Option 1 “Ring Sizing Tool” on Amazon

1 Pack US Ring Sizer Measuring Gauge, 1-17 US Rings Size, Plastic Finger Sizing Measure Tool, Reusable, 4.6 out of 5 stars 47 ratings
Amazon’s Choice for “ring measure” $2.99 & FREE Returns 1 PCS, Color Black, US ring soft ruler, measuring range 1-17, standard ring size measurement. Quickly and accurately determine the ring size that suits you, avoiding unnecessary trouble caused by improper size.

Option 2