| Millennium: | 1st millennium |
|---|---|
| Centuries: | |
| Decades: | |
| Years: |
| 821 by topic |
|---|
| Leaders |
| Categories |
| Gregorian calendar | 821 DCCCXXI |
| Ab urbe condita | 1574 |
| Armenian calendar | 270 ԹՎ ՄՀ |
| Assyrian calendar | 5571 |
| Balinese saka calendar | 742–743 |
| Bengali calendar | 228 |
| Berber calendar | 1771 |
| Buddhist calendar | 1365 |
| Burmese calendar | 183 |
| Byzantine calendar | 6329–6330 |
| Chinese calendar | 庚子年 (Metal Rat) 3518 or 3311 — to — 辛丑年 (Metal Ox) 3519 or 3312 |
| Coptic calendar | 537–538 |
| Discordian calendar | 1987 |
| Ethiopian calendar | 813–814 |
| Hebrew calendar | 4581–4582 |
| Hindu calendars | |
| - Vikram Samvat | 877–878 |
| - Shaka Samvat | 742–743 |
| - Kali Yuga | 3921–3922 |
| Holocene calendar | 10821 |
| Iranian calendar | 199–200 |
| Islamic calendar | 205–206 |
| Japanese calendar | Kōnin 12 (弘仁１２年) |
| Javanese calendar | 717–718 |
| Julian calendar | 821 DCCCXXI |
| Korean calendar | 3154 |
| Minguo calendar | 1091 before ROC 民前1091年 |
| Nanakshahi calendar | −647 |
| Seleucid era | 1132/1133 AG |
| Thai solar calendar | 1363–1364 |
| Tibetan calendar | 阳金鼠年 (male Iron-Rat) 947 or 566 or −206 — to — 阴金牛年 (female Iron-Ox) 948 or 567 or −205 |
Year 821 (DCCCXXI) was a common year starting on Tuesday (link will display the full calendar) of the Julian calendar.