Why use this calculator
Stock water is the one input that cannot run out, even for a day, and most water crises on farms start with an optimistic guess about what a tank or dam holds. Round tanks are simple to measure, but dams are not: their sloped sides mean a dam that looks half full by depth may hold only a third of its volume.
The old rule of thumb of length times width times depth times 0.4 can be badly wrong for small, deep or steep dams. This calculator uses the prismoidal formula with your actual batter slope, converts the answer into every unit you are likely to meet on a tank plate, a licence or a builder's quote, and turns it into days of supply at your daily draw.
How to use it
- Choose the storage type first, since each shape uses different measurements.
- Enter the inside diameter for a round tank in metres.
- Enter the length at the top water line for a dam, or the inside length for a rectangular tank. Enter the width the same way.
- Enter the water depth at full supply, from the floor to the overflow or spillway.
- Enter the dam batter slope as horizontal metres per vertical metre; it is ignored for tanks.
- Enter your daily water draw in litres to see days of supply.
What each result means
- Volume in litres
- Total water held when full to the depth you entered, the figure to compare with tank ratings and stock needs.
- Litres per centimetre of depth
- How many litres one centimetre of water depth holds, so a dipstick or a level gauge reading can be turned straight into litres. For a dam it applies at the top water line.
- Volume in kilolitres
- The same volume in thousands of litres, the unit water carters and many council and utility bills use.
- Volume in megalitres
- The volume in millions of litres, the unit used for dam storage, water licences and irrigation allocations in Australia.
- Volume in cubic metres
- Geometric volume in cubic metres, the unit an earthmover uses when quoting a dam by the cubic metre of excavation.
- Volume in imperial gallons
- Volume in imperial gallons, the unit on older Australian tank plates and some historical farm plans.
- Days of supply at your draw
- How long a full storage lasts at your daily draw, before any rain, inflow, evaporation or seepage.
The formula
Round tank:
V (m³) = π x (diameter / 2)² x depth
Rectangular tank:
V = length x width x depth
Dam, prismoidal formula:
- Floor length Lb = L minus 2 x s x D
- Floor width Wb = W minus 2 x s x D, where L and W are top water line dimensions in metres
- D is full depth
- S is the batter slope; V = D / 6 x (L x W + 4 x Lm x Wm + Lb x Wb), with Lm and Wm the dimensions at half depth
Litres = m³ x 1,000; megalitres = m³ / 1,000; imperial gallons = litres / 4.546
Days of supply = litres / daily draw
How this number is used in practice
For stock water planning in Australia, state departments suggest budgeting enough storage to get through the longest dry spell you expect, which in many districts is well over six months. A mature beef cow drinks around 40 to 70 litres a day and a dry sheep 3 to 7 litres, and both figures can double in hot weather or on dry, salty feed.
Multiply by head, add house and spray water, and compare the result with this storage. The days of supply figure is the number to watch as numbers or weather change.
Dams lose water in ways tanks do not. Evaporation across much of inland Australia is 1.5 to 2.5 metres a year, so a shallow dam can lose more to the sky than stock drink. Deep dams with a small surface area hold water far better than wide shallow ones of the same volume.
The table of part depths shows why a dam at half depth is not half full: with sloped sides most of the water sits in the top of the profile, which is also the layer evaporation takes first. Size dams for depth, fence them where you can and pipe water to troughs.
Worked examples
Round tank 3 m across, 2.5 m of water, 2,000 L a day draw
17,671 litres (17.67 cubic metres), 8.8 days of supply.
Farm dam 40 m by 30 m at the top water line, 3 m deep, 3 to 1 batters, 5,000 L a day
2,034,000 litres, or 2.034 megalitres, and 406.8 days of supply before evaporation.
Concrete trough 3 m long, 0.6 m wide, 0.5 m of water, 500 L a day
900 litres, 1.8 days of supply.
When people use it
- Checking how many days of stock water a tank or dam holds going into summer
- Comparing a dam builder's quote in cubic metres with the water it will store
- Sizing a new poly or steel tank for a house, shed or stock trough
- Estimating dam storage in megalitres for water licence or farm plan paperwork
- Planning fire fighting reserves for a property bushfire plan
Tips
- Measure water depth to the overflow or spillway, not the tank wall or dam crest.
- Allow 5 to 8 millimetres a day of evaporation from a dam surface in summer when budgeting.
- Deep, narrow dams lose far less to evaporation than wide shallow ones of the same volume.
- Tank outlets sit above the floor, so the last 5 to 10 per cent of a tank is usually not drawable.
- Check stock water quality as dams drop, because salinity and algae rise as volume falls.
What it assumes, and where it stops being accurate
- Dam volume assumes a rectangular dam with the same batter slope on all sides and a flat floor.
- Irregular, turkey nest and gully dams differ from this shape. A survey gives the true volume for licences or sales.
- Days of supply ignores rain, inflow, evaporation, seepage and the undrawable volume below the outlet.
- Round tanks are treated as straight sided cylinders; corrugated walls and domed floors change volume by a few per cent.
- Gallons are imperial gallons of 4.546 litres, the unit used on older Australian tanks, not US gallons.
Terms used on this page
- Batter
- The sloped inside wall of a dam, described as horizontal distance per unit of vertical depth.
- Top water line
- The level of water when a dam is full to its spillway, where length and width are measured.
- Megalitre
- One million litres or 1,000 cubic metres of water.
- Prismoidal formula
- A volume formula that averages the top, middle and base areas of a shape with sloping sides.
Questions people ask
- Why not just use length times width times depth for a dam?
- Because the sides slope. A dam with 3 to 1 batters is much narrower at the floor than at the top, so the simple box volume overstates storage, often by half or more. The prismoidal formula used here averages the top, middle and floor areas, which is the standard method for dams with straight sloping sides.
- How much water do my stock need each day?
- As a planning guide, a dry beef cow drinks 40 to 70 litres a day, a lactating cow up to 100 litres, a dry sheep 3 to 7 litres and a horse 40 to 60 litres. Needs rise sharply in hot weather and on saline water or dry feed. Your state department publishes stock water guides for local conditions.
- What is a megalitre?
- A megalitre is one million litres, or 1,000 cubic metres. It is the standard unit for dam storage, irrigation allocations and water licences in Australia. A dam holding 2 megalitres of water covers a family home's use for many years but may last only a few months for a larger cattle herd in summer.
- What batter slope do farm dams have?
- Inside batters of 2.5 to 1 up to 3 to 1 are common in clay soils, with flatter slopes in less stable soils. The figure is horizontal distance per metre of depth. If the calculator says the floor width would be negative, the depth and slope cannot fit the top dimensions you entered.
- Why does the calculator show imperial gallons?
- Many older Australian tanks were sold by the imperial gallon, which is 4.546 litres, before metric conversion in the 1970s. A tank plate reading 5,000 gallons holds about 22,700 litres. US gallons are smaller at 3.785 litres, so be careful comparing overseas figures with local tanks.
Use this with
Water Tank & Dam Volume Calculator answers one part of the question. These pick up where it stops: Stocking Rate Calculator, Irrigation Water Calculator, Rainfall & Soil Moisture Calculator, Hay & Silage Feed Calculator and Water Intake Calculator.
Where these figures come from
Figures and rules on this page follow Australian law and ATO guidance.
- NSW Department of Primary Industries. Farm budgets, drought feeding, fencing, farm dam and soil water guidance
- Agriculture Victoria. Extension notes on feeding livestock, farm water supplies and soil moisture
- Primary Industries and Regional Development, Western Australia. Dryland cropping costs, farm water planning and soil water holding data
- Bureau of Meteorology. Daily temperature, rainfall and evaporation records for Australian stations
Water Tank & Dam Volume Calculator produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.