Water Tank Size Calculator

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Calculate how many litres a rectangular or cylindrical water tank holds from its dimensions, or estimate the storage size required from people, daily water use and days of backup.

Inputs

Choose calculation

What do you want to calculate?

Tank shape

Dimension unit

Tank dimensions

Use internal water dimensions, not external masonry or plastic dimensions.

Internal length

Internal width

Internal water depth / height

Usable fill level (%)

Use 100% for geometric brim-full capacity, or reduce this for freeboard / operating level.

How to use this calculator: Water Tank Size Calculator

Calculate how many litres a rectangular or cylindrical water tank holds from its dimensions, or estimate the storage size required from people, daily water use and days of backup. The example below is calculated by this page's real engine from the displayed inputs.

1Use Tank capacity in litres when you know the tank dimensions. Use Required tank size when you know the number of users and storage days.
2Select rectangular for sumps and box tanks, or cylindrical for vertical round tanks. Dimensions can be entered in feet, metres, centimetres or inches.
3Use the clear internal dimensions that water can occupy. Wall thickness and external mould dimensions should not be included.
4Leave 100% to find geometric brim-full capacity or reduce the percentage to allow for freeboard and normal operating level.
5The result shows usable litres, gross litres, cubic metres, cubic feet and gallon equivalents for cross-checking.

Input guide and example values

Use values from the same measurement basis and time period. Conditional fields appear only when the related option is selected.

Input	Example value	Why it matters
What do you want to calculate?	Tank capacity in litres — I know the dimensions	Select the option that matches the real installation or scenario.
Tank shape	Rectangular / square tank	Select the option that matches the real installation or scenario.
Dimension unit	Feet (ft)	Select the option that matches the real installation or scenario.
Internal length	6	Measured or known internal length used by the calculation engine.
Internal width	4	Measured or known internal width used by the calculation engine.
Internal water depth / height	4	Measured or known internal water depth / height used by the calculation engine.
Usable fill level	100 %	Use 100% for geometric brim-full capacity, or reduce this for freeboard / operating level.

Formula inputs & variables for Water Tank Size Calculator

These are the named quantities used by this calculator. When the source formula does not define a mathematical symbol, OneCalcApp keeps the real input label instead of inventing one.

Variable / input	Unit	Meaning in this calculation
What do you want to calculate?	—	Select the option that matches the real installation or scenario.
Tank shape	—	Select the option that matches the real installation or scenario.
Dimension unit	—	Select the option that matches the real installation or scenario.
Internal length	—	Measured or known internal length used by the calculation engine.
Internal width	—	Measured or known internal width used by the calculation engine.
Internal water depth / height	—	Measured or known internal water depth / height used by the calculation engine.
Internal diameter	—	Measured or known internal diameter used by the calculation engine.
Usable fill level	%	Use 100% for geometric brim-full capacity, or reduce this for freeboard / operating level.

Formula, derivation and worked example

Indian standard IS 1172 allows 135 litres per person per day for a house with full plumbing (about 70–100 lpcd where usage is modest). Multiply by the number of people, add outdoor use, then multiply by how long you want to survive without supply. Most homes split storage: an underground sump for a day or two and a smaller overhead tank for daily use.

Daily demand = people × litres per person + extra use

Tank capacity = daily demand × days of storage

Recommended size = next standard tank above the requirement

Rectangular tank capacity (L) = length × width × height (m³) × 1000

Cylindrical tank capacity (L) = π × diameter² ÷ 4 × height (m³) × 1000

Usable capacity = gross geometric capacity × fill level %

Substitution steps

1. Gross tank volume
L × W × H
= 2.7184 m³
2. Gross litres
m³ × 1000
= 2,718 L
3. Usable capacity
gross × 100%
= 2,718 L

Computed example results

Usable water capacity: 2,718 litres; 100% fill level
Gross geometric capacity: 2,718 litres; 2.718 m³
Usable volume: 2.718 m³
In cubic feet: 96 ft³
In US gallons: 718.1 US gal
In UK gallons: 598 UK gal
Approx. water mass: 2,718 kg; water only; approximately 1 kg per litre

Understanding the result

Read the main result together with supporting checks, assumptions, limits and intermediate values.

For a manual check, repeat the first equation, confirm the units and change one input at a time.

Assumptions and calculation scope

Usable fill level is assumed to be entered in %; converting from another unit before entry avoids changing the numerical meaning of the calculation.
What do you want to calculate? is assumed to match the real case. Choosing a different available option can change the method, factor or interpretation used by this calculator.
Water Tank Size Calculator applies the displayed Daily demand = people × litres per person + extra use relationship to the entered construction inputs. Factors that are not exposed as inputs or stated assumptions are outside this calculator's calculation scope.

Next logical calculator

Continue with Underground Water Tank Calculator

Underground Water Tank Calculator is directly connected from Water Tank Size Calculator as a source-defined continuation or comparison.

Open Underground Water Tank Calculator

How to calculate water tank capacity in litres

For a rectangular tank, multiply the internal length, width and water depth to get volume. When dimensions are in metres, the answer is cubic metres and each cubic metre equals 1,000 litres. For example, a 2.0 m × 1.5 m × 1.0 m tank has a gross volume of 3.0 m³ or 3,000 litres.

For dimensions in feet, the calculator first converts each measurement to metres before calculating litres. This avoids the common mistake of mixing feet, centimetres and metres in one formula.

Cylindrical water tank capacity formula

A vertical round tank uses cylinder volume: π × diameter² ÷ 4 × height. Use the internal diameter and the actual water height. Multiply cubic metres by 1,000 to get litres.

Plastic tank catalogue capacity may differ slightly from a simple cylinder because moulded tanks can have domed tops, ribs, tapered walls or a shaped bottom. Use the manufacturer's stated capacity for procurement and the geometric calculation for planning or checking.

Gross capacity, usable capacity and freeboard

Gross capacity is the volume when the geometric tank is filled to the entered full height. In practice, an operating tank may leave freeboard below the roof, overflow or float-valve level. The usable-fill percentage lets you account for that without changing the physical dimensions.

For structural design, remember that one litre of water has a mass close to one kilogram. A 5,000 litre full tank therefore contains roughly 5,000 kg of water before adding the self-weight of the tank and supporting structure.

Choosing a household water tank size

When dimensions are not known yet, switch to Required tank size. Multiply the number of users by the selected litres-per-person-per-day allowance, add extra daily use and multiply by the desired storage duration. The calculator then rounds up to a practical standard capacity.

Local supply frequency, building type, fixture count, garden use, fire-water requirements and municipal rules can change the design requirement, so use the demand mode as a planning estimate rather than a universal code value.

Formula

Daily demand = people × litres per person + extra use
Tank capacity = daily demand × days of storage
Recommended size = next standard tank above the requirement
Rectangular tank capacity (L) = length × width × height (m³) × 1000
Cylindrical tank capacity (L) = π × diameter² ÷ 4 × height (m³) × 1000
Usable capacity = gross geometric capacity × fill level %

1 cubic metre = 1,000 litres

Once the tank volume is known in m³, multiply by 1,000. A 2.5 m³ tank holds 2,500 litres geometrically.

Use internal dimensions

Capacity is based on the space occupied by water. External wall, insulation or mould dimensions can overstate the litres if they are used directly.

Water load is approximately 1 kg per litre

A 2,000 L full tank contains about 2 tonnes of water, while a 5,000 L tank contains about 5 tonnes, excluding tank self-weight.

Worked example

1Family of 4 at 135 lpcd = 540 litres per day.
2Plus 60 litres for the garden = 600 litres per day.
3For 2 days of storage → 1,200 litres, so a 1,500 litre tank is chosen.

Tips

Split the storage: sump for bulk, overhead tank for one day's use — smaller roof load.
Keep 15–20% free space; tanks are rated to the brim, not to the working level.

Frequently asked questions

How many litres does a family of 4 need?

About 500–600 litres a day, so a 1,000 litre tank covers roughly two days of normal use.

Should the overhead tank hold everything?

No. Keep the overhead tank to about one day's demand and store the rest in an underground sump — it is cheaper and safer for the slab.

Does drinking water need separate storage?

Only 5–10 litres per person per day is for drinking and cooking; the rest is bathing, flushing and washing.

How do I calculate water tank capacity in litres?

For a rectangular tank multiply internal length × width × water height in metres, then multiply by 1,000. For a round tank use π × diameter² ÷ 4 × height × 1,000.

How many litres are in 1 cubic metre?

Exactly 1,000 litres. Therefore 5 m³ equals 5,000 litres.

How do I calculate a tank in feet?

The calculator converts feet to metres automatically. If doing it manually, 1 cubic foot is about 28.3168 litres.

What dimensions make a 1,000 litre rectangular tank?

Any internal dimensions whose product is 1 m³ give about 1,000 litres, for example 1 m × 1 m × 1 m before freeboard.

Should I use gross or usable tank capacity?

Use gross capacity to describe geometric volume. Use usable capacity for operating storage after allowing for freeboard, overflow and the normal minimum water level.

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