Dam volume and water storage calculators

Method & worked example

How much water does a rectangular farm dam hold?

For a dam with straight, uniform side slopes, calculate the top, middle and bottom areas. Multiplying surface area by maximum depth overstates storage.

Which dimensions should you measure?

Use the dimensions at the water level being assessed, the actual water depth and the internal side slopes. This idealised example assumes a level rectangular bottom and uniform slopes on all four sides. Irregular dams need a surveyed stage–storage relationship; do not treat their bounding rectangle as the water volume.

Stored volume is not all usable water

Deduct inaccessible water below the outlet or pump intake. Allow separately for evaporation, seepage, reserve storage and the operating water level. Freeboard above that level is not working storage.

Start with TNK-004 dam volume, then check evaporation, spillway capacity and tank overflow. For demand, use the irrigation volume example.

Field reference: Agriculture Victoria: measuring water in a dam.

Static educational example · Updated 1 October 2026 · Methods and limitations. Project calculations run in the subscription application; New accounts require approval.

Remaining water, not just full capacity

How much water is left in a partly filled dam?

Use the same dam as the example above: a level 30 m × 10 m floor, uniform 3 horizontal to 1 vertical side slopes, and a full water depth of 5 m. The table shows total stored water before deductions for inaccessible water, reserve or future losses.

At water depth h metres, the water surface is (30 + 6h) m long and (10 + 6h) m wide. Apply the same prismoidal formula using the bottom, half-depth and current water-surface areas. These values assume the stated geometry; they are not a lookup table for dams with different dimensions, sediment deposits or irregular banks.

Is a dam half full when the water is at half depth?

Not usually. With sloping sides, the surface area shrinks as the level falls. In this example, 2.5 m of water holds 1.6875 ML, about 28% of the 6 ML full volume. Use the dimensions at the current water level or a surveyed depth-to-storage table, rather than multiplying full capacity by the fraction of depth.

How do I convert dam volume from cubic metres to litres or megalitres?

One cubic metre (m³) equals 1,000 litres. One megalitre (ML) equals 1,000 m³ or 1,000,000 litres. Multiply m³ by 1,000 for litres, or divide m³ by 1,000 for ML. For example, 1,687.5 m³ is 1,687,500 litres or 1.6875 ML.

From stored water to a water budget

Use the volume at the current depth, deduct water that cannot be accessed and any reserve, then account for expected inflow, demand, evaporation and seepage over the same period. If the dam shape is uncertain, obtain a survey and a storage curve before relying on it for a critical supply.

Field guidance: DPIRD: measuring current dam volume and water losses. Its field-estimation methods provide context; the table above is calculated from the explicitly defined geometry.

Static educational example · Updated 9 October 2026 · Methods and limitations. Run project calculations with an approved Tenselo account.