Size Classes (Bu) & How Koi Grow

Shows sort fish by length classes called bu; growth itself depends on lineage, water and time — this page keeps the two apart.

At Japanese koi shows, entries are sorted into size classes called bu, running from small young fish to the largest classes above 80 cm. The class decides whom a fish competes against, so the same fish can be a strong entry in one year and a small entry in its new class the next.

Growth in nishikigoi is real but never guaranteed. Fish grow fastest when young, continue more slowly for years, and can pass a metre in exceptional cases — the 2026 All Japan champion measured 100 cm at six years old. Lineage sets the potential; conditions decide how much of it is reached.

This guide states size facts and show classes only. It does not promise growth rates for individual fish or prescribe feeding to force growth — those are husbandry prescriptions, outside this site's scope.

Sources: Wikipedia (CC BY-SA — summary in our own words, credit only); All Japan Nishikigoi Promotion Association official; Ministry of Agriculture, Forestry and Fisheries (MAFF) and Finance Ministry trade statistics as published; Japanese Agricultural Standard (JAS) for nishikigoi variety names and definitions (2022); Niigata Prefecture, Ojiya City, Nagaoka City (Yamakoshi) and Nishikigoi no Sato official; Japan National Tourism Organization (JNTO) visitor statistics; USDA APHIS and Australian DAFF official import guidance. Checked 6 November 2026.