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Land & plots

Rectangular pond section capacity calculator

Calculate water-level capacity for aligned rectangular sections with vertical or linearly sloping sides and entered freeboard.

Your numbers

Plain decimal, up to 6 decimal places.
Plain decimal, up to 6 decimal places.
Plain decimal, up to 6 decimal places.
Plain decimal, up to 6 decimal places.
Plain decimal, up to 6 decimal places.
Plain decimal, up to 6 decimal places.

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Your result

Ready when you are

Enter your numbers and select Calculate.

Try it, understand it

Explore the result

Compare the geometric capacity to the top with an entered lower water level, then try vertical walls. These examples update the inputs and can restore your previous settings.

Worked example comparison
ExampleOutputWhat to notice
Entered freeboard of 0.5 mGeometric capacity below entered water level238.888889 m³Water depth is 2.5 m; capacity is 2150/9 ≈ 238.888889 m³ under the stated sloping-section model.
Water level at the topGeometric capacity below entered water level330 m³With zero freeboard, the same geometric section contains 330 m³. This is not a recommended operating level.
A 10 × 4 m vertical-wall sectionGeometric capacity below entered water level100 m³At 2.5 m water depth, the constant 40 m² section gives 100 m³.

Check your understanding

Why does 0.5 m freeboard not remove top area × 0.5 in the sloping example?

The horizontal section shrinks as you move down from the top. Its removed layer does not have constant top area.

Is geometric capacity the same as available usable water?

No. Silt, losses, pump level, drawdown and other operational conditions are outside this geometry calculation.

Behind the numbers

How this calculation works

Interpolate the length and width of each horizontal rectangular section from the bottom to the top, then integrate their product to the entered water level. Removing freeboard changes both depth and the water-surface rectangle.

THE FORMULAV = h/6 × [A(bottom) + 4A(mid-depth) + A(water level)], with section dimensions varying linearly

A worked example

With top 20 × 10 m, bottom 10 × 4 m, total depth 3 m and freeboard 0.5 m, modeled water capacity is 2150/9 ≈ 238.888889 m³. Full geometric capacity to the top is 330 m³.

Good to know

Aligned rectangular sections with independently linear length/width variation. Top dimensions must be no smaller than the bottom. No embankment, liner, stability, inflow, seepage, evaporation, silt, pumping or usable drawdown design. Freeboard is an input assumption, not a recommended value.

Reference: NIST: exact international-foot and acre factors (opens in a new tab) ↗

Check the land-unit definitions and sources

The standard factors use the international foot and an acre of 43,560 sq ft. Cent means 1/100 acre; gunta means 1/40 acre; the selected Chennai ground means 2,400 sq ft.

The Assam report defines bigha as 14,400 sq ft, katha as 2,880 sq ft and lecha as 144 sq ft. The cited West Bengal court order mentions a 720-sq-ft katha convention. These are explicit presets, not an automatic definition for every record or every place using the same name. Match your record to the stated convention, or use its verified custom factor.

The acre relationships are converted using exact base factors. Rounded or unrelated regional values in a reference table are not substituted for these definitions.

The horizontal rectangle changes as the water level rises

For the example’s 20 × 10 m top and 10 × 4 m bottom, the dimensions halfway up the full 3 m depth are 15 × 7 m. That middle area is 105 m². The prismoidal calculation gives 3/6 × (40 + 4×105 + 200) = 330 m³ to the top.

Length and width each change linearly with depth, so their product is quadratic. The midpoint-weighted expression integrates that quadratic exactly under this stated shape model. It also handles vertical walls when top and bottom dimensions are equal.

The two dimension ratios need not match. The example’s length doubles from bottom to top while width multiplies by 2.5, so this is not assumed to be a pyramid frustum with geometrically similar end rectangles. The section model remains explicit rather than guessing a universal basin shape.

Freeboard removes a changing layer, not just a constant depth

With freeboard 0.5 m, water depth is 2.5 m and the water-level rectangle is 55/3 by 9 m. Integrating only to that height gives 2150/9 m³, approximately 238.888889 m³. Capacity excluded by freeboard is about 91.111111 m³.

Multiplying the full top area by the freeboard would instead remove 100 m³. That assumes the upper layer has constant top area, which this sloping-wall model does not. For a vertical-wall rectangle, constant-area depth multiplication is appropriate.

Soil fill volume is a separate material-state and delivery question. Geometric water capacity does not determine loose excavation quantity, embankment volume or usable drawdown. Keep those different models and their assumptions separate.

A few questions, answered

Does the tool recommend freeboard or side slopes?

No. You supply the dimensions and water-level offset. No safe or appropriate construction dimensions are inferred.

What happens when freeboard equals total depth?

The modeled water depth and capacity are zero.

Is this a measurement of my actual pond?

No. It integrates the supplied rectangular-section model. An irregular surveyed basin needs a different measured surface or section model.

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