How do I determine the right volume of my pond?

Here is a simple rule:

For round ponds: Length x width x depth / 2 = pond volume in m³

For rectangular ponds: Length x width x depth = pond volume in m³

How do I determine the right size of pond liner for my garden pond?

For this there are two simple formulas:

Length of a liner section: Length + 2 x depth (at the deepest point of the excavation) + 2 x 50 cm = length of the liner Width + 2 x depth (at the deepest point of the excavation) + 2 x 50 cm