Age groups are calculated using UK Athletics rulings. Most age groups define the category at the age of the athlete on 31st August within the current competition year. e.g. Bob is 11 on 31st August 2025, Bob is now an U13 for the whole of the 2025 season. ​ For Masters Age Groups, you move into the new category based on your age on the first day of the competition. This calculator will estimate to the nearest month only.