Water Tank Sizing: How to Calculate the Capacity You Need

How to Calculate Water Tank Capacity

Water tank sizing is one multiplication: tank capacity (m³) = daily demand (m³) × days of storage, where days of storage is how long the building should run on the tank if the mains supply fails. There is no universal rule for the days; it depends on local outages and how critical water is to the building.

Then check any regulatory minimum, add the fire reserve and round to a panel layout. The building services engineer sets the final figure. The calculator at the end of this guide turns people, litres per person and days into a volume, with example GRP and steel tank sizes.

Litres, Cubic Metres, Tonnes and Gallons

One cubic metre (m³) is 1,000 litres, and that volume of water weighs roughly one tonne, so “10-tonne”, “10 m³” and “10,000-litre” tanks are the same size.

• Litres ÷ 1,000 = m³ (2,500 L = 2.5 m³).
• 1 m³ is about 264 US gallons or 220 imperial gallons.
• Internal width × length × height in metres = m³ (2 × 3 × 2 m = 12 m³).

The tonne figure also gives the load: a full 10 m³ tank puts about 10,000 kg of water on its base.

How to Work Out Daily Water Demand

Your own water bill or meter is the most reliable source, because consumption per person varies widely between buildings and seasons.

• Bills: divide billed m³ by the days in the period. 18 m³ over 30 days is 0.6 m³ (600 litres) a day; use the highest period of the year.
• Meter: read the main meter at the same time on several days and divide the difference by the days. Best for schools, hotels and factories.
• Occupancy (new buildings): occupants × litres per person per day, with the per-person figure from current bills (monthly m³ × 1,000 ÷ 30 ÷ household size) or your engineer.

We do not quote a “typical” litres-per-person figure: a measured value beats a general average.

Minimum Tank Volumes Under Turkish Regulations

In Turkey, Article 36 of the Planned Areas Zoning Regulation (Planlı Alanlar İmar Yönetmeliği) makes a water tank mandatory in areas with an implementation zoning plan and sets minimum volumes by building type. The actual volume must reflect the building's use, daily demand and fire suppression system, so these are floors, not targets.

Building type	Minimum tank volume
Residential, up to 10 independent units	3 m³
Residential, more than 10 units	3 m³ + 0.50 m³ for each unit above 10
Public buildings (schools, hospitals, hotels, offices, etc.) and high-rise buildings	15 m³
Very high-rise buildings	30 m³
Other buildings	5 m³

Source: Planlı Alanlar İmar Yönetmeliği, Article 36 (consolidated text on mevzuat.gov.tr as of 22 October 2026). High-rise: building height over 21.50 m or structure height over 30.50 m; very high-rise: over 51.50 m or 60.50 m. Article 36's term “yüksek katlı yapı” is not defined separately, so this guide applies the high-rise (“yüksek yapı”) definition.

The article also requires a booster set wherever water cannot be distributed by gravity from the storey the tank is on (see Aquapump booster sets). Outside Turkey, your local building code and design engineer set the minimum.

Worked Examples: Villa, Apartment Block, School and Factory

The consumption figures below are illustrative assumptions; use your own readings.

• Villa: 15 m³ a month is 0.5 m³ a day, so two days of storage needs 1 m³, but the 3 m³ residential minimum applies in a Turkish planned area.
• 16-unit apartment block (not high-rise): the minimum is 3 + (16 − 10) × 0.50 = 6 m³. A peak month of 240 m³ means 8 m³ a day: 8 m³ for one day, 16 m³ for two; use whichever is larger, the minimum or the demand figure.
• School: the public-building minimum is 15 m³. At 9 m³ per teaching day, two days is 18 m³; size on teaching days, not on the yearly average.
• Factory: add staff and process water. A plant using 4 m³ + 26 m³ a day that wants one day of autonomy needs at least 30 m³; take process demand from the machine supplier or a sub-meter.

None of these figures include fire water.

Converting m³ into Tank Dimensions

A sectional tank's volume is width × length × height, each a multiple of the panel size. Aquasan GRP panel tanks use 1,000 × 1,000 mm panels. Width and length follow the 1,000 mm panel module, and height is chosen from 1.0 to 5.0 m in 0.5 m steps (catalogue pp. 5, 8, 11). The catalogue's partitioned-tank example also shows 2 × 1.5 × 2 m tanks (p. 13), so half-metre plan dimensions may be possible; confirm the exact footprint for your project with us. Galvanized and stainless steel tanks use 1,080 × 1,080 mm panels, 540–4,320 mm high in 540 mm steps (p. 16).

Required	GRP example layout (W × L × H)	GRP volume	Steel layout (catalogue p. 15)	Steel geometric volume
5 m³	1 × 2 × 2.5 m	5.00 m³	2.16 × 2.16 × 1.08 m	5.04 m³
10 m³	2 × 2 × 2.5 m	10.00 m³	2.16 × 2.16 × 2.16 m	10.08 m³
20 m³	2 × 4 × 2.5 m	20.00 m³	4.32 × 2.16 × 2.16 m	20.16 m³
30 m³	3 × 4 × 2.5 m	30.00 m³	3.24 × 4.32 × 2.16 m	30.23 m³
50 m³	5 × 5 × 2 m	50.00 m³	5.40 × 4.32 × 2.16 m	50.39 m³
90 m³	6 × 5 × 3 m	90.00 m³	6.48 × 4.32 × 3.24 m	90.70 m³

GRP layouts are calculated examples; other combinations give the same volume. Steel layouts come from the catalogue's layout drawings; intermediate sizes are made to project requirements (p. 15).

Leave at least 500 mm to walls, columns and ceiling (pp. 9, 16) and allow for the height of the concrete base and steel skid. More in our sectional tank sizes guide and tank material comparison.

Gross vs Net (Usable) Capacity

Panel dimensions give the gross (geometric) capacity; usable water is a little less. The level stays below the overflow, so the top of the tank holds air, and water below the outlet is not drawn off; in a GRP tank it is emptied through the drain panel at the lowest point of the floor (p. 11).

Connection heights vary by project, so the gross–net gap is not fixed. Your demand is a net figure: ask for the net capacity in the quote. Avoid generous oversizing, as water stands longer in a tank that is too big, and clean the tank regularly; see the cleaning and maintenance notes in our drinking water storage tank guide.

Fire Water Is Sized Separately

Fire-fighting water is calculated separately and held as a dedicated reserve. Under Article 92 of Turkey's Regulation on Fire Protection of Buildings (Binaların Yangından Korunması Hakkında Yönetmelik), the fire reserve in a tank serving water-based systems may be used for nothing else, and the volume is based on 30, 60 or 90 minutes for low-, medium- and high-hazard buildings. One exception: where fire hose cabinets are the only water-based system and the occupancy is not high hazard, they may be connected to the domestic water system with no separate system required.

A partitioned tank keeps both reserves in one footprint: a partition panel divides a GRP tank into two compartments (p. 11). Because separate tanks need a 1 m maintenance gap, two 2 × 1.5 × 2 m tanks hold 12 tonnes, while a 2 × 4 × 2 m partitioned tank on the same floor area holds 16 tonnes (p. 13). For fire pumps, see Aquapump fire booster sets.