Sizing a water tank for an industrial site comes down to one question: how much water do you actually need? To calculate water tank capacity, start with the application (fire protection, process water, potable supply, or cooling), factor in peak demand and regulatory minimums, then check what physically fits on site. Get those three right and the capacity figure follows. Franklin Hodge has been engineering industrial water tank systems for over 50 years, and the underlying principles haven’t changed.
What has changed is how fast you can get from rough sizing variables to a working number. We’ve launched a brand-new water tank capacity calculator, the first online tool of its kind in the UK industrial tank market, that runs the figures for you in seconds based on real Franklin Hodge engineering specs. Keep reading for the full sizing logic, then jump straight into the tool.
What Size Water Tank Do You Need for an Industrial Site?
Tank capacity is driven by use case before anything else.
For fire protection, capacity follows BS EN 12845 and your site’s hazard classification. Light, ordinary (Groups 1 to 4), and high hazard occupancies each carry their own minimum water volume and required flow duration. The stakes here are not abstract. A UK Parliament submission from BAFSA, drawing on Optimal Economics’ analysis of fire service data, confirms automatic sprinkler systems are 99% effective at controlling fires and operationally reliable in 94% of activations. That performance only holds if the tank delivers the specified flow for the full hazard duration. Our LPCB and FM Global certified fire protection tanks, the Firestore® and Firetainer® ranges, are sized and built to those standards.
For potable supply, peak demand drives the calculation rather than daily average. Typical UK benchmarks run 40 to 60 litres per person per day in offices and 350 litres or more per bed per day in hospitals. Most facilities hold 24 hours of peak demand as a minimum.
Process water tracks operational demand, which varies sharply by sector. Cooling tower make-up is sized around evaporation, drift losses, and blowdown. The application sets the floor. Every other variable adjusts the number from there.
What Factors Determine the Right Tank Capacity for Your Site?
Once the application baseline is set, a handful of site-specific variables move the figure up or down:
- Peak demand profile: Not the daily average. The maximum draw across any given hour.
- Regulatory minimums: BS EN 12845 for fire protection, BS EN 13280 and WRAS for potable supply, plus any local authority or insurer requirements.
- Redundancy: Whether you need resilience against mains supply interruption.
- Footprint and headroom: What physically fits, including access for installation and ongoing inspection.
- Refill rate: Under BS EN 12845, sprinkler tanks must refill within 36 hours, which feeds directly into the capacity you specify.
Pull these numbers together before specifying. Without them, you cannot land on an accurate figure.
How to Use the New Franklin Hodge Water Tank Capacity Calculator
Once you have a working sense of the variables, our new water tank capacity calculator runs the figures for you. It is built around our actual tank designs, so the number you get back is effective capacity. The usable volume after fittings and suction connections are accounted for, not raw geometry.
The flow is straightforward:
- Select your tank shape: Cylindrical or rectangular.
- Choose your approval where relevant: LPCB or FM, for fire protection installations.
- Enter your dimensions: Diameter and height for cylindrical, grid length and width for rectangular, plus infill and suction connection sizes from DN50 to DN450.
- Click calculate: The tool returns your effective capacity in cubic metres, based on Franklin Hodge design parameters.
- Adjust until you hit your target: Tweak diameter, height, or grid size until the figure matches the capacity your site needs.
- Save and submit: Create an account to keep the calculation, then submit it as an enquiry for our engineering team to review.
You arrive at the enquiry stage with a working number our engineers can validate against your application, rather than a rough estimate that must be rebuilt from scratch.
Cylindrical vs Rectangular Industrial Water Tanks: Which Fits Your Site?
Tank shape matters as much as volume.
Cylindrical tanks are the default for open ground, compounds, and external installations. They use material efficiently and suit most fire protection and potable applications. Our galvanised Firestore® and aluminium Liquistore® cylindrical ranges can be designed for high wind speeds and seismic loads, (on application at time of enquiry), with marine-grade aluminium options for coastal or marine-tropical sites.
Rectangular water storage tanks earn a place where a circular footprint will not fit. The Liquitainer® (potable water storage) and Firetainer® (LPCB-approved fire protection storage) are sectional, site-assembled from hot-dip galvanised steel panels with a 300gms per side zinc coating, and LPCB approved to a height of 5.35 metres, which exceeds comparable sectional rectangular systems on the UK market. That makes them the workable option for basements, plant rooms, enclosed lofts, and layouts that need the tank to encapsulate roof support columns.
Both shapes can hit the same capacity; they just get there differently.
What Happens if Your Industrial Water Tank Is the Wrong Size?
Sizing errors are expensive on both sides.
Under-sizing creates compliance failure on fire protection systems, downtime on process water, and exposure on insurance sign-off. For LPCB-approved sprinkler tanks, an undersized installation will not pass certification. Replacing it later costs significantly more than getting the spec right at the start. The same logic applies in reverse on potable systems: undercapacity at peak demand means rationing exactly when the site needs supply most.
Oversizing has its own costs. Wasted capital footprint that could be used elsewhere, and on potable systems, stagnation risk where stored water turnover is too slow to stay fresh.
Right-sizing is a commercial decision before it is an engineering one. The capacity figure on the spec’ sheet shapes budget, footprint, and operational resilience for the full life of the tank.
Get a Tailored Quote from Our Engineering Team
With a working capacity figure in hand from the tank calculator, the next step is converting that number into a specified tank ready for your site. Our engineering team reviews the saved calculation against your application, hazard classification, and site constraints, then confirms the right tank type for the brief, whether that is a cylindrical Firestore®, a rectangular Firetainer®, or one of our potable Liquistore® systems, and flags any variables that affect the final spec’. Request a quote when you’re ready to move forward.
