What size pond pump do I need for my garden pond?

garden pond

Choosing the right sized pond pump is one of the most important decisions you need to make when building a garden pond, get it wrong and you will be heading for all sorts of trouble.

In this article we will help you choose the right pump for your garden pond, it’s not as easy as it sounds as there will be many variables that we need to discuss in the process, but stay with me, and we will get you through this.

Nearly all garden ponds will require the use of a pond pump to circulate the pond water, the exception to this is if you have a wildlife pond with plenty of plants and no fish, if this is the case then you could get away without using a pump.

As soon as you add fish to your pond the circumstances change and a good pond pump is needed to keep the quality of the water clean and safe for your fish to live in.

Choosing a pump.

Before you choose a pump for your pond you need to establish a few things, firstly and most importantly you need to know how much water is in your pond.

You can use the pond water volume calculator from bradshawsdirect.co.uk by clicking here.

With the volume of water calculated you would think it would be easy to find a pump to suit your pond, but nothing is that easy.

The next calculation has to do with the head of water that the pump needs to handle, this is simply the height the pump has to lift the water from the top of the pump.

The head is the height of water at its highest point from the pump in your pond to the top of a waterfall, a stream or to your filter system.

To calculate, measure the height of water from the pond pump to its highest point, if the highest point is a waterfall 120 cm above the pond pump the head is 120 cm, simple, but we are not quite done (obviously if your reading this article you might not have a pump yet so measure from where the pump will sit on the bottom of your pond).

You also need to take into account the length of pipe the pump has to pump water to get to the top of the waterfall, if you need 300 cm of pipe to get to the top of the waterfall/stream or filter system this will affect the head calculations. For every 300 cm of pipe add 30 cm to the head, so if you had 120 cm of height and 300 cm of pipe you would have a head of 150 cm.

As the head increases due to the height of the water and length of piping so the pump’s flow rate decreases, so what you think is the right sized pump could end up being under powered for the amount of water you need to circulate if you don’t take the head into consideration, are you still with me.

Every pond pump has its maximum flow rate rated at a zero head, as you add head the flow rate goes down, sometimes to half the original flow rate, luckily most pond pumps have a flow chart that shows the flow at different levels of head so it make it easier when selecting your pump.

All pumps will also have a maximum head height, this is the height of water that the pump can achieve, once it reaches this level the flow of water will stop.

Now you have all this information you think you are ready to buy your pump, not so fast, we need to talk about circulation per hour.

Circulation per hour is how much water your pump will circulate in one hour, so if you bought a pump that matched the volume of water in your pond including the head, then it will circulate all the water in your pond every hour, but do you need to circulate your water every hour, that’s another variable we need to look at.

There are many different opinions on how often you should circulate your pond water, some say once an hour, others say twice an hour, but we like to do things a bit different, we look at the size of your pond to determine how often the water should be circulated.

Pump for small ponds.

As crazy as it sounds a small pond upto 4500 litres (1000 gallons) is a lot harder to look after and maintain than a larger pond.

The reason comes down to dilution, with less water in the pond, all the toxins from fish waste and all the debris that rots at the bottom of the pond gets concentrated as it can’t dilute in the limited amount of water, before you know it you have one very messy pond that your fish will not like, in fact if left too long your fish could die.

To get around this we recommend you circulate the water twice every hour, to do this you need a pump that is twice the size of volume of water plus head.

Let’s say you have calculated a pond pump of 3400 litres (750 gallons) of water is needed to circulate the water once per hour, now you need to double the size of pump to 6800 litres (1500 gallons) of water to circulate the water twice per hour.

Pump for medium ponds.

This is the easy one to work out, a medium pond 4500 litres to 9000 litres (1000 to 2000 gallons) will need to circulate all the water once per hour, there is enough water in the pond to dilute the waste without it getting out of hand.

So say you have a pond that requires a 6800 litres (1500 gallon) pump, all you need to do is buy a 6800 litres (1500 gallon) pump, simple.

Pump for large ponds.

Now we get on to large ponds above 9000 litres (2000 gallons), because there is so much more water in a large pond the dilution of waste is easily taken up with the amount of water that’s available.

So a large pond only needs to circulate all its water every 2 hours, so if you calculated you needed a pond pump of 13600 litres (3000 gallons) you can half it and buy a 6800 litres (1500 gallon) pump.

As you can see we have shown you three different size of pond, but they all use the same sized pump which is a bit surprising when you think about it, a large pond can be just as cheap to run and a lot easier to maintain than a small pond, so bear that in mind if you are about to build a pond.

Once you know the volume of water in your pond, the head of water at its highest point and the amount of times you need to circulate that amount of water in an hour it’s time to pick a pond pump that matches your requirements.

You can use the charts that come with most pond pumps to figure out the pump that is most suitable for your pond, but if in doubt always go for the larger option, you can never have too much filtration but you can have too little.

Check out our article on submersible pond pumps here.

Now you know how to choose the right pump for your own pond you might think you are done, but there are a few other factors to look at.

Matching your pump to your filter system.

Once you have worked out the right sized pump for your pond you need to match your filter system to the pump.

Its no good having a filter system that can’t cope with the amount of water your pump is sending through it.

Its very simple to do, find a filter system that matches the flow rate of your pump and it will work in harmony, doing what it does best, so if your pump size is 1500 gallons per hour find a filter system that is 6800 litres (1500 gallons) per hour.

UV Carifier.

A lot of filter systems come with a UV Clarifier, but if you want a separate one you will need to match it to the pumps flow rate.

A UV Clarifier kills all the single cell algae that turns your pund green, the pond water passes a UV Light that kills them, if you fit a UVC that is rated at 2700 litres (600 gallons) of water and your pump has a flow rate of 6800 litres (1500 gallons) the water will flow through it so fast the algae won’t be killed.

Waterfalls.

The amount of flow from your pump will determine how much water cascades down your waterfall.

If you have matched a pump with your pond and filter system that also supplies the water to your waterfall that’s the amount of water available.

If you want more water cascading down your waterfall you can add a separate pump to work alongside your filter pump, or you can run one pump for your filter system and a second pump for your waterfall.

The good thing about having a dedicated pump for your waterfall is the ability to turn it of when not needed, just leaving the main filter pump running for maintaining the water quality, this can also reduce the head if the filter system is sitting low down.