Use this guide to help you estimate the capacity of your dam in litres.
1,000 litres = 1 cubic metre of water
1,000 cubic metres = 1 megalitre
1,000,000 litres = 1 megalitre of water

To work out the size/capacity of your dam it is a very simple equation.
L x W x D x 0.45 = Capacity in cubic metres.
Everything needs to be done in metres for convenience. You are after a simple (Length “L”) x (Width “W”) x (Depth “D”) equation that will give you a cubic metre result. Basically the surface area by the depth. You will then need to multiply by a reduction factor of 45% (0.45).

The average farm dam is generally an odd shape, but that’s not a big concern as you are only estimating, so just treat it as a square or rectangle to get the surface area. Just pace it out, roughly 1 large pace is 1 metre.

For example if this is your dam just pace on the shore A and B. Just imagine the dam was a rectangle, so think straight lines. Multiply these 2 together and you have the surface area in square metres (m²).
For example if A= 20 mtrs & B = 50 Mtrs So 20 x 50 = 1000 m².

Then, you need to look at the depth.

You need to find the deepest spot in the dam, most dams slope down to the centre if a standard turkey nest dam. If you are a valley dam where you have a small wall and then flood back up a valley you will need to do extra calculations of all the extra shallow water.
If your depth “D” is say 3 mtrs deep then you multiply the surface area by the depth. This gives you a cubic metre equation. For example Surface Area 1000 m² x 3 = 3000 m³.
This is not the final size; you need to multiple by the factor (.45). The factor allows for the slope and is the factor that would be recognized by most Government departments for rough calculations. Just multiply your 3000 m³ by 0.45 = 1,350 m³ or 1.35 MegaLitres (ML). This is the final capacity of your dam.
Therefore: Dam 50 mtrs by 20 mtrs and 3 mtrs deep has a capacity of 1,350 m³ or 1.35ML