St-Calais was a Norman, and a native of Bayeux; he may have been a member of one of its clerical dynasties. His mother's name, Ascelina or Anselma, is given in Durham's records; his father, whose name is unknown, became a monk at the monastery of Saint-Calais in Maine, and may previously have been a knight. Although St-Calais is generally referred to as Saint Calais or St-Calais, the main source of information about his life, the monastic chronicle of Symeon of Durham, does not call him such.
St-Calais studied under Odo, Bishop of Bayeux, the half-brother of the future William I of England, who was then Duke of Normandy. Other bishops educated at Bayeux around this time included Archbishop Thomas of York and Samson, Bishop of Worcester. Symeon of Durham considered St-Calais to be well-educated in classical literature and the scriptures; at some point St-Calais also acquired a knowledge of canon law.