|804 by topic|
|Ab urbe condita||1557|
|Balinese saka calendar||725–726|
|Chinese calendar||癸未年 (Water Goat)|
3500 or 3440
— to —
甲申年 (Wood Monkey)
3501 or 3441
|- Vikram Samvat||860–861|
|- Shaka Samvat||725–726|
|- Kali Yuga||3904–3905|
|Japanese calendar||Enryaku 23|
|Minguo calendar||1108 before ROC|
|Seleucid era||1115/1116 AG|
|Thai solar calendar||1346–1347|
930 or 549 or −223
— to —
931 or 550 or −222
Year 804 (DCCCIV) was a leap year starting on Monday (link will display the full calendar) of the Julian calendar.
- Battle of Krasos: Emperor Nikephoros I refuses to pay the tribute imposed by Caliph Harun al-Rashid of the Abbasid Caliphate. A Muslim-Arab expeditionary force invades Asia Minor. During a surprise attack, Nikephoros suffers a major defeat against the Saracens at Krasos in Phrygia. According to Arabian sources, the Byzantines lose 40,700 men and 4,000 pack animals, while Nikephoros himself is almost killed, but saved by the bravery of his officers.
- Abbasid caliph Harun al-Rashid marries Abbasa, the daughter of Abbasid prince and official Sulayman.
- Summer – Emperor Charlemagne finishes the conquest of Saxony. The Carolingian administration in the north is restored and the diocese of Bremen is re-established. Venice, torn by infighting, switches allegiance from Constantinople to King Pepin of Italy, son of Charlemagne.
- Obelerio degli Antenori becomes the ninth doge of Venice, after his predecessor Giovanni Galbaio flees to Mantua, where he is killed.
- The Gymnasium Carolinum in Osnabrück is founded by Charlemagne (the oldest school in Germany).
- Kūkai, Japanese Buddhist monk, travels in a government-sponsored expedition to China, in order to learn more about the Mahavairocana Sutra. He brings back texts of Shingon (Esoteric Buddhism).
- Priest Saichō, patriarch of Tendai Buddhism, visits China and reportedly brings back tea seeds (or 805).
- The Inscription of Sukabumi from Eastern Java marks the beginning of the Javanese language.
- Ludger, Frisian missionary, becomes the first bishop of Münster, and builds a monastery there.
- Bayazid Bastami, Persian Sufi (d. 874)
- Fujiwara no Yoshifusa, Japanese regent (d. 872)
- May 19 – Alcuin, bishop and advisor to Charlemagne
- October 1 – Richbod, archbishop of Trier
- Saint Abundantia, Christian saint
- Giovanni Galbaio, doge of Venice (approximate date)
- Ibrahim al-Mawsili, musician and singer (b. 742)
- Lu Yu, Chinese author of The Classic of Tea (b. 733)
- Ragnar Lothbrok Legendary Norse Viking hero and Scandinavian King.
- ^ Bosworth 1989, p. 248; Mango & Scott 1997, p. 660
- ^ Treadgold 1988, p. 135
- ^ Nicolle 2014, p. 83.
- ^ The building of the Gymnasium Carolinum, Osnabrück Archived May 19, 2006, at the Wayback Machine Accessed October 2008
- Bosworth, C. E., ed. (1989). The History of al-Ṭabarī, Volume XXX: The ʿAbbāsid Caliphate in Equilibrium: The Caliphates of Mūsā al-Hādī and Hārūn al-Rashīd, A.D. 785–809/A.H. 169–192. SUNY Series in Near Eastern Studies. Albany, New York: State University of New York Press. ISBN 978-0-88706-564-4.
- Mango, Cyril; Scott, Roger (1997). The Chronicle of Theophanes Confessor. Byzantine and Near Eastern History, AD 284–813. Oxford University Press. ISBN 0-19-822568-7.
- Nicolle, David (2014). The Conquest of Saxony AD 782–785. ISBN 978-1-78200-825-5.
- Treadgold, Warren (1988). The Byzantine Revival, 780–842. Stanford, California: Stanford University Press. ISBN 978-0-8047-1462-4.