Calculating Pond Volume
Before we start, it is important to know how much water you have. If the pond isn't a perfect square or rectangle, it may not be 100% accurate, but it is a good guideline.
For irregular shaped ponds, take the furthest points and work it out to a square or rectangle. Then you can take off 10 - 20% (example) for shelves or rounded corners perhaps. You are much better off over calculating than under for this part.
In feet: Length x Width x Depth
then multiply by 6.23 = gallons, then multiply gallons x 4.54 = litres
Pond size - 13(L) x 9(w) x 3(d)
= 351 x 6.23 = 2186.73 gallons then multiply by 4.54 = 9927.74 total pond size
Where everyone goes wrong
If for example you have worked out your pond volume to be 5000 litres, a filter advertising that it will filter a 5000 litre pond will not be able to do so.
This filter would perhaps filter an indoor pond with no sunshine, fish, plants etc.
In reality, a 5000 litre filter would just about manage a 2500 litre pond at best with a few fish and some sunshine.
What to do
As a general rule, the bare minimum would be to double up. If you can afford it and you have the space, do yourself a favour invest in the biggest filter possible.
Not only will it keep the water clear and healthy, you may save yourself a few grey hairs with less maintenance and blockages.