Farm Dam Calculator: Size, Volume and Build Cost
Short answer: A farm dam calculator multiplies the water surface area by the maximum depth and a 0.4 batter factor to give the volume in cubic metres, then divides by 1,000 for megalitres. A 60 m by 40 m dam that is 5 m deep holds about 4.8 ML. Big Ditch builds earth dams from $12,000 per ML + GST, falling to $5,000 per ML above 24 ML.
Farm dam calculator
Start with the size you have in mind, the dimensions of the hole, or the stock you need to water. The calculator works out the capacity in megalitres and prices construction at Big Ditch’s published 2026 rates, so you have a working budget before you book a site inspection.
Your dam
One megalitre (ML) is one million litres, or 1,000 cubic metres of water.
Measure at full supply level. For a gully dam, length runs up the gully from the wall and width is measured along the wall.
Stock rates are average daily figures from NSW and Victorian agriculture departments. The evaporation allowance is a Big Ditch rule of thumb from 24 years of building dams. Most farms size for two to three years of supply so the dam carries them through a drought.
Enter values greater than zero.
Your estimate
Build budget
Estimated all-in budget including GST
- Construction, excluding GST
- Rate applied
- Dam design, excluding GST
- $2,750.00
- GST on construction and design
- Site inspection, including GST
- $500.00
- Total including GST
Mobilisation and the fuel surcharge are quoted separately for each site. This is a budget guide only. Your fixed-price quote follows the $500 site inspection, once we have seen access, soil, catchment and spillway options.
How the volume is worked out
The calculator uses the standard farm dam formula published by Water NSW and state agriculture departments: surface area at full supply level, multiplied by maximum depth, multiplied by 0.4. The 0.4 batter factor allows for the sloping internal walls of a dam built at a 3:1 batter. Divide the cubic metres by 1,000 to get megalitres.
Rectangular area is length times width. Round area is 3.1416 times the radius squared. A triangular gully dam uses half of length times width.
How the price is worked out
Construction is priced per megalitre, excluding GST, mobilisation and fuel surcharge. Rates step down as dams get bigger, and a transition rate applies just above each break so a larger dam never costs less than a smaller one. Dams under 1 ML are budgeted at the 1 ML price.
| Dam size | Headline rate (ex GST) | Example |
|---|---|---|
| Up to 3 ML | $12,000 per ML | 1 ML: $12,000 |
| Over 3 to 5 ML | $8,000 per ML | 5 ML: $40,000 |
| Over 5 to 10 ML | $7,000 per ML | 10 ML: $70,000 |
| Over 10 to 20 ML | $6,000 per ML | 20 ML: $120,000 |
| Over 20 ML | $5,000 per ML | 30 ML: $150,000 |
Dams above 250 ML are priced individually after inspection.
How big should a farm dam be?
Size a farm dam for the driest stretch you need to get through, not an average year. Most graziers plan for two to three years of supply, because a dam that only carries you through one dry summer leaves nothing in reserve when the following autumn fails. Add up daily use, multiply by the number of days in that period, then add an allowance for evaporation and seepage.
The NSW Government livestock water table puts adult dry sheep at up to 6 litres per head per day, ewes with lambs at up to 10, yearling cattle at up to 80 and lactating cows at up to 100. Agriculture Victoria lists summer figures well above those averages, such as 112 litres a day for adult dry cattle, so allow extra if your stock rely on the dam through a hot summer.
Worked example: 200 head of cattle
- Daily use: 200 head at 80 litres = 16,000 litres a day.
- Two years of supply: 16,000 litres x 730 days = 11.68 ML.
- Evaporation allowance for coastal or tablelands country: 11.68 ML + 30% = about 15.2 ML.
- Construction budget: 15.18 ML at $6,000 per ML = $91,080 + GST, plus design and the site inspection.
Run the same numbers for Western NSW and the evaporation allowance doubles the net figure, which is why inland dams are dug deeper with a smaller surface area for the same capacity.
How to measure the volume of an existing dam
Water NSW uses one formula for every dam shape: volume in cubic metres = 0.4 x surface area x maximum depth, then divide by 1,000 for megalitres, according to its harvestable rights guidance. The 0.4 is a batter factor that allows for the sloping walls. The NSW Government farm dam guide suggests 0.3 or lower for shallow dams with flatter sides.
- Rectangular or square: surface area = length x width.
- Round: surface area = 3.1416 x radius x radius.
- Triangular gully dam: surface area = length x width / 2, with width measured along the wall.
- Measure at full supply level, and find the deepest point with a weighted rope marked at one metre intervals.
Will the dam fill?
Capacity is only useful if the catchment can fill it. Runoff depends on rainfall, slope, soil and ground cover, and the same hectare of catchment can yield several times more water on the coast than on the western plains. We check catchment area and the spillway path at the site inspection, because a dam that is too big for its catchment rarely fills, and one that is too small for its catchment needs a wider spillway.
What else decides the design?
- Clay: the bank and floor need soil with at least 20% clay to hold water without a liner or bentonite treatment.
- Batters: we build internal batters at 3:1 and external batters at 2:1 for a stable wall that stock and machinery can use.
- Spillway: sized for a 1 in 100 year flood with at least 0.5 m of freeboard between full supply level and the top of the wall.
- Licensing: in NSW you can usually build a dam within your harvestable right without a licence. The limit depends on your property size and location, so check it with Water NSW before you settle on a size.
Frequently asked questions
How many litres are in a megalitre?
One megalitre is one million litres, or 1,000 cubic metres. An Olympic swimming pool holds about 2.5 ML.
How much does a 1 ML farm dam cost?
Construction is $12,000 + GST. With the $2,750 + GST design fee and the $500 site inspection, the all-in budget is $16,725 including GST, before mobilisation and fuel surcharge. See the full breakdown on our farm dam cost guide.
Why is a 3.5 ML dam the same price as a 3 ML dam?
Rates step down at 3, 5, 10 and 20 ML. Just above each break a transition rate applies so that a slightly larger dam never costs less than a smaller one. A 3.5 ML dam is charged at the 3 ML total of $36,000 + GST.
Does the estimate include the fuel surcharge?
No. Every quote carries a separate fuel surcharge line, calculated from the rolling seven day average diesel price under clause 4 of our Terms of Trade. It shows $0.00 when diesel is at or below $2.20 a litre.
Who builds the dam?
Big Ditch Aquatecture has been building farm dams since 2002. In 24 years we have built more than 341 dams across NSW, Queensland and Victoria.
Book a site inspection with Big Ditch and we will turn this estimate into a fixed-price quote for your property.