Inner length:

21 yards: 13cm

22 yards: 13.5 cm

23 yards: 14.1 cm

24 yards: 14.7 cm

25 yards: 15.3cm

26 yards: 15.9 cm

27 yards: 16.5 cm

28 yards: 17.3 cm

29 yards: 18cm

30 yards: 18.5cm