How deep should a koi pond be?

Dear Ben,

How deep should a Koi pond be? And why does a Koi pond need to be a certain depth? Is it because Koi need a certain gallonage of water, or if the pond is really long and wide, will that make up for it?

Whenever you hear koi keepers chatting, you can’t help but notice how keen we are to share our pond’s vital statistics. Firstly, we’ll make a point of mentioning the volume of the pond and then qualify that by stating it’s depth. Whether that’s because we think that the depth of the pond is the best measure of how serious a koi keeper we are – or that it reflects on our own physique or physical strength, recognising how difficult it must have been to dig. But more importantly, depth is definitely a key vital statistic for any koi pond because it has a fundamental effect on how koi will fair during this stay in the pond and how well the pond itself will function.

I’ll answer your 2nd question first ‘why does a koi pond need to be a certain depth?’ Depth of a pond affects how a pond operates in several different ways.

1. Temperature stability. As you will know from your own experience, a small cup of tea cools quicker than a larger mug. This is because it has a greater surface area to volume ratio, and that there is comparatively less volume of stored heat in the smaller cup. In a similar way, a deep pond not only enables a pond to hold more water, but also helps to keep the pond water more temperature-stable by increasing the volume of the pond, relative to its surface area (where heat is lost). This has obvious benefits for the koi in winter where the larger volume of water to start with retains its heat for longer and secondly the chilling effect at the water’s surface, relative to the pond’s overall volume is greatly reduced. You will notice that once ambient temperature falls below 8-10C, your koi will stop feeding and will become less active, adopting typical cool weather behaviour, settling to the lower layers of your pond. As water cools down further, between 4 degrees C and freezing, the water actually gets less dense and floats above the denser 4-degree water, eventually to form ice if the air temperature drops below freezing. Koi and other pondfish take full advantage of this phenomenon by sitting on the pond bottom. In very still and deeper water a stable boundary can form between the less dense 0-4 degrees C water and the denser warmer water.

This barrier is called a thermocline and the phenomenon of different layers forming in water is called stratification.

However, even in the deepest artificial koi ponds a thermocline is not likely to form due to the mixing action of pumps and filters. Even so, any pockets of warmer water in this dynamic situation will still have a tendency to sink providing a warmer retreat for the koi. If this strange phenomenon did not occur then all aquatic life would freeze to death on the bottom of ponds each winter.

So, the deeper your pond, the more stable and insulated will your water be at the bottom of the pond. There is a cost to this benefit of winter stability, and that is the pond will warm up more slowly in the summer. This again is a function of the ratio between the pond’s surface area (through which it is warmed by the air and sun) and the overall volume of the pond. However, koi will benefit from more stable temperatures over daily cycles, as well as from the stability they experience within each season.

  1. Another benefit of digging a deeper pond is that you can squeeze more pond volume into a pond of the same sized footprint (or area). By packing in more volume, you are able to offer your koi better, more stable water quality by virtue of ‘the solution to pollution is dilution’ phenomenon. This means that as a new pond matures, the deeper pond is, the less likely it is to suffer from an extreme case of new pond syndrome compared to say a shallower pond of the same area. Making a pond ‘long and wide’ will enable a pond to hold more volume, but by using this approach to gain more volume, you will not benefit from the advantages of a deeper pond.


  1. More typical environment. Experience has shown that koi will grow faster in ponds containing a greater volume, but will also be more likely to develop deeper bodies, enjoying the vertical scope that a deep pond provides. The opportunity for your koi to swim up and down improves their overall body shape and provides your koi with a water depth that their physiology is accustomed to and arguably benefits from.


How deep should a koi pond be?

Just as there are no strict formulae for stocking rates at which koi can be kept in a pond, the same can be said for pond depth; there  are however recommendation based on a wide body of experiences. Minimum pond depths of 4’ are quoted and widely regarded as the safe depth to keep koi at (from an overwintering point of view). Having said that, the vast majority of garden ponds in the UK are 3’ deep or less – most of which contain koi that over winter satisfactorily. But from a koi purist’s point of view – 4’ is a minimum.

BOXOUT: Top tips for designing and building a Koi pond?

  1. Build you pond as large as possible the first time – you’’ probably regret it if you don’t.
  2. Dig it at least 4’ deep which means that if you are going to opt for a bottom-drain, gravity-fed system, you will need to dig down at least 6’.
  3. When building a gravity-fed filter system, make sure pipe diameters to the filter are as large as possible and that pipe-runs are as short as possible.
  4. To help aid mixing and reduce dead spots, use natural, sweeping curves rather than angular corners.
  5. Deep ponds are of course, dangerous pond for children. Make sure you are always aware when children and non-swimmers are in your garden. Put a fence around your pond if necessary.

Kill blanketweed and string algae.