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When did Louis I become emperor?

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Illuminated manuscript portrait of Louis the Pious as a Christian Roman emperor, holding a staff and shield, with Latin text arranged around and over the figure. — Louis I Louis the Pious as a Christian Roman Emperor, portrait from De laudibus sanctae crucis by Rabanus Maurus.

Louis I, also known as Louis the Pious, became coemperor with his father, Charlemagne, in 813. Following his father's death, Louis succeeded him as sole emperor in 814.12