You will need to know the volume of water in your pond sooner or later and probably sooner because it plays a role in choosing the right pump, the right filter and the right UVC. Good pond designs need good pond volume information.
You will need to know the volume of garden ponds under the following particular circumstances:

Pump specifications for basic pond designs
Treatment of garden ponds under differing pond water conditions - eg blanketweed treatments.

Addition of medicines to garden ponds. Accurate volume knowledge is critically important in these circumstances.
It is ideal to measure the volume of a pond when it is filled the first time using a metre or a pre-determined flow from a hose pipe for example. If this method is chosen store the information in a safe place where you can find it months later.

Using a hose pipe to MEASURE pond volume

The garden hose is turned on and the amount of time taken to fill a 10 gallon or other known volume container is taken.
The garden pond is then filled without changing the position of the tap supplying the hosepipe.
The amount of time taken to fill the garden ponds is recorded
The volume is calculated as follows:
Time to fill garden ponds is divided by time to fill fixed volume container. This result is then multiplied by the volume of the container.
For example time taken to fill a 10 gallons container is 1.8 minutes.
Time taken for garden ponds filling is 23.9 minutes.
Then garden ponds volume is 132.7 gallons ( 23.9 divided by 1.8 multiplied by 10).

How to CALCULATE pond volume.

It is very difficult sometimes to do this with great accuracy but for most garden ponds it is normally not critical if we are 10% or so out.
For regular shaped garden ponds based upon squares and rectangles the calculation is simple using fixed formulae. More complex regular shaped garden ponds such as trapezoid, triangular, hexagonal and so on are included on the FREE calculator's site as are the calculations for the simple round, square, rectangular shaped garden ponds also.