Constantine the Great (; ; 27 February 272 AD – 22 May 337 AD), also known as Constantine I or Saint Constantine (in the Orthodox Church as Saint Constantine the Great, Equal-to-the-Apostles), was a Roman Emperor of Illyrian-Greek origin from 306 to 337 AD. He was the son of Flavius Valerius Constantius, a Roman Army officer, and his consort Helena. His father became "Caesar", the deputy emperor in the west, in 293 AD. Constantine was sent east, where he rose through the ranks to become a military tribune under Emperors Diocletian and Galerius. In 305, Constantius raised himself to the rank of "Augustus", senior western emperor, and Constantine was recalled west to campaign under his father in Britannia (Britain). Constantine was acclaimed as emperor by the army at Eboracum (modern-day York) after his father's death in 306 AD, and he emerged victorious in a series of civil wars against Emperors Maxentius and Licinius to become sole ruler of both west and east by 324 AD. 

As emperor, Constantine enacted many administrative, financial, social, and military reforms to strengthen the empire. The government was restructured, and the civil and military authorities were separated. A new gold coin was introduced to combat inflation known as the solidus. It became the standard for Byzantine and European currencies for more than a thousand years.
When was Constantine declared Emperor?
was acclaimed as emperor by the army at Eboracum (modern-day York) after his father's death in 306 AD