There are many factors to consider when selecting pump size, including pond size, number and type of water features, head height, chosen filtration system, distance of filtration system from the pond and intended fish and plant stock. To work out an appropriate pump capacity you should follow these steps.
To calculate the volume of water for a basic rectangular pond you multiply length x width x depth x 1000. Throughout this site we work with litres per hour (lph).
Example: if your pond is 2m long by 1m wide x 0.5m deep you have 1000 litres of water.
For uneven shaped ponds it is best to take your measurements from the widest points and deepest area. Having a pump or filter slightly larger than your requirements is preferable to undersizing. Please see our fact sheet “Calculating Pond Volume” for calculating the volume of water in ponds of various shapes.
For most ponds with Koi we recommend that you pass the water in your pond through your filter every hour, therefore at a minimum your pump should be big enough to do this. So a 5000 litre pond would need a minimum of a 5000lph pump.
Your filtration system's capacity also needs to meet your desired flow rate. If you have 5000 litres of water your filter needs to be able to handle a flow of more than 5000 litres per hour for your water to turn over hourly. The flow returning from your filter will be less than the flow entering the filter.
When calculating the required pump size for a filtration system or water feature the Total Dynamic Head (TDH) refers to the height the water needs to be lifted from the pump to the final outlet. The actual calculated TDH includes
There are some good resources available online if you want to delve into all of the fine detail of calculating accuate TDH, however, we will try to simplify things here with some guidelines for the typical garden pond.
Pump manufacturers Max Head Height refers to the maximum pumping height of the pump. The stated head height of a pump is the minimum height at which you receive zero flow. So a pump with a 4m head will have zero flow at 4m. Therefore it is important to know the height of your intended water feature. If you need 3000 litres of water to run your water feature then you need to ensure the pump you select is able to pump 3000 litres of water at the height of the feature. Pump flow rates for different head heights are provided on this website, please refer to these when selecting a pump to ensure you select the right pump for your pond.
Hose size and length as well as the number of joins (elbows, T pieces etc) also affect flow. The larger the hose size the better the flow. There is no value in buying a large pump and then connecting it to small hose as flow is restricted and back pressure is placed on the pump, in many cases reducing the lifespan of the pump or causing the pump’s auto cut out to engage continually. When you browse a pump on this website the recommended hose size is shown in the detailed view under accessories.
Working out the exact loss of flow for varying hose sizes and types is very complex. For most garden ponds we allow for 10cm of lift for every 1m of hose length ie if you have 6m of tubing you need to add 60cm to your head height calculations. Any elbow or T piece will substantially reduce flow (add 25% height for each 90 degree elbow), so where possible avoid the use of these and use flexible tubing to curve around corners.
To make things easier, below is an approximate TDH calculator. This calculator takes into account the non-linear nature of increasing friction as the flow rate increases in pipes.
Ensure the pump you select has some excess flow to your requirements to allow for anything you may have missed and for slowing of the pump from debris etc.
If you have chosen to keep your pond clean via a biological filter (pressure or gravity filter) then you must size your pump appropriately for the filtration system. If you do not have enough flow through your filter you will not turn your water often enough and therefore your filter will not clean your pond appropriately. Too much flow through your filter will result in the water passing through the filter system too quickly, often bypassing the sponges and not giving the UV clarifier a chance to kill algae. This rapid flow can result in a cloudy pond and cause damage to pressure filters or overflow of gravity filters.
The recommended maximum flow is the maximum amount of water that should pass through the filter in litres per hour. In many situations separate pumps should be used for the filtration system and water feature. It is often not possible to push enough water through a filter to effectively run water features such as large waterfalls and waterwalls. Additionally you may not wish to run these features all of the time, whereas biofilters need to be run 24 hours a day to maintain oxygen levels to healthy bacteria in the filter.
As stated earlier pumps connected to biofilters need to be run 24 hours a day. This means if you would like to run a water feature or fountain off the same pump then the feature or fountain will invariably be running 24 hours a day as well. Running a water feature or fountain 24 hours a day is often not desirable due to noise, increased rates of evaporation and additional power use. A single large pump used to run multiple equipment will often use substantially more power than a couple of smaller pumps. With rising energy costs this is a very important consideration when designing your pond system. In many cases the initial increase in cost of purchasing multiple pumps is saved in power costs within months, particularly if you only run your water feature some of the time.
Here is a calculator which will help calculate your average cost of running a pump.
Please note the results of this calculation are approximate only. If you have "smart power" add up all of the units for each tariff rate for Total units.