domestic or commercial use. It typically supplies bathrooms, hot water cylinders, and sometimes kitchens. It must be fitted with a float-operated inlet valve, overflow, and screened vent.
BS 8558 recommends annual inspection and cleaning as necessary. Tanks with poor turnover or visible sediment should be cleaned more frequently. Legionella risk assessments may mandate more frequent cleaning for high-risk systems.
GRP (Glass Reinforced Plastic) tanks are constructed from fibreglass-reinforced polymer panels. They are lightweight, corrosion-resistant, WRAS-approved for potable water, and available in modular sectional designs for capacities from a few hundred to several million litres.
Key measures include: sizing tanks for adequate turnover (β€24 hours), insulating to maintain water below 20Β°C, preventing stagnation in dead legs, regular cleaning and disinfection, and avoiding oversized storage. Follow HSG 274 Part 2 guidance.
Peak water demand is the maximum simultaneous water flow rate expected in a building. It's calculated using fixture unit methods or diversity factors. Storage tanks must be sized to meet peak demand without depletion, accounting for mains replenishment.
No. Potable and non-potable water must be stored in completely separate tanks with no cross-connection. This is a fundamental requirement of the Water Supply Regulations to prevent contamination of drinking water.
Measure internal dimensions where possible. For cylindrical tanks, measure internal diameter and internal height. For rectangular tanks, measure internal length, width, and height. Subtract wall thickness if measuring externally. Use our calculator with these measurements.
There is no universal minimum; it depends on occupancy and water demand. However, BS EN 806 suggests a minimum of 24 hours' storage at design occupancy demand. A small office with 20 staff at 90 L/day would need at least 1,800 litres.
Cold water must be stored below 20Β°C to inhibit bacterial growth. In warm climates or heated plant rooms, tank insulation is critical. Larger tanks have more thermal mass and warm up more slowly, but this shouldn't be used as a justification for oversizing.
A float valve (ball valve) controls the water level in a storage tank. It opens when the water level drops and closes when the tank is full. It must be sized correctly for the inlet pressure and flow rate to ensure adequate replenishment.
Water weighs 1,000 kg per cubic metre (1 kg per litre). A 5,000-litre tank contains 5 tonnes of water. Add the tank's own weight for total structural loading. This is critical for rooftop and elevated tank installations.
A dual-compartment tank has an internal dividing wall creating two separate chambers. This allows one compartment to be taken offline for cleaning or maintenance while the other continues to supply water β essential for hospitals and critical facilities.
First calculate volume in cubic metres: V = Ο Γ rΒ² Γ h. Then convert: multiply by 219.969 for UK gallons or 264.172 for US gallons. Our calculator performs both conversions automatically.
Nominal capacity is the manufacturer's stated tank size. Actual (useable) capacity accounts for the air gap at the top (typically 50β100mm), the unusable water below the outlet, and any internal fittings that displace water. Actual capacity is typically 85β95% of nominal.
Hospitals use 250β350 L per bed per day with a minimum 24-hour storage, often 48 hours for critical care facilities. Dual-compartment tanks with full standby capacity in each compartment are standard. Emergency reserves for fire-fighting may also be required.
The wetted surface area is the internal surface area of the tank in contact with water. It's important for heat loss calculations, corrosion protection requirements, and disinfection dosing. For a full rectangular tank: 2(LW + LH + WH).
Fire water storage is sized according to BS 9251 (residential) or BS 9990/BS EN 12845 (commercial/industrial). Requirements depend on building size, fire risk category, and sprinkler system design. Fire storage is additional to domestic/commercial storage.
The Water Supply Regulations require a physical air gap of at least 20mm or twice the internal diameter of the inlet pipe (whichever is greater) between the inlet and the spillover level of the tank. This prevents backflow contamination.
Subtract twice the wall thickness from each external dimension to estimate internal dimensions. For a GRP tank with 8mm walls and external dimensions 2000Γ1500Γ1800mm, internal β 1984Γ1484Γ1784mm. Then apply standard volume formulas.
Turnover rate = Daily water consumption Γ· Tank volume. A rate of 1.0 means the tank is fully emptied and refilled daily. Rates below 0.5 indicate potential stagnation issues. BS EN 806 recommends turnover rates β₯1.0 for potable water tanks.
Measure internal length, width, and height in metres. Multiply: L Γ W Γ H to get cubic metres. Multiply by 1,000 to convert to litres. Example: 2m Γ 1.5m Γ 1.2m = 3.6 mΒ³ = 3,600 litres.
A weir tank incorporates an overflow weir that maintains a constant water level while allowing excess water to spill over. Common in swimming pool balance tanks andζδΊ industrial processes. Not typically used for potable water storage.
Calculate: Number of apartments Γ average occupancy Γ daily water demand per person Γ storage duration. For 50 apartments at 2.5 persons Γ 120 L/day Γ 24 hours = 15,000 litres. Add common area demand (typically 10β20% extra).
WRAS-approved GRP, stainless steel (grade 304 or 316), and certain polyethylene grades are suitable for potable water. GRP is most common for larger tanks due to cost and corrosion resistance. All materials must comply with BS 6920 for water contact.
Domestic: 100β150 L per person per day. Offices: 90β120 L per person. Hotels: 150β200 L per guest. Schools: 60β90 L per pupil. These include all uses β drinking, washing, toilet flushing, and cleaning. Meter readings provide actual data.
Effective (useable) capacity is the volume between the low-water alarm level (or pump suction level) and the normal maximum operating level. It excludes dead storage below the outlet and the freeboard space at the top of the tank.
Warehouse water demand is typically low (45β60 L per person per day) as most water use is for welfare facilities. For 30 staff: 30 Γ 60 Γ 1 day = 1,800 litres minimum. Include any process water or fire reserve requirements separately.
Tank volume does not directly affect pressure. Pressure in gravity systems depends on the elevation difference between the tank water level and the outlet. Larger tanks maintain more stable water levels during drawdown, providing more consistent pressure.
Overhead tanks provide gravity pressure and are easier to access. Underground tanks save space, protect water from temperature extremes, and are less visually intrusive. The choice depends on site constraints, building height, and whether pumps are acceptable.
50 FAQs covering water tank volume calculations, storage sizing, plumbing engineering standards, and hydraulic system design. Updated regularly to reflect current UK regulations.
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