A koi through the years
The life cycle of koi
From tiny fry to a familiar face at feeding time. Follow one Sanke as it grows.
One journey. Plenty of possibilities.
The ruler shows each illustrated length, from nose to tail. These are strong-growth examples, not averages or promises. Age names follow growing seasons; check the hatch season and current size with the dealer.
How to read the sizes ↓-
About 4–5 weeks
Fry
Fry detail · enlarged2.5 cm illustrated size
View full image ↗- Early fry guide
- 2–2.5 cm
Size shown is an example, not an average.
A tiny, translucent body, large eyes and the first hints of colour.
Find young koi in Scout →Fry are not listed separately. This opens Tosai, our youngest browsing category.
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First growing year
Tosai
35 cm illustrated size
View full image ↗- Observed listing range
- 17.5–42.5 cm
Size shown follows our strong growth example, not the sample median.
A lean youngster with a lot of growing ahead. Colour is still finding its feet.
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Second growing year
Nisai
50 cm illustrated size
View full image ↗- Observed listing range
- 30–62.5 cm
Size shown follows our strong growth example, not the sample median.
The frame lengthens, the shoulders fill out and the pattern becomes easier to read.
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Third growing year
Sansai
63 cm illustrated size
View full image ↗- Observed listing range
- 46–72 cm
Size shown follows our strong growth example, not the sample median.
A young adult with more body and a stronger tail. Look at balance as well as length.
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Fourth growing year
Yonsai
70 cm illustrated size
View full image ↗- Observed listing range
- 57.5–75 cm
Size shown follows our strong growth example, not the sample median. Range based on six listed koi.
A fuller shape gives the familiar pattern more presence. Growth still varies widely.
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Fifth growing year
Gosai
76 cm illustrated size
View full image ↗- Broad pond guide
- 55–80 cm
A strong growth example, not an average or a promised size.
The adult frame carries the pattern with confidence. Length alone never tells the whole story.
Find koi aged five and above →Scout groups ages five and above as Older. Ask the dealer for the exact age.
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Sixth growing year
Rokusai
80 cm illustrated size
View full image ↗- Broad mature guide
- 60–90+ cm
Adds 4 cm in this example. Actual yearly growth may be smaller or stop.
A mature koi with a well-filled frame. Its care and genetics matter more than another birthday.
Find koi aged five and above →Scout groups ages five and above as Older. Ask the dealer for the exact age.
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Seventh growing year
Nanasai
83 cm illustrated size
View full image ↗- Broad mature guide
- 60–90+ cm
Adds 3 cm in this example. Actual yearly growth may be smaller or stop.
Changes become gentler. Enjoy its shape, skin and the way it moves through the pond.
Find koi aged five and above →Scout groups ages five and above as Older. Ask the dealer for the exact age.
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Eighth growing year
Hassai
86 cm illustrated size
View full image ↗- Broad mature guide
- 60–90+ cm
Adds 3 cm in this example. Actual yearly growth may be smaller or stop.
Settled proportions, with smaller changes from season to season. Bigger every year is not a rule.
Find koi aged five and above →Scout groups ages five and above as Older. Ask the dealer for the exact age.
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Ninth growing year
Kyusai
88 cm illustrated size
View full image ↗- Broad mature guide
- 60–90+ cm
Adds 2 cm in this example. Actual yearly growth may be smaller or stop.
A familiar pattern on an established adult. Good condition matters at every size.
Find koi aged five and above →Scout groups ages five and above as Older. Ask the dealer for the exact age.
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Tenth growing year
Jussai
89 cm illustrated size
View full image ↗- Broad mature guide
- 60–90+ cm
Adds 1 cm in this example. Actual yearly growth may be smaller or stop.
Years of watching turn a koi into a familiar companion. The pace of growth is usually slower.
Find koi aged five and above →Scout groups ages five and above as Older. Ask the dealer for the exact age.
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Beyond ten years
Older koi
90 cm illustrated size
View full image ↗- Broad mature guide
- 60–90+ cm
Later maturity at 90 cm in this example. This final step can span several years; some koi grow larger.
Enjoy the koi it has become. Mature koi share a wide size range, whatever their exact age.
Find koi aged five and above →Scout groups ages five and above as Older. Ask the dealer for the exact age.
How to read these sizes
The illustrated lengths follow an example of strong growth, not a database average or a prediction for an individual koi. Breeder records show Sanke growing from 35 cm Tosai to 53 cm Nisai, and another from 54 cm Nisai to 65 cm Sansai. Our later steps illustrate continued growth that gradually slows; their exact yearly increments are chosen for this guide, not measured averages.
The ranges beneath the portraits provide separate buying context. Tosai through Yonsai use the smallest and largest dealer listings in a published sample, with only six Yonsai. Gosai and older use broad pond guidance, so the mature range overlaps across ages. Fry guidance describes early selection, not the entire first year. Genetics, sex, growing season and care all affect size. The ruler ends at 90 cm for comparison, not as a biological limit.
Size references: Cuttlebrook’s fry development ↗ · Dealer listing sample and pond size guidance ↗. Growth references: Marugen’s recorded development ↗ · Sakai on continued growth and genetics ↗.