Ring size chart in mm: US ring size to inside diameter and circumference
By HeyGema™ · Updated
In short
A US size 6 ring is 16.5 mm across the inside and 51.9 mm around. Each step of a full size adds about 0.8 mm to the diameter and 2.6 mm to the circumference.
US ring size in mm
Inside diameter and circumference of a ring for each US size, from the US ring-size scale. The last column shows how much of that width a round 1 carat diamond covers.
- 3
- Inside diameter: 14.1 mm
- Inside circumference: 44.2 mm
- Diameter in inches: 0.55 in
- 1 carat round covers: 46%
- 3.5
- Inside diameter: 14.5 mm
- Inside circumference: 45.5 mm
- Diameter in inches: 0.57 in
- 1 carat round covers: 45%
- 4
- Inside diameter: 14.9 mm
- Inside circumference: 46.8 mm
- Diameter in inches: 0.59 in
- 1 carat round covers: 43%
- 4.5
- Inside diameter: 15.3 mm
- Inside circumference: 48.0 mm
- Diameter in inches: 0.60 in
- 1 carat round covers: 42%
- 5
- Inside diameter: 15.7 mm
- Inside circumference: 49.3 mm
- Diameter in inches: 0.62 in
- 1 carat round covers: 41%
- 5.5
- Inside diameter: 16.1 mm
- Inside circumference: 50.6 mm
- Diameter in inches: 0.63 in
- 1 carat round covers: 40%
- 6
- Inside diameter: 16.5 mm
- Inside circumference: 51.9 mm
- Diameter in inches: 0.65 in
- 1 carat round covers: 39%
- 6.5
- Inside diameter: 16.9 mm
- Inside circumference: 53.1 mm
- Diameter in inches: 0.67 in
- 1 carat round covers: 38%
- 7
- Inside diameter: 17.3 mm
- Inside circumference: 54.4 mm
- Diameter in inches: 0.68 in
- 1 carat round covers: 37%
- 7.5
- Inside diameter: 17.7 mm
- Inside circumference: 55.7 mm
- Diameter in inches: 0.70 in
- 1 carat round covers: 36%
- 8
- Inside diameter: 18.1 mm
- Inside circumference: 57.0 mm
- Diameter in inches: 0.71 in
- 1 carat round covers: 36%
- 8.5
- Inside diameter: 18.5 mm
- Inside circumference: 58.2 mm
- Diameter in inches: 0.73 in
- 1 carat round covers: 35%
- 9
- Inside diameter: 18.9 mm
- Inside circumference: 59.5 mm
- Diameter in inches: 0.75 in
- 1 carat round covers: 34%
- 9.5
- Inside diameter: 19.4 mm
- Inside circumference: 60.8 mm
- Diameter in inches: 0.76 in
- 1 carat round covers: 33%
- 10
- Inside diameter: 19.8 mm
- Inside circumference: 62.1 mm
- Diameter in inches: 0.78 in
- 1 carat round covers: 33%
- 11
- Inside diameter: 20.6 mm
- Inside circumference: 64.6 mm
- Diameter in inches: 0.81 in
- 1 carat round covers: 31%
- 12
- Inside diameter: 21.4 mm
- Inside circumference: 67.2 mm
- Diameter in inches: 0.84 in
- 1 carat round covers: 30%
How to measure your ring size at home
- 1
Cut a strip of paper
About 6 mm wide and 15 cm long. A piece of string works too.
- 2
Wrap it around the finger
Use the base of the finger the ring will sit on, when your hands are warm and at their usual size.
- 3
Mark and measure
Mark where the end meets the strip, lay it flat and measure the length in mm. That is your circumference.
- 4
Find the nearest size
Look up the circumference in the table. If you are between sizes, go up when the ring has a wide band, since wide bands fit tighter.
- 5
Or measure a ring that fits
Lay a ring that fits you on a ruler and measure across the widest point of the inside. That is the inside diameter.
Why the stone size matters here
The width of the finger at the ring is what a stone is compared with. A round 1 carat diamond is 6.5 mm across: 43% of a size 4 finger and 34% of a size 9. See what size diamond for each ring size.