How to Measure for a Pond Filter

The correct filtration is essential if you want a healthy pond. While over-filtration will not harm your pond, inadequate filtration can kill plants and fish, and leave the water murky instead of healthy and clear. There are a number of factors to consider when choosing the correct filter size for your pond. The pond’s volume, shape, location, plants, and number of fish all affect how well the filter can perform. If your pond is not currently stocked, the minimum requirement would be a filter that can run the entire volume of your pond each hour.

Sizing a Pond Filter

The first step is to find out the volume of your pond if you don’t already know it. You can either use a metre-stick or a stiff measuring tape to measure the length, depth, and width of the pond. If your pond is an irregular shape or has different depths, take a few measurements and then average them together. To do this, add the measurements for each part of the pond, such as breadth, and divide by the number of measurements that you took. Remember to make note of the measurements as you take them.

Calculating the Volume

To calculate the volume of water for your basic rectangular shaped pond, you can multiply depth x width x length x 1000 (working in litres per hour, or lph). For instance, if your pond is 2 metres long x 1-metre-wide x 0.5 metres deep, you will have 1,000 litres of water.

If you have an uneven pond shape, it is a good idea to take the measurements from the deepest area and widest points. Having a pump that is slighter larger than your requirements is much better than having one that is too small.


When it comes to calculating pond volumes for different pond shapes, most results will be approximations since the sides of ponds are not typically vertical, and the bottom of the pond may not be completely flat.

You also need to consider your filtration system’s capacity which needs to meet the desired flow rate. For example, if you have 5,000 litres of water, your filter will need to be able to handle a flow of over 5,000 litres per hour for the water to turn over hourly. The result will be that the flow returning from the filter will be less than that entering the filter.

Remember to Compensate for Fish

When sizing up your pond for a pond filter from, be sure to adjust the effective volume in order to compensate for fish. For instance, if there are less than 100 fish per 3785 litres of water, you will not require additional filtration. However, if you have 125 fish per 3785 litres, you will have to increase the filter capacity by 25%. Alternatively, you could multiply the effective volume by 1.25 which will give you the total filtration needed.

Finally, once you have all your measurements, you should purchase a pond filter that is rated to run 1.5 times the total filtration you require.