| Centuries: | 4th century · 5th century · 6th century |
| Decades: | 390s 400s 410s 420s 430s 440s 450s |
| Years: | 418 419 420 421 422 423 424 |
| 421 by topic | |
| Politics | |
| State leaders – Sovereign states | |
| Birth and death categories | |
| Births – Deaths | |
| Establishment and disestablishment categories | |
| Establishments – Disestablishments | |
| Gregorian calendar | 421 CDXXI |
| Ab urbe condita | 1174 |
| Armenian calendar | N/A |
| Bahá'í calendar | -1423 – -1422 |
| Bengali calendar | -172 |
| Berber calendar | 1371 |
| Buddhist calendar | 965 |
| Burmese calendar | -217 |
| Byzantine calendar | 5929 – 5930 |
| Chinese calendar | 庚申年十一月十二日 (3057/3117-11-12) — to —
辛酉年十一月廿二日(3058/3118-11-22) |
| Coptic calendar | 137 – 138 |
| Ethiopian calendar | 413 – 414 |
| Hebrew calendar | 4181 – 4182 |
| Hindu calendars | |
| - Bikram Samwat | 477 – 478 |
| - Shaka Samvat | 343 – 344 |
| - Kali Yuga | 3522 – 3523 |
| Holocene calendar | 10421 |
| Iranian calendar | 201 BP – 200 BP |
| Islamic calendar | 207 BH – 206 BH |
| Japanese calendar | |
| Korean calendar | 2754 |
| Thai solar calendar | 964 |