Choosing the correct sized pump to feed a filter system for your pond can seem quite a difficult and complex task. One reliable method is to calculate a flow-rate, in litres per hour (Lph), for your pond. This is an appropriate rate, at which the pond water will be passed through a suitable filter.When calculating for this method, there are three variables which should be considered; the volume of water in the pond, the type of pond (in relation to the amount and size of fish in the pond and quantity of plants), and the amount of sunlight the pond receives.

1.Pond Volume

A bigger pond will generally need a bigger pump. To work out the volume of your pond, you will need to know the following dimensions of the pond in metres;

• The average length of the pond (L),
• The average width of the pond (W) and
• The average depth of the pond (Av. D).

If the pond dimensions are not calculated in metres, it is usually easier to convert the given measurements before you attempt to work out the volume.The length (L), width (W), and average depth (Av. D) are multiplied together. This will give you the pond volume in metres cubed (m³). To convert m³ into litres you need to multiply m³ by 1000.The formula for calculating volume is; L x W x Av. D x 1000E.g. a pond is 5 metres long, 3 metres wide and has an average depth of 1 metre;

Volume = L x W x Av. D x 1000 = 5 x 3 x 1 x 1000 = 15,000 LitresThis calculation is fine if your pond is square, or ‘square-ish’, but if the pond is triangular the formula should be;

(L x W x av. D x 1000) x 0.5,

and if the pond is circular the calculation should be;

3.14 x ((0.5 x L) x (0.5 x W)) x Av. D.Once you have calculated you pond's volume, you can use this to work out an appropriate flow-rate...

2. Type of Pond

Fish require feeding, and they produce waste. This increases the amount of pollutants present in the water which can affect the water quality, therefore a pond with a lot of fish will require filtering more frequently to keep the pond water clear, clean and healthy. Plants aid in filtering a pond, a pond with a lot of plants generally doesn’t need as high a flow-rate through a filter as one with few.Pond types are divided into three categories for working out flow-rates;

Koi Pond

This is a pond with high stock-rates of fish, or with very large fish, and no, or very few, plants. This type of pond will accumulate debris and toxins relatively quickly and therefore will need filtering at a higher rate. The entire volume of water in a Koi pond should be passed through the filter once an hour to keep the water clear and healthy.E.g. If the pond volume is 6000 Litres, the pump flow-rate should be 6000 Litres per hour.

Goldfish Pond

A Goldfish pond is a type of pond which has an average amount of smaller to medium sized fish, and some areas of planting. This type of pond will not produce waste as quickly as a Koi pond and will only need to pass through a filter once every 1 ½ hours to keep it clean. This can be calculated by dividing the pond volume by 1.5;E.g. If the pond volume is 6000 Litres, the pump flow-rate should be 4000 Litres per hour.

Nature Pond

This is a pond with no or very few fish, and a lot of planting. A nature pond will not accumulate toxins and debris as quickly as other types of pond, and water will only need to pass through a filter once every 2 hours. This can be calculated by dividing the pond volume by 2;E.g. If the pond volume is 6000 Litres, the pump flow-rate should be 3000 Litres per hour.When you have decided upon the pond type which would more closely describe your pond, you can begin to work out a flow-rate and look towards taking sunlight into account...

3. Sunlight

Pond plants need the sun to establish and thrive, but too much sunlight will encourage the growth of algae in the pond, turning the water green. The more sunlight a pond receives, the greener it will become. Passing the water through a UVC will help to rid the pond of suspended algae. If the pond receives a lot of sunlight it will need to pass through the UVC more frequently to help combat the algae.If a pond receives more than 50% of the days sunlight it is advised that the flow-rate should be increased by 25%. This can be calculated by multiplying the flow-rate (after taking the amount of fish into account) by 1.25.E.g. If the flow-rate is calculated at 6000 Litres per hour, and receives more than half the days sunlight, the pump flow-rate should be increased to 7500 Litres per hour.

You should now have a flow-rate in Lph that you can use to find a pump which will be suitable for your pond.

Flow-rate Flow Chart

To help calculate an adequate flow-rate for pond filtration, the flowchart below can be used;

Once you have attained a suitable flow-rate you can find a pump which can deliver this amount of water to a filter. Make sure you take into account the total head which the pump may have to achieve to reach the filter.

Some pumps are designed to supply water features or fountains, at the same time as passing water through a filter. When diverting water to a feature you will be reducing the flow-rate through the filter, which could result in the pond becoming under-filtered.

As a rule of thumb, to compensate for this loss of flow, simply add 100Lph to the flow-rate for every 10cm of fountain or water feature height which the pump will be supplying.

E.g. If the pond's calculated flow-rate is 6000Lph, and you want a pump to supply a 50cm tall fountain or water feature, the flow-rate should be increased to 6500Lph.When choosing a filter you should aim to find a unit which can handle the amount of water been fed to it, and is specified by the manufacturer for the size and stock rates of your pond.