Option one:
Download a detailed size checking app, just do it step by step and you will know your size.
Place the ring on the marked surface,
The size of the circle on the marked surface should be adjusted to the size of the ring. Should be accurate on the ring line.

second option:
Take your ring to the extent that is good for you and a ruler.
Measure the inside diameter of the ring
If it comes out 1.65 cm (16.5 mm) the measurement is 6,
If it comes out 1.73 cm (17.3 mm) the size is 7.
If it came out 1.82 cm (18.2 mm) the size is 8.