Tie a piece of string around your finger, then measure the length (mm)

Ring Sizes
Ring Size	J	K	L	M	N	O	P	Q	R	S	T	U	V	W	X	Y	Z
Size in mm	48.75	50	51.25	52.5	53.75	55	56.25	57.5	58.75	60	61.25	62.5	63.75	65	66.25	67.5	68.75