How Much Water Storage Does an Industry or Town Need?

The water volume an industry or municipality should store is calculated by multiplying daily demand by the days of reserve you want to cover, then adding the fire reserve if the tank feeds that system. As a guide, a town usually stores between 0.5 and 2 days of consumption and an industrial plant between 0.25 and 1 day of its process flow, adjusted for criticality and supply reliability. This article gives you the full method, a worked example and a reference table by population and by flow rate.

The method in three steps

Sizing the reserve is not a magic number: it follows from three linked decisions. The base formula is:

V_storage = (Daily demand × Days of reserve) + Fire reserve

Each term is defined as follows:

• Daily demand: for potable water, population × per-capita allowance; for industry, the sum of process, sanitary and auxiliary flows over a day of operation.
• Days of reserve: how many days of consumption you want available against a pumping outage, a stoppage at the treatment plant, or a demand peak.
• Fire reserve: the volume required by code for the fire system (system flow × required autonomy time). It is added separately because it is a dormant volume that is not consumed in normal operation.

Step 1: compute the daily demand

For a municipality, demand starts from the served population and a per-capita allowance. A reference range in the region is 120 to 200 liters per person per day, higher where there is more commercial consumption or a warm climate. A peak-day factor (typically 1.2 to 1.5) is then applied so you size on the peak day, not the average.

For industry, demand is built by adding each use: process water, cooling, CIP cleaning, boilers, sanitary. Measure or estimate the peak flow, not just the average, because many processes concentrate consumption in a few hours.

Step 2: choose the days of reserve

This is the project criterion that moves the volume the most. It depends on source reliability and how critical it is to never run out of water:

• Municipality with reliable pumping and a nearby source: 0.5 to 1 day usually suffices as balancing and operating reserve.
• Municipality with a distant source, long pumping runs or frequent power cuts: 1 to 2 days or more.
• Continuous-process industry (slaughterhouse, brewery, dairy): 0.25 to 1 day, because a stoppage for lack of water halts production; autonomy against outages is prioritized.
• Data centers and critical uses: their own criterion, with redundancy; the project for the largest data center in Latin America, in Uruguay, solved its reserve with three bolted steel tanks erected in 28 days.

Step 3: add the fire reserve and freeboard

If the tank feeds the fire main, the code-required volume is added. That calculation has its own method (system flow times the required autonomy time), which we develop in the dedicated article on how to size a water storage tank. Finally, the geometric volume of the tank is sized 5 to 10% above the usable volume, to leave freeboard and dead volume.

Worked example: a town of 20,000 people

1. Population: 20,000 people.
2. Allowance: 160 L/person/day. Average demand = 20,000 × 160 = 3,200,000 L/day = 3,200 m³/day.
3. Peak day: factor 1.3 → design demand = 3,200 × 1.3 = 4,160 m³/day.
4. Days of reserve: the project adopts 1 day (moderately reliable source). Reserve = 4,160 m³.
5. Fire: add 150 m³ for the fire system.
6. Usable volume: 4,160 + 150 = 4,310 m³.
7. Geometric volume: +7% freeboard → ~4,610 m³. Adopt a tank of 4,500 to 5,000 m³ depending on the standard panel.

A single bolted steel tank covers this case; for redundancy, many municipalities prefer two tanks of half the volume, so one can be drained for maintenance without cutting service.

Reference table: indicative reserve volume

Approximate usable volumes by population (1 day of reserve, 150 L/person/day allowance, 1.3 peak-day factor) or by industrial flow (0.5 day of reserve). Indicative values for preliminary design; the real volume depends on the days-of-reserve criterion and the fire reserve.

User	Design demand	Reserve adopted	Approx. usable volume (m³)
Community 2,000 people	390 m³/day	1 day	400
Town 5,000 people	975 m³/day	1 day	1,000
City 20,000 people	3,900 m³/day	1 day	4,000
City 50,000 people	9,750 m³/day	1 day	10,000
Industry 20 m³/h	480 m³/day	0.5 day	240
Industry 50 m³/h	1,200 m³/day	0.5 day	600
Industry 100 m³/h	2,400 m³/day	0.5 day	1,200

Common mistakes when sizing the reserve

• Sizing on the average: ignoring the peak day leaves the system short during summer or campaign peaks.
• Forgetting the fire reserve: the fire volume is dormant; if it is not separated, it gets consumed in operation and is missing when needed.
• A single tank with no redundancy: any cleaning or repair forces a service cut; two tanks handle maintenance without interruption.
• Not planning for growth: a bolted steel tank can add rings or expand in diameter, so size it for the project horizon.

If you want us to review your real demand, your days-of-reserve criterion and the fire volume, Tech Tank sizes and supplies the tank turn-key. See our potable water (ETA) solutions and the conceptual basis in what a bolted steel tank is.

Frequently Asked Questions

How many days of water should a municipality store?

As a rule, between 0.5 and 2 days of peak-day consumption. Municipalities with reliable pumping and a nearby source manage with 0.5 to 1 day; those with a distant source, long pumping runs or frequent power cuts justify 1 to 2 days or more. The fire reserve is added separately.

And how much water should an industry store?

It depends on how costly it is to stop production for lack of water. Continuous-process industries typically store between 0.25 and 1 day of their daily flow, prioritizing autonomy against supply outages and process consumption peaks.

How do I calculate daily water demand?

For potable water: population × allowance (120 to 200 L/person/day), multiplied by a peak-day factor of 1.2 to 1.5. For industry: the sum of all use flows (process, cooling, cleaning, sanitary) over a day of operation, accounting for the peak flow.

Why is the fire reserve added separately?

Because it is a dormant volume that must not be consumed in normal operation: it has to be available on the day of a fire. If it is not separated, it is spent with daily consumption and missing when needed. Its calculation follows NFPA 22.

Is one large tank or several smaller ones better?

Several tanks provide redundancy: you can drain one for cleaning or repair without cutting service. A single large tank is cheaper per m³ but forces a supply interruption at each maintenance. The decision depends on how critical the supply is.

What regulations apply in Paraguay?

For potable water and sanitation systems, verify current regulations and the environmental license with MADES, under Law 294/93 and Water Resources Law 3239/07. The structural design of the bolted steel tank follows the AWWA D103 standard.