What is freeboard and why does it matter?

Freeboard is the vertical distance between the full supply level and the top of the embankment — typically 0.5-1.0 m. Always calculate storage capacity to the full supply level, not to the embankment crest, or you will overstate usable capacity.

How accurate is this formula for a licence application?

The prismatoid formula is within 2-3% for well-measured dimensions with regular geometry. This is acceptable for most licence applications under about 50 ML. For larger storages, commission a licensed surveyor.

What is a turkey nest dam?

An above-ground storage built by constructing embankments on flat ground — water is pumped in from a bore or watercourse. Common in flat country where natural fall is insufficient for a conventional gully dam.

Dam Capacity Calculator — Free Agriculture Calculator

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The new turkey nest is graded and the surveyor isn't back for three weeks. You need the capacity in megalitres for the water licence application before the deadline this Friday.

Dam Capacity Calculator

Water

Dam shape

Length / diameter (m)

Imperial: enter feet

Width (m)

Leave blank for circular

Average depth (m)

Imperial: enter feet

Volume (m³) = L × W × D × Shape factor Shape factors: Rectangular 0.6 · Circular 0.6 · Irregular 0.5 (accounts for sloping sides)
Stock water requirements (L/day): Cattle 50L · Sheep 5L · Horses 45L
Allow 30% reduction from nominal capacity for sediment accumulation and evaporation.

Reference: NSW DPI — Farm Dams · DAFF — Water on Farms

ℹ️ Results are estimates for planning purposes. Verify with current standards and a qualified professional.

1 What this calculator does

Calculates farm dam or turkey nest water storage capacity in megalitres from measured dimensions. Supports rectangular/trapezoidal (standard farm dam) and circular profiles. Shows volume at 100%, 75% and 50% fill.

2 Formula & professional reasoning

Trapezoidal dam (Prismatoid formula):
Volume (m3) = (Depth / 3) x [A_top + A_bottom + sqrt(A_top x A_bottom)]
Volume (ML) = Volume (m3) / 1,000

Farm dams are typically trapezoidal — wider at top, narrowing to floor. The prismatoid formula correctly calculates volume for this shape. The simpler average-of-top-and-bottom method overestimates volume by 5-15% — unacceptable for water licence applications. Using only the top area (treating as a rectangle) can overstate capacity by 30-50%.

3 Worked examples

⚠️ Illustrative example only — not clinical or professional instruction.

Basic

Turkey nest dam from dimensions

Given: Top: 80 m x 60 m (4,800 m2) | Bottom: 70 m x 50 m (3,500 m2) | Depth: 3.5 m

Working: sqrt(4,800 x 3,500) = sqrt(16,800,000) = 4,099 m2 | Volume: (3.5/3) x (4,800+3,500+4,099) = 1.167 x 12,399

Answer: 14,471 m3 = 14.5 ML

💡 Round down to 14 ML for licence application — never overstate storage capacity.

Standard

Embankment dam — surface area known

Given: Surface at full supply: 2.2 ha (22,000 m2) | Bottom: 0.4 ha (4,000 m2) | Depth: 4.2 m

Working: sqrt(22,000 x 4,000) = 9,381 m2 | Volume: (4.2/3) x (22,000+4,000+9,381) = 1.4 x 35,381

Answer: 49,533 m3 = 49.5 ML

💡 For licence applications requiring >95% accuracy, commission a licensed surveyor.

Advanced

Volume at partial fill

Given: Full capacity: 49.5 ML | Current water level: 75% of full supply depth

Working: Due to tapered shape, 75% depth gives less than 75% of full volume. Use prismatoid at actual depth.

Answer: Approximately 37.0 ML at 75% depth

💡 Tapered dams hold proportionally less at a given depth fraction than a rectangular tank.

4 Sanity check

Quick rule of thumb

100 m x 100 m x 4 m deep (tapered) = approx 25 ML

A flat-sided rectangle the same size would be 40 ML — taper makes a big difference.

Water licence

Most AU states require licence for dams storing more than 1 ML

Check with state Department of Water before construction.

Freeboard allowance

Full supply level is typically 0.5-1.0 m below embankment crest

Calculate capacity to full supply level — NOT to the top of the bank.

Evaporation loss

Open water: 1,500-3,000 mm/year in inland Australia

A 50 ML dam may lose 3-8 ML/year to evaporation.

5 Common errors

Error	Cause	Consequence	Fix
Using only the top surface area	Ignoring tapered sides	Volume overstated 30-50%	Always use both top AND bottom area in the prismatoid formula.
Measuring to embankment crest instead of full supply level	Using total depth including freeboard	Capacity overstated	Full supply level is typically 0.5-1.0 m below the embankment crest.
Confusing m3 and ML	Not performing the division by 1,000	Licence application in wrong units	1 ML = 1,000 m3. Water licences in Australia use megalitres.
Not surveying before licence application	Using estimated dimensions	Licence for wrong volume — compliance issue	For a formal licence application, use GPS or total station surveyed dimensions.