Understanding Japanese Koi Measurements: CM to Inches & KG to Pounds

If you’re shopping for Japanese koi, you’ve probably noticed something different about the way they’re described. Instead of seeing a koi listed as 18 inches, you may see 45 cm. And when weight is discussed, you’ll usually see kilograms (kg) rather than pounds.

Why?

Because Japan uses the metric system, while the United States still commonly uses inches and pounds the customary system.

For American koi keepers, learning a few simple conversions makes it much easier to understand Japanese koi listings, breeder information, growth records, and even medications and pond products.

At Koi Boyz, we work with both American-bred koi and imported Japanese koi, so here’s an easy guide to understanding both systems.

Why Does Japan Measure Koi in Centimeters?

Japan officially adopted the metric system during the Meiji era as the country modernized and expanded international trade. Over time, metric measurements became the standard throughout Japanese commerce and everyday life.

Today, Japan measures length primarily in millimeters, centimeters, and meters, while weight is commonly expressed in grams and kilograms.

That includes the Japanese koi industry.

Since Nishikigoi — the ornamental carp we call koi — were developed in Japan, many of the traditions and terminology used by koi breeders around the world originated there.

Japanese breeders therefore typically describe the length of a koi in centimeters (cm).

In the United States, however, most people naturally visualize fish size in inches.

Fortunately, converting between the two is easy.

Converting Centimeters to Inches

The exact conversion is:
1 inch = 2.54 centimeters
To convert centimeters to inches:
Centimeters ÷ 2.54 = Inches
For example:
50 cm ÷ 2.54 = 19.7 inches
So a Japanese koi listed as 50 cm is approximately 19.7 inches long.

Quick Koi Size Conversion Chart

Centimeters	Approx. Inches
10 cm	3.9 in
15 cm	5.9 in
20 cm	7.9 in
25 cm	9.8 in
30 cm	11.8 in
35 cm	13.8 in
40 cm	15.7 in
45 cm	17.7 in
50 cm	19.7 in
55 cm	21.7 in
60 cm	23.6 in
65 cm	25.6 in
70 cm	27.6 in
75 cm	29.5 in
80 cm	31.5 in
90 cm	35.4 in
100 cm	39.4 in

A simple shortcut is to remember that about 25 cm is about 10 inches, 50 cm is about 20 inches, and 75 cm is about 30 inches.

What About Kilograms and Pounds?

Japan also uses kilograms instead of pounds.
The conversion is:
1 kilogram = approximately 2.205 pounds
To convert kilograms to pounds:
Kilograms × 2.205 = Pounds
For example:
5 kg × 2.205 = 11.0 lbs

Quick KG to Pounds Conversion Chart

Kilograms	Approx. Pounds
0.5 kg	1.1 lbs
1 kg	2.2 lbs
2 kg	4.4 lbs
3 kg	6.6 lbs
4 kg	8.8 lbs
5 kg	11.0 lbs
6 kg	13.2 lbs
7 kg	15.4 lbs
8 kg	17.6 lbs
9 kg	19.8 lbs
10 kg	22.0 lbs
15 kg	33.1 lbs
20 kg	44.1 lbs

 

An easy shortcut:
Double the kilograms and add about 10%.
So if something weighs 10 kg:
10 kg × 2 = 20 lbs
Add roughly 10% = about 22 lbs

Why Koi Keepers Should Learn the Metric System

If you become serious about Japanese koi, you'll see metric measurements everywhere.

Japanese breeder listings, auction information, certificates and growth records commonly describe koi using centimeters. Metric measurements also appear frequently when discussing water treatments, medications, pond volume, shipping, and other aspects of koi keeping.

Understanding metric measurements allows you to compare koi more easily without constantly wondering what the numbers mean.

For example, if a breeder says:
“This Sansai Showa is 68 cm.”

An experienced koi keeper can quickly recognize that the fish is roughly:
26.8 inches

That immediately gives you a much better idea of the koi's size.

A Little Koi History

Koi are domesticated ornamental varieties of the common carp, Cyprinus carpio.

Although carp have been raised as food fish for centuries, the colorful ornamental koi we recognize today became especially associated with farmers in the Niigata region of Japan.

During the 1800s, farmers selectively bred carp that displayed unusual colors and patterns. Over generations, this selective breeding helped produce the foundation for many of today's famous koi varieties.

These ornamental carp became known as Nishikigoi, often translated as “brocaded carp.”
Eventually, koi keeping spread beyond Japan.

Today, koi are raised and enjoyed throughout the United States and around the world, but Japan remains enormously influential in the hobby. Many famous bloodlines, breeders, terminology, judging standards, and traditions have Japanese origins.

That's why American koi enthusiasts frequently encounter a mixture of two measurement systems:

🇯🇵Japan: centimeters and kilograms
🇺🇸United States: inches and pounds

Learning both is simply part of learning the language of koi.

The Koi Boyz Rule of Thumb

You don't need to pull out a calculator every time you see a Japanese koi listing.

Just remember:
CM → Inches: Divide by 2.54
KG → Pounds: Multiply by 2.205

And keep these three koi sizes in your head:
25 cm ≈ 10 inches
50 cm ≈ 20 inches
75 cm ≈ 30 inches

Once you've been around koi long enough, you'll probably find yourself doing something funny:

Someone will tell you their koi is 24 inches long…
…and you'll automatically start thinking:

“So, about 61 cm.”

Welcome to the world of Japanese koi!
— Koi Boyz

Passionate about koi. Dedicated to helping the hobby grow.