The deciding factor for choosing which equipment to use is the volume of water in the pond.

The first thing to do is calculate this using the following formula:

Average length (m) x average width (m) x average depth (m) x 1000 = pond volume in litres

Add on 25% if the pond depth is less than 70cm. Add on another 25% if the pond is in sunlight during any period of the day. The figure that you arrive at is the size of filter that you are looking for.

**For example:** if the answer you get is 5500, you would chose a filter with this rating. If the size of your pond falls between 2 filter sizes, then always chose the bigger size as it is better to over filter than under filter. The pump you'd get with the filter, as a general rule, is rated at approximately half the size of the physical pond volume and should mean that at least half the pond is filtered every hour.