Underground Water Tank Calculator

ConstructionInstant resultsFormula shown100% in your browser

Work out how many litres an underground sump holds from its length, width and depth — and how many days it will last.

Inputs

Length (ft)

Width (ft)

Depth (ft)

Free space at top (ft)

How to use this calculator: Underground Water Tank Calculator

Work out how many litres an underground sump holds from its length, width and depth — and how many days it will last. The example below is calculated by this page's real engine from the displayed inputs.

1Confirm that the Underground Water Tank Calculator matches the quantity or design check you need.
2Enter Length, Width, and Depth using the units printed beside each field.
3Check every value before calculating, especially decimal points and measurement units.
4Calculate, then follow the substituted equations in the worked example and compare the result with any stated limit.
5Read the assumptions, warnings and cited references before using the result for a financial, medical or engineering decision.

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
Length	8 ft	Measured or known length used by the calculation engine.
Width	6 ft	Measured or known width used by the calculation engine.
Depth	6 ft	Measured or known depth used by the calculation engine.
Free space at top	1 ft	Optional input; leave the supplied default only when it matches your case.

Formula inputs & variables for Underground Water Tank 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
Length	ft	Measured or known length used by the calculation engine.
Width	ft	Measured or known width used by the calculation engine.
Depth	ft	Measured or known depth used by the calculation engine.
Free space at top	ft	Optional input; leave the supplied default only when it matches your case.

How the Underground Water Tank Calculator works

The Underground Water Tank Calculator uses Length, Width, Depth, and Free space at top to calculate Water capacity, Usable depth, Volume, Gross capacity (brim full), and Days for a family of 4. Its engine applies Volume (cu ft) = length × width × usable depth; the worked values below come from that same live calculation rather than a separately typed example.

With Length 8 ft, Width 6 ft, Depth 6 ft, and Free space at top 1 ft, the main worked-example result is Water capacity = 6,796 litres.

How each Underground Water Tank Calculator input is used

Length

The Underground Water Tank Calculator worked example uses Length = 8 ft. This value is passed directly into the calculation, with an allowed minimum 0.5.

Width

The Underground Water Tank Calculator worked example uses Width = 6 ft. This value is passed directly into the calculation, with an allowed minimum 0.5.

Depth

The Underground Water Tank Calculator worked example uses Depth = 6 ft. This value is passed directly into the calculation, with an allowed minimum 0.5.

Free space at top

The Underground Water Tank Calculator worked example uses Free space at top = 1 ft. This value is passed directly into the calculation, with an allowed minimum 0.

Underground Water Tank Calculator formulas and result interpretation

Formula 1: relationship used

In the Underground Water Tank Calculator, Volume (cu ft) = length × width × usable depth. The quantities in this relationship come from the named inputs or from an earlier calculation step shown in the worked example.

Formula 2: relationship used

In the Underground Water Tank Calculator, Capacity (litres) = volume × 28.317. The quantities in this relationship come from the named inputs or from an earlier calculation step shown in the worked example.

Formula 3: relationship used

In the Underground Water Tank Calculator, 1 m³ = 1000 litres = 35.31 cu ft. The quantities in this relationship come from the named inputs or from an earlier calculation step shown in the worked example.

Water capacity

For the displayed Underground Water Tank Calculator worked example, Water capacity is 6,796 litres. 6.8 m³ Verify Length, Width, and Depth and their units before relying on this output.

Usable depth

For the displayed Underground Water Tank Calculator worked example, Usable depth is 5 ft. Verify Length, Width, and Depth and their units before relying on this output.

Volume

For the displayed Underground Water Tank Calculator worked example, Volume is 240 cu ft. Verify Length, Width, and Depth and their units before relying on this output.

Gross capacity (brim full)

For the displayed Underground Water Tank Calculator worked example, Gross capacity (brim full) is 8,155 litres. Verify Length, Width, and Depth and their units before relying on this output.

Days for a family of 4

For the displayed Underground Water Tank Calculator worked example, Days for a family of 4 is 12.6 days. at 135 litres per person per day Verify Length, Width, and Depth and their units before relying on this output.

Underground Water Tank Calculator accuracy, checks and limitations

Underground Water Tank Calculator units check: confirm Length (ft), Width (ft), Depth (ft), and Free space at top (ft) before calculating.
Underground Water Tank Calculator result check: compare Water capacity, Usable depth, Volume, Gross capacity (brim full), and Days for a family of 4 with the substituted formula steps and the displayed rounding precision.
Underground Water Tank Calculator: Use this result for preliminary design and cross-checking. Confirm the applicable code edition, manufacturer data, site conditions and qualified-engineer approval before final design or installation.

Formula, derivation and worked example

A rectangular tank holds length × width × depth. Water is never filled to the very top, so subtract a free space (freeboard) of about 0.5 ft for an overhead tank and 1 ft for a sump. One cubic foot holds 28.317 litres, which is the only conversion you need.

Volume (cu ft) = length × width × usable depth

Capacity (litres) = volume × 28.317

1 m³ = 1000 litres = 35.31 cu ft

Substitution steps

1. Usable depth
depth − freeboard
= 5 ft
2. Volume
L × W × usable depth
= 240 cu ft
3. Capacity
cu ft × 28.317
= 6,796 litres

Computed example results

Water capacity: 6,796 litres; 6.8 m³
Usable depth: 5 ft
Volume: 240 cu ft
Gross capacity (brim full): 8,155 litres
Days for a family of 4: 12.6 days; at 135 litres per person per day

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

Length is assumed to be entered in ft; converting from another unit before entry avoids changing the numerical meaning of the calculation.
Underground Water Tank Calculator applies the displayed Volume (cu ft) = length × width × usable depth relationship to the entered construction inputs. Factors that are not exposed as inputs or stated assumptions are outside this calculator's calculation scope.

Common mistakes when using Underground Water Tank Calculator

Do not mix units for Length (ft), Width (ft), Depth (ft). A unit mismatch changes the input magnitude even when the typed number looks reasonable.
Do not replace the displayed Volume (cu ft) = length × width × usable depth relationship with a different convention without also changing the underlying assumptions; compare like-for-like methods when checking the result.
Do not treat Water capacity = 6,796 litres from the worked example as a universal answer. It belongs to the displayed example inputs and must be recalculated for the actual case.

When the Underground Water Tank Calculator is useful

Underground Water Tank Calculator is designed for cases where Length, Width, Depth, Free space at top are known and you need Water capacity, Usable depth, Volume. The page keeps the live calculator, calculation method and worked example together so the result can be checked instead of treated as a black-box number.

Use the calculator for the scope described by its inputs and notes. The displayed method is Volume (cu ft) = length × width × usable depth. If the real project or decision needs factors that are not represented here, treat the result as an estimate and add the missing checks separately.

Length and Width: what changes the answer

The worked example uses Length = 8 ft, Width = 6 ft, Depth = 6 ft, Free space at top = 1 ft. With those values, Water capacity is 6,796 litres. Changing an input should be interpreted according to that field's unit, range, option and hint rather than by the number alone.

For this calculator, the main input roles are: Length (ft): Measured or known length used by the calculation engine. Width (ft): Measured or known width used by the calculation engine. Depth (ft): Measured or known depth used by the calculation engine. Free space at top (ft): Optional input; leave the supplied default only when it matches your case.

How to sanity-check a Underground Water Tank Calculator result

Start by confirming the entered values and units, then compare the substituted working with the displayed formula or calculation steps. Pay particular attention to Water capacity, because it is the first worked-example output shown by the live engine.

Finally, compare the result with the assumptions, warnings and related calculators on this page. A nearby calculator can be useful as a cross-check when it measures the same workflow from a different input or output direction.

Next logical calculator

Continue with Water Tank Size Calculator

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

Open Water Tank Size Calculator

Formula

Volume (cu ft) = length × width × usable depth
Capacity (litres) = volume × 28.317
1 m³ = 1000 litres = 35.31 cu ft

Worked example

1A sump 8 ft × 6 ft × 6 ft with 1 ft freeboard has a usable depth of 5 ft.
2Volume = 8 × 6 × 5 = 240 cu ft.
3Capacity = 240 × 28.317 ≈ 6,796 litres.

Tips

Design the depth first — deeper tanks use less slab area and less concrete.
Add an air vent and a manhole of at least 600 mm for cleaning.

Frequently asked questions

How many litres in one cubic foot?

28.317 litres. So 35.31 cu ft makes exactly 1,000 litres.

Why leave free space at the top?

For the inlet, the float valve and to stop overflow splashing. About 0.5–1 ft is normal.

Can I use metres instead of feet?

Enter your dimensions in feet — 1 metre is 3.28 feet, so a 3 m long tank is 9.84 ft.

What inputs does the Underground Water Tank Calculator use?

It uses Length, Width, Depth, and Free space at top. Follow the unit printed for each field and choose any selectable option to match the real scenario.

What does the Underground Water Tank Calculator calculate?

It calculates Water capacity, Usable depth, Volume, Gross capacity (brim full), and Days for a family of 4. With the displayed default inputs, Water capacity is 6,796 litres.

Which formula does the Underground Water Tank Calculator use?

The primary relationship is Volume (cu ft) = length × width × usable depth. The page also shows substituted values and calculation steps so the result can be checked independently.

How can I verify a Underground Water Tank Calculator result?

First verify the units for Length, Width, and Depth. Then compare the substituted formula steps with Water capacity and its displayed precision.

What are the limitations of the Underground Water Tank Calculator?

Underground Water Tank Calculator: Use this result for preliminary design and cross-checking. Confirm the applicable code edition, manufacturer data, site conditions and qualified-engineer approval before final design or installation.

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