Our sizing is the Australian Standard with letters.
This is measured by the inside diameter of the ring.
J(15.6mm) - K(16mm) - L(16.4mm) - M(16.8mm) - N(17.2mm) - O(17.6mm) - P(18mm) - Q(18.4mm) - R(18.8mm) - S(19.2mm) - T(19.5mm) - U(19.8mm)
V(20.2mm) - W(20.7mm) - X(21mm) - Y(21.4mm) - Z(21.8mm) - Z+1(22.2mm)
*1/2 sizes are in between the size listed and the size above.