[5] After leaving school, Chalmers secured a place at the University of Bath, where he would train under Dan Cossins.

[6] In 2015, Chalmers made his first junior appearance for Great Britain, competing in the under-20 4 × 400 m event at the Loughborough International.

[8] In 2016, Chalmers competed at the U20 World Championships in Bydgoszcz,[9] 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.

[11] 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.

[4] Ahead of the 2017 Island Games, Chalmers announced he was pulling out in order to focus on making selection for Great Britain at the 2017 World Championships.