| Next prime | 643 |
| Previous prime | 631 |
| Octal | (1201)8 |
| Hexadecimal | (281)16 |
| Prime gap | 2 |
| 29th Twin prime (the small number) | The big number is 643 . |
| 24th Prime triplet (the smallest number) | The others are 643 and 647 . |
| 57th Sexy prime (the small number) | The big number is 647 . |
| 55th Prime of the Form 4n+1 | 42 + 252 |
| 25th Euler prime number | |
| 28th Sophie Germain prime |