How Deep Should a Koi Pond Be?

6 min read

Koi ponds are very beautiful, attractive places to relax and enjoy your beautiful koi fish.

They are a great place to enjoy with your family or guests and even a great conversation starter if you build one and get to know the intricacies.

However, when you do decide to build one, it takes careful planning and you need to do it right. One of the most important questions I will answer for you is, how deep should a koi pond be?

An outdoor koi pond should be at least 18 inches deep in its shallowest area. The deepest parts should be 4-6 feet to provide different heat layers, prevent total freezing, keep the koi pond from becoming too hot, provide room for the ecosystem, and deter predators.

Some people actually do have an indoor koi pond which can be really cool, and since it’s indoors, the depth isn’t as important because the indoor temperature is better regulated. However, most people have koi ponds outdoors, so let’s talk about those first.

The most basic guideline for around 4 grown koi is the depth mentioned above. Make the pond at least 4 ft by 6 ft in length and width and start with at least 1000 gallons of water.

You should have at least 10 gallons of water for each inch of length of the koi fish you have.

Use this pond volume calculator to determine the volume of your koi pond.

A lot of people make mistakes when they build their first koi pond. One of the most important mistakes is making their koi pond too shallow. This can cause problems for your koi fish during the summer and winter.

How Deep Should a Koi Pond Be in the Winter?

It can cause problems during winter because the top water will freeze no matter what the depth of your pond is.

In most of the U.S., at least 18 inches will suffice. If you live in the coldest parts of the U.S. add an extra half foot or so when you build your pond. When building a koi pond, it never hurts to go a little bit deeper than the minimum requirements.

The deep part of your koi pond will dramatically come into play in the wintertime. It is said that koi fish hibernate during the winter. While this isn’t completely true, koi fish do go into a state similar to hibernation during winter. Their metabolism slows down and they stop eating.

They stay pretty much inactive in the deepest parts of the pond where the water stays at a constant cold temperature. This is why it is so important to have the proper depth for your pond during winter. The main goal is to keep your pond from freezing to the bottom.

Tip: Make sure your pond is deep enough that it goes below the freeze line wherever you live.

Tip: A proper de-icer will leave a hole in part of the pond to allow dangerous gases to escape.

It is also important not to circulate your water during the winter. Any fluctuations in your pond’s water may harm your koi that are trying to stay in the one area in your pond that does not change temperature.

Some people even keep their koi in a winter fish tank indoors just to keep them safe. This is more feasible if you have a basement or an unused room in your home.

How Deep Should My Koi Pond Be in the Summer?

Your pond has to be deep enough in the summertime for the opposite reason as winter. You don’t want the water to get too hot. The depth requirements are the same in summer as they are in winter for opposite reasons.

Your koi will also change color during the summer months if they receive too much sunlight. They will also be uncomfortable because koi are very active during the summer versus the winter. One of the reasons is they are feeding like crazy to store enough fat in their bodies to survive winter.

Tip: Make sure your pond has plenty of shade in the summer for your fish to hide from the sun.

Other Reasons Why Koi Pond Depth is Important


Some amount of water will always evaporate from any body of water. You will need to top off your koi pond every now and then because of this. If your koi pond is not deep enough, especially at the shallow end, the water will evaporate much faster

Under Gravel Filter

If your pond is going to have an under-gravel filter then you need to make sure when you build it that you add another foot to your depth to account for the filter. So, your pond will need to be at least 30 inches in the shallow part and 5 – 7 feet in the deepest part. This is to allow enough room for the filter, pipes, and braces for all of the equipment that will be used.

Indoor Koi Pond

Obviously, if your koi pond is an indoor pond, then the temperature is not as important during the summer and winter. So, the depth is not very important either. However, you should still have a deeper part in your pond, so that your koi fish have room to move around.

They will also want to stay in the deeper waters at night because they will feel more secure. They sleep at night in the deeper waters where they feel safe from predators.


predators are why your koi pond should be deep

The shallower your koi pond is, the easier it will be for predators to get to them.

Birds will leave koi ponds alone for the most part, but the most infamous koi predator is the heron. It will try to swoop down and grab your koi from shallow waters. They will also stand in the shallow water and wait until the koi becomes preoccupied and quits paying attention to the heron. Other predators such as raccoons will try to snatch your koi from shallow waters.

This is why you should try to keep your whole pond a foot deeper than the minimum requirements if you can when you first build it. Keep the sides of your koi pond deep so predators have a hard time getting to your koi which warns your koi when they are struggling in the deep water so your koi can get away


Your koi pond has to be deep enough to have enough space for your ecosystem to work.

Biological filtration removes excess nutrients from your water by removing sediment and debris. This keeps algae away and keeps your fish healthy.

Your ecosystem also involves plants. Many people think plants are just for looks but they are actually a very important part of your pond. Plants consume excess nutrients and use them to grow. This keeps algae away.

Plants also infuse your water with oxygen which flushes out nitrates and carbon dioxide. This is important for your fish’s health.

Your ecosystem will not have enough room to work without the overall proper depth of your pond.


If your pond has the proper depth for your ecosystem, then your koi fish will also have some natural food that they can eat. This will help keep your koi fish healthier with the addition of any food that you feed them.

Precast Garden

Some beginners try to use a precast pond, but they are not for koi. These ponds are mainly for decorations and maybe some smaller fish like goldfish. Koi fish simply get too big for these precast garden ponds.


Even though the recommended minimum depth is 18 inches, we recommend keeping your koi pond between 4 – 6 feet. This is because koi fish like to dive.

4 – 6 feet is also a great amount of space for the beneficial bacteria to work as described above. Some people like to make their pond even deeper so they can swim in it but don’t go over 9 – 10 feet in depth. If you go too deep, the beneficial bacteria could become overwhelmed and algae could overtake your pond.

Koi ponds are great! Koi fish can live for a very long time. They can come to trust you to the point that you can feed them out of your hand or even pick them up out of the water.

You can build your own koi pond or hire someone to do it so you can enjoy these great fish.

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Pet Aquariums

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