Inner diameter is 18.3 mm (about 8 1/4 US size).
Isn't it your size? Find your size on the link below:
US Size 5 (UK size J 1/2)
Inner Diameter = 15.70 mm; 0.618 in
Inner Circumference = 49.32 mm; 1.942 in
US Size 5.5 (UK size L)
Inner Diameter = 16.10 mm; 0.634 in
Inner Circumference = 50.58 mm; 1.991 in
US Size 6 (UK size M)
Inner Diameter = 16.51 mm; 0.650 in
Inner Circumference = 51.87; 2.042 in
US Size 6.5 (UK size N)
Inner Diameter = 16.92 mm; 0.666 in
Inner Circumference = 53.16 mm; 2.093 in
US Size 7 (UK size O)
Inner Diameter = 17.35 mm; 0.683 in
Inner Circumference = 54.51 mm; 2.146 in
US Size 7.5 (UK size P)
Inner Diameter = 17.75 mm; 0.699 in
Inner Circumference = 55.76 mm; 2.195 in
US Size 8 1/4 (in this page)
Inner Diameter = 18.30 mm; 0.720 in
Inner Circumference = 57.49 mm; 2.263 in
US Size 8.5 (UK size Q 3/4)
Inner Diameter = 18.53 mm; 0.729 in
Inner Circumference = 58.21 mm; 2.292 in
US Size 9 (UK size R 3/4)
Inner Diameter = 18.89 mm; 0.744 in
Inner Circumference = 59.34 mm; 2.336 in
US Size 9.5 (UK size S 3/4)
Inner Diameter = 19.41 mm; 0.764 in
Inner Circumference = 60.98 mm; 2.401in
US Size 10 (UK size T 1/2)
Inner Diameter = 19.84 mm; 0.781 in
Inner Circumference = 62.33 mm; 2.454 in
US Size 10.5 (UK size U 1/2)
Inner Diameter = 20.20 mm; 0.795 in
Inner Circumference = 63.46 mm; 2.498 in
US Size 11 (UK size V 1/2)
Inner Diameter = 20.68 mm; 0.814 in
Inner Circumference = 64.97 mm; 2.558 in
US Size 11.5 (UK size W 3/4)
Inner Diameter = 21.08 mm; 0.830 in
Inner Circumference = 66.22 mm; 2.607 in
US Size 12 (UK size Y)
Inner Diameter = 21.49 mm; 0.846 in
Inner Circumference = 67.51 mm; 2.658 in
US Size 12.5 (UK size Z)
Inner Diameter = 21.89 mm; 0.862 in
Inner Circumference = 68.77 mm; 2.707 in
If you need an other inner diameter, send me a message and I adjust it for you, thanks.