| Centuries: | 6th century BC - 5th century BC - 4th century BC |
| Decades: | 450s BC 440s BC 430s BC - 420s BC - 410s BC 400s BC 390s BC |
| Years: | 424 BC 423 BC 422 BC - 421 BC - 420 BC 419 BC 418 BC |
| 421 BC by topic | |
| Politics | |
| State leaders - Sovereign states | |
| Birth and death categories | |
| Births - Deaths | |
| Establishments and disestablishments categories | |
| Establishments - Disestablishments | |
| Gregorian calendar | 421 BC |
| Ab urbe condita | 333 |
| Armenian calendar | N/A |
| Bahá'í calendar | -2264 – -2263 |
| Berber calendar | 530 |
| Buddhist calendar | 124 |
| Burmese calendar | -1058 |
| Chinese calendar | 2216/2276 ([[Sexagenary cycle|]]年) — to —
2217/2277([[Sexagenary cycle|]]年) |
| Coptic calendar | -704 – -703 |
| Ethiopian calendar | -428 – -427 |
| Hebrew calendar | 3340 – 3341 |
| Hindu calendars | |
| - Vikram Samvat | -365 – -364 |
| - Shaka Samvat | N/A |
| - Kali Yuga | 2681 – 2682 |
| Holocene calendar | 9580 |
| Iranian calendar | 1042 BP – 1041 BP |
| Islamic calendar | 1074 BH – 1073 BH |
| Japanese calendar | |
| Korean calendar | 1913 |
| Thai solar calendar | 123 |