Calvin Howarth was one of the jewels in the Boroughmuir crown in season 2000-2001 and was instrumental in their BT Cellnet Cup final win over Melrose at Murrayfield. He joined Boroughmuir in 1998, having played for the North Harbour provincial team at under-19, under-21 and adult levels as well as the New Zealand Youth Development XV. While with Boroughmuir, he played for Scottish Students and Edinburgh's Inter-District Championship team.
He signed for Edinburgh Rugby, but he was later loaned to Glasgow Rugby before joining permanently.
He has recently completed residence qualification for Scotland. He made his debut for Glasgow against Bridgend in April 2002.