Tectonic

Option 1: I don't have a printer

1. Cut a thin strip of paper 10-12cm long.
2. Wrap the strip of paper tightly around your desired finger.
3. Using a pen or pencil, mark the point where the paper overlaps itself.
4. Lay the strip of paper on a flat surface and use a ruler to measure the distance from the end of the strip to the marking you made in millimeters. This will give you the circumference of your finger.
5. Using the below chart, convert your measurement into standard US sizing.
6. If you are in between sizes, it’s best to round up rather than down.

Option 2: I have a printer 

1. Print our ring sizer PDF (here) on standard 8.5” x 11” paper at 100% scale (ensure scaling is turned off in your print dialog box).
2. Measure the grey section — it should be 3.5 inches from end to end.
3. Cut out the sizing tool (the top section that looks like a ruler), and wrap the piece of paper tightly around your desired finger. The number that aligns closest with the ‘Read Size Here’ line will be your size.
4. Alternatively, if you have a ring that fits you well (or a ring you know fits the person you are gifting to), place the ring on top of each sizing circle until you find the matching size. The matching circle should fit perfectly inside the ring.
5. If you are in between sizes, it’s best to round up rather than down.