How Big Do Koi Fish Get? (& How To Make Koi Grow Faster?)

How Big Do Koi Fish Get

The color and pattern of a koi fish can make it the star of any water body!

In fact, it’s their appearance that makes them a favorite among fish keepers and hobbyists around the world. But that doesn’t mean you can get one right now.

Unless you consider their size and weight, it’s practically impossible for you to provide them with the ideal thriving conditions. So, in today’s guide, we will answer a crucial question- how big do Koi fish get?

Let’s start!

How Big Do Koi Fish Get?

The exact length and weight of a koi fish depends on various factors like its genetics, the nutrition it receives, the population, and environment of the pond or aquarium. 

On average, they may grow anywhere between 20 and 24 inches long, weighing from 9 to 12 pounds. But some varieties can grow to be even longer and heavier. For instance, there was the “big girl” koi that weighed a mammoth 90 pounds and measured 40 inches long at the age of 17 years. 

For the better understanding of our readers, we have mentioned the different Koi varieties and their sizes in the following sections.

How Big Do Butterfly Koi Get? 

Also known as longfin koi or dragon carp, this variety can grow between 36 and 40 inches, with the most common size being 24 inches. However, they may weigh more than 20 pounds.

The body and fins of butterfly koi get pretty large, and they grow relatively slowly throughout their lifespan. If you plan to keep other fish with this variety, then make sure they don’t feed on the fins of the koi. 

How Big Do Japanese Koi Fish Get?

Perhaps, Japanese koi are the most famous variety due to their color, and they can grow between 22 and 26 inches long. Likewise, the average weight is 12 pounds, but some can grow to be heavier than that. Due to intensive and selective breeding, this variety tends to grow very fast in breeding farms.

How Big Do Domestic Koi Fish Get?

With the slowest growth rate among all the other varieties (due to genetics), domestic koi grow to be only 12 to 15 inches long, making them extremely suitable for home aquariums. 

How Big Do Jumbo Koi Fish Get?

As you may have guessed from the name, “jumbo” koi is the biggest and largest variety, measuring at least 30 inches long and weighing well over 20 pounds. 

On top of that, there are some larger jumbo varieties that may get about 36 inches long and weigh between 22 and 26 pounds. A few record-breakers have also grown up to 52 inches! So, they should always be housed in outdoor ponds with ample space and depth.

It’s also worth noting that jumbo koi is a selectively bred variety, and only a few chosen young koi receive the required intensive care to grow that big.

How Fast Do Koi Fish Grow Full Size?

Again, the exact growth rate of a Koi fish will depend on the variety that you have your eyes on. For instance, the smaller Koi carp will generally grow faster and may attain their full length in or under 2 years. 

However, larger varieties like the Japanese or jumbo koi can take 3 years to grow to their full length. In ideal conditions, all varieties should grow at least 6 to 8 inches long at the end of the first year.

That said, let’s take a look at the difference in the growth rate of koi in an aquarium and pond.

How Fast Do Koi Grow In An Aquarium?

With good water quality, temperature, and adequate nutrition, Koi fish can add about a little less than an inch per month. Or, in other words, they can become about 8 inches in 10 months.

Here, we should mention that keepers should always consider the maximum length of the koi variety that they wish to house in the aquarium. Although they won’t grow bigger than they are supposed to, it’s important to give them enough space and depth for free survival.

How Fast Do Koi Grow In Pond?

When it comes to rearing Koi in a pond, they usually grow about 5 inches long (average) each year. Now, some varieties may also add less than that, while others may add more than 5 inches in length per annum.

As they start attaining maturity, the growth rate (in terms of length) slows down and they start adding more girth to their bodies.It may also be helpful to know that koi fish will usually attain their maximum length at the end of the third or fourth year, depending on what variety you have opted for. And as they grow in length, they add weight, leading to bulkier and heavier bodies.

How To Make Koi Grow Faster?

Size Of Pond

Undoubtedly, one of the most important considerations is the size of the pond based on the koi variety and its final length. Smaller ponds will invariably make your koi struggle for space, stressing them out during their growth stage and preventing them from attaining the full size.

A full-sized jumbo koi may require up to 50 gallons of water in a pond that’s at least 3 feet deep. And the more koi you have, the more should the water and depth of the pond be. 

As a rule of thumb, keep 10 gallons of water per inch of koi fish. Additionally, consider the requirements of the other fish varieties, if any.

Proper Filtration

All fish varieties require good quality water, and koi fish is no exception. Bad quality water will put undue stress on koi, making them unhealthy, and even killing them before time. 

In this regard, we’d strongly recommend installing a 3-stage filtration system that can take care of mechanical, biological, and chemical filtration. 

While mechanical filtration will eliminate solid waste particles, biological filtration removes ammonia and nitrate compounds. And with chemical filtration, you can get rid of toxins, chemicals, or colors present in the water.

Moreover, the filtration system should process the entire water at least 3 to 5 times per hour.

Ideal Water Conditions

Koi thrive in ponds that have a pH level between 7 and 7.5, and the ideal water temperature should be around 70-degree Fahrenheit. Keep in mind that waste accumulation (like extra fish food and decaying vegetation) can alter the pH level. 

Besides, they are a resilient variety, so a water hardness level of around 80ppm (parts per million) should keep them healthy. You can get a water testing kit to monitor the water conditions on a regular basis.


Koi generally have a very fast metabolism rate, so they require feeding at least thrice a day. But in doing so, ensure that the quantity of food is such that they can finish it within 5 minutes per feed.

Talking about nutrition, they prefer a nice mix of proteins, vitamins, carbohydrates, and fats. Hence, you can feed them different things like peas, soybeans, melon, rice, shrimp, meat, lettuce, etc. 

However, their digestion can slow down significantly during winters, which is when you should switch to a low protein diet. It’s during the summer months that their metabolism hits the peak with a high need for a protein-based diet.

Balance Population

Given the size of most Koi fish varieties, it doesn’t come as a surprise that they need a lot of space, be it in the aquarium or pond. Overcrowding the water will hamper their growth, and it may even kill the younger ones.

Moreover, koi are typically a messy species, especially in overcrowded habitats. It won’t take them long to kill the “good” nitrifying bacteria, which acts as a natural filter for keeping biological waste like ammonia at bay to prevent the water from turning toxic.

In addition, they tend to swim at every level of the water and hibernate below the surface during winters. But overcrowding the pond or aquarium with too many koi or other fish varieties, for that matter, can put them under undue stress.

Koi Genetics

No matter how good the conditions and nutrition are, koi won’t grow taller or larger than what their genes allow. Japanese koi have high-quality genetics, and many breeders in the west have selectively bred this variety to create many modern domestic koi that can attain full maturity at the age of 2. 

Besides, their growth rate is at highest during the first year, which is a genetically defined feature.

Final Thoughts

That has brought us to the end of our guide today.

Although it can appear intimidating at first, rearing koi fish is actually a straightforward affair. But then again, every fish requires some maintenance effort, so there’s no shortcut here.

All you need to ensure is that they have enough food, space, and consistent good water quality. And don’t forget the bonding time, especially while feeding them. Trust us; the joy of your koi fish feeding directly out of your hands is second to none!

Until we meet again!