Family name Koiwai (Koiwai 
) , in the entire world, is a very rare cognomen. The cognomen Koiwai is habitual in the United States, where it is an extremely rare surname, Brazil, and Canada. Not in comparison with a country's population, it is the most numerous in the United States, Brazil, and India. Very seldom, Koiwai is the forename as well as the last name.
| VARIANT | COUNTRY |
|---|---|
| Koiwai | China, United States, India, Canada, Brazil |
| sadao koiwai 1893 (b. 1893) |
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| eichi karl koiwai judoka (b. 1920) |
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| keiichiro koiwai researcher ORCID 0000-0002-3582-7572 |
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| kotaro koiwai researcher |
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| keiichiro koiwai researcher (ORCID 0000-0002-8890-8229) |
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| frederic koiwai |
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