Family name Caius (Caio 
, Caius 
, Гай 
) , all over the world, is quite a common cognomen. The cognomen Caius is habitual in Portugal, where it is quite a rare surname, Ukraine, and Angola. In absolute terms, it is commonest in Brazil, Ukraine, and Russia. Predominantly, Caius is the forename as well as the last name.
| VARIANT | COUNTRY |
|---|---|
| Caio | Portugal, Italy, Angola, United States, Brazil |
| Caius | Romania, Italy, Nigeria, United States, Brazil |
| Гай | Kazakhstan, Armenia, Russia, Belarus, Ukraine |
|
john caius English physician (1510–1573), GB (b. 1510) |
|
| antonius caius author (16th century) (b. 1501) |
|
|
| thomas caius Master of University College, Oxford (b. 1505) |
|