The singer is said to have met the Hollister model after a friend of his bandmate Harry Styles set them up on a date, according to The Sun.
"Louis can't believe his luck," a source said. "He is in awe of how fit she is. They went to see The Inbetweeners Movie together the other day and they've been out for a few secret dates. He's getting stuck in.
"She is a couple of years older than him but the lads in the band think she's the female version of Louis. She is gobby, loud and confident but pretty laid back at the same time.
"She isn't afraid to bend the rules either - just like Louis. It looks like she could be serious girlfriend material."
Calder is currently studying politics and sociology at a Manchester University.
One Direction are currently on course for their first number one single this Sunday with 'What Makes You Beautiful'.
> One Direction: 'Cheeky tweets make us smile' - video
Watch the music video for One Direction's 'What Makes You Beautiful' below: