How To Calculate Water Volumes For Any Garden Fish Pond of Any Shape
If your pond is not a simple rectangle or square the calculation is more
difficult. If you are not totally accurate do not be too concerned because we
are looking for an approximate volume. However you should aim to be within
10% to 15% accuracy.
In making approximations the simplest way to do this is to assume the pond is
surrounded by a rectangle and then ask yourself what proportion of the rectangle
is covered by water. See the sketch below to assist in understanding this
measurement technique.
Lets say the proportion is 50% and the depth is once more constant at 0.5 metres
and the rectangle around the pond is 3 metres long x 2.8 metres wide. The volume
is calculated as follows:
3.0 x 2.8 x 0.5 x 50/100 = 2,1 cu.metres = 2.1 x 1,000 = 2,100 litres
If the depth is not uniform then use the average depth to calculate the volume.
Another more accurate approximation can be made by counting the small squares
into which the pond has been divided as in the diagram above. This is the method
explained in detail:

The pond was first drawn to scale using graph paper or something similar like a
spreadsheet (this sketch was done on a spreadsheet):

Each square in the sketch has a constant size. As you can see the total squares
covered in the total rectangular area is 11 x 12 = 132 squares.

The total yellow squares must now be estimated.
We estimate about 21 full squares
are covered by the yellow bits. If you get a bit different it does not matter
much.

Now count the white squares we get 56.

This means you can now calculate the number of squares covered by the blue pond
as follows:

= 53 squares

If each square is 0.2 x 0.2 metres then area covered by pond is as follows:

53 x 0.2 x 0.2 = 2.12 sq metres

If average depth of pond is 0.5 metres then pond volume is

2.12 x 0.5 x 1,000 = 1,060 litres
Knowing the volume of your pond is important if you want to specify other
equipment correctly. Once again if you are not sure seek help.
While considering pond volume 1,000 litres of water is a good volume to hold
about 1 kg of fish.
Here is that conversion table again.
this page reprinted with permission of
The Pond Professor 