Ring Size Difference in mm (0.8 mm per Size Explained)
Knowing how big one size actually is makes every other sizing decision easier — from ordering online to deciding whether a ring is worth resizing.
Quick answer
One full US ring size = about 0.8 mm more inside diameter, or about 2.5 mm more inside circumference. A half size is 0.4 mm of diameter.
Conversion table
| Inside diameter | Circumference | US | UK |
|---|---|---|---|
| 14.00 mm | 44.0 mm | 3 | F |
| 14.40 mm | 45.2 mm | 3.5 | G |
| 14.80 mm | 46.5 mm | 4 | H |
| 15.20 mm | 47.8 mm | 4.5 | I |
| 15.60 mm | 49.0 mm | 5 | J½ |
| 16.00 mm | 50.3 mm | 5.5 | K½ |
| 16.45 mm | 51.7 mm | 6 | L½ |
| 16.90 mm | 53.1 mm | 6.5 | M½ |
| 17.30 mm | 54.3 mm | 7 | N½ |
| 17.70 mm | 55.6 mm | 7.5 | O½ |
| 18.10 mm | 56.9 mm | 8 | P½ |
| 18.50 mm | 58.1 mm | 8.5 | Q½ |
| 19.00 mm | 59.7 mm | 9 | R½ |
| 19.40 mm | 60.9 mm | 9.5 | S½ |
| 19.80 mm | 62.2 mm | 10 | T½ |
| 20.20 mm | 63.5 mm | 10.5 | U½ |
| 20.60 mm | 64.7 mm | 11 | V½ |
| 21.00 mm | 66.0 mm | 11.5 | W½ |
| 21.40 mm | 67.2 mm | 12 | X½ |
| 21.80 mm | 68.5 mm | 12.5 | Z |
| 22.20 mm | 69.7 mm | 13 | Z+1 |
Step sizes in each system
- US: 1 full size = 0.8 mm diameter ≈ 2.5 mm circumference.
- US: 1 half size = 0.4 mm diameter ≈ 1.3 mm circumference.
- UK: 1 full letter ≈ 0.4 mm diameter, so two letters equal one US size.
- EU (ISO): 1 unit = 1 mm of circumference ≈ 0.32 mm of diameter.
Can you feel half a size?
Most people can. A half size is the difference between a ring that spins freely and one that sits still. Under 0.4 mm the difference is usually only noticeable on wide or heavy bands.
What this means for resizing
Jewellers size most plain bands up or down by two sizes safely, which is about 1.6 mm of diameter. Beyond that the band thins out or a new section of metal has to be added.
Frequently asked questions
How many mm is half a ring size?
About 0.4 mm of inside diameter, or roughly 1.3 mm of inside circumference.
Is 1 mm a full ring size?
Slightly more than one. One full US size is 0.8 mm of diameter, so 1 mm is about one and a quarter sizes.
Keep reading
For an expensive ring, confirm the size with a jeweller before ordering — see the ring size FAQs for resizing advice.