In 2016, Chalmers competed at the U20 World Championships in Bydgoszcz, where he set a then personal best of 46.51 seconds in the semifinals; he narrowly missed out on the finals, eventually finishing in 9th. Representing Bath in the 400 m final at the 2017 BUCS Championships, Chalmers won his fourth consecutive BUCS 400 m title. His winning time of 45.71 broke the seventeen-year-old championship record, and set a new Guernsey record. In June later that year, Chalmers improved his own Guernsey record time, setting a new personal best of 45.64 as he won the 400 m final at the U23 England Athletics Championships in Bedford.