Take a soft tape measure, now wrap the tape measure around your finger
Ring sizing advice — MAIRI . CO
TURNADO Loose Ring snaffle 16 mm – Stainless steel, size 135 mm (5 1/4″) | Herm. Sprenger USA
Amazon.com : Shuokecrafts 5pcs Top-Binding Ring Binder Mechanism, 2 D-Rings, 26mm Base Width,133mm= 5 .24 inch Sheet Size, 8.5” Edge of a Letter Size Page , No Boosters (25mm 5pcs) : Office Products
Tiffany & Co. Sterling Silver Narrow Atlas Collection Ring Size 5 1/4 - A | eBay
Ring Size Chart
Ring Sizing Kit Plastic Belt Ring Sizer size 1 to Size 17 - Etsy