| Millennium: | 1st millennium |
|---|---|
| Centuries: | 8th century – 9th century – 10th century |
| Decades: | 790s 800s 810s – 820s – 830s 840s 850s |
| Years: | 818 819 820 – 821 – 822 823 824 |
| 821 by topic | |
| Politics | |
| State leaders – Sovereign states | |
| Birth and death categories | |
| Births – Deaths | |
| Establishment and disestablishment categories | |
| Establishments – Disestablishments | |
| Gregorian calendar | 821 DCCCXXI |
| Ab urbe condita | 1574 |
| Armenian calendar | 270 ԹՎ ՄՀ |
| Bahá'í calendar | -1023 – -1022 |
| Bengali calendar | 228 |
| Berber calendar | 1771 |
| Buddhist calendar | 1365 |
| Burmese calendar | 183 |
| Byzantine calendar | 6329 – 6330 |
| Chinese calendar | 庚子年十一月廿四日 (3457/3517-11-24) — to —
辛丑年十二月初四日(3458/3518-12-4) |
| Coptic calendar | 537 – 538 |
| Ethiopian calendar | 813 – 814 |
| Hebrew calendar | 4581 – 4582 |
| Hindu calendars | |
| - Bikram Samwat | 877 – 878 |
| - Shaka Samvat | 743 – 744 |
| - Kali Yuga | 3922 – 3923 |
| Holocene calendar | 10821 |
| Iranian calendar | 199 – 200 |
| Islamic calendar | 205 – 206 |
| Japanese calendar | |
| Korean calendar | 3154 |
| Thai solar calendar | 1364 |