The Two Critical Measurements: Inside Diameter vs Circumference

International ring sizing standards (ISO 8653) are strictly based on the inner diameter of the ring band in millimeters. Even a fractional discrepancy of 0.4 mm represents an entire ring size (e.g., US Size 6 is 16.5 mm, while US Size 7 is 17.3 mm).

Step-by-Step: Capturing the Perfect Calibration Photo

Select a Rigid Reference: Place a standard bank card (credit card, debit card, or driver's license) flat on a desk. Under international standard ISO/IEC 7810 ID-1, all cards are exactly 85.60 mm wide.
Position the Ring: Place an existing, well-fitting ring flat directly beside the card on the same level surface.
Shoot Directly Overhead (0° Tilt): Hold your phone camera directly parallel to the table. Avoid capturing at an angle, as perspective foreshortening will distort the circular inner opening into an ellipse.
Upload to ImageMeasure AI: Upload your photo to ImageMeasurementAI.online.
Calibrate Scale: Select the Scale Calibration tool (S), trace the top edge of the credit card, and enter 85.6 mm.
Measure Inner Diameter: Switch to the Circle Tool (C) or Distance Line (L). Click the inside edges of the band to read the internal diameter in millimeters.

International Ring Size Conversion Table

Inside Diameter (mm)	Inside Circumference (mm)	US & Canada	UK & Australia	EU & France	Japan
14.9 mm	46.8 mm	Size 4	H	47	7
15.7 mm	49.3 mm	Size 5	J 1/2	49	9
16.5 mm	51.9 mm	Size 6	M	52	11
17.3 mm	54.4 mm	Size 7	O	54	14
18.1 mm	57.0 mm	Size 8	Q	57	16
19.0 mm	59.5 mm	Size 9	S	60	18
19.8 mm	62.1 mm	Size 10	T 1/2	62	20
20.6 mm	64.6 mm	Size 11	V 1/2	65	23

Measuring Without a Physical Ring: The Actual Size Screen Ruler

If you don't have a photo of a ring, you can also wrap a thin strip of paper around your finger, mark where it overlaps, and measure that length on our Actual Size Screen Ruler to get your exact circumference in millimeters.