Ring size by diameter means measuring straight across the inside of a ring. A 16.5 mm inner diameter is US 6, 17.3 mm is US 7 and 18.1 mm is US 8; each whole size adds about 0.8 mm. Find the band your measurement falls into below, in millimetres or inches, or type it into the ring size calculator.
Diameter ranges for every ring size
Each row shows the band of measurements that is closest to that size. If you measured 17.4 mm, it falls in the 17.10–17.50 mm band, so you are a US 7.
| Measured inner diameter | In inches | US size | UK size | EU size |
|---|---|---|---|---|
| 13.90–14.30 mm | 0.547–0.563 | 3 | F | 44 |
| 14.30–14.70 mm | 0.563–0.579 | 3.5 | G | 46 |
| 14.70–15.10 mm | 0.579–0.594 | 4 | H | 47 |
| 15.10–15.50 mm | 0.594–0.610 | 4.5 | I | 48 |
| 15.50–15.90 mm | 0.610–0.626 | 5 | J½ | 49 |
| 15.90–16.30 mm | 0.626–0.642 | 5.5 | K½ | 51 |
| 16.30–16.70 mm | 0.642–0.657 | 6 | L½ | 52 |
| 16.70–17.10 mm | 0.657–0.673 | 6.5 | M½ | 53 |
| 17.10–17.50 mm | 0.673–0.689 | 7 | N½ | 54 |
| 17.50–17.90 mm | 0.689–0.705 | 7.5 | O½ | 56 |
| 17.90–18.30 mm | 0.705–0.720 | 8 | P½ | 57 |
| 18.30–18.75 mm | 0.720–0.738 | 8.5 | Q½ | 58 |
| 18.75–19.20 mm | 0.738–0.756 | 9 | R½ | 60 |
| 19.20–19.60 mm | 0.756–0.772 | 9.5 | S½ | 61 |
| 19.60–20.00 mm | 0.772–0.787 | 10 | T½ | 62 |
| 20.00–20.40 mm | 0.787–0.803 | 10.5 | U½ | 63 |
| 20.40–20.80 mm | 0.803–0.819 | 11 | V½ | 65 |
| 20.80–21.20 mm | 0.819–0.835 | 11.5 | W½ | 66 |
| 21.20–21.60 mm | 0.835–0.850 | 12 | X½ | 67 |
| 21.60–22.00 mm | 0.850–0.866 | 12.5 | Z | 68 |
| 22.00–22.40 mm | 0.866–0.882 | 13 | Z+1 | 70 |
| 22.40–22.80 mm | 0.882–0.898 | 13.5 | Z+2 | 71 |
| 22.80–23.20 mm | 0.898–0.913 | 14 | Z+3 | 72 |
| 23.20–23.60 mm | 0.913–0.929 | 14.5 | Z+4 | 74 |
| 23.60–24.00 mm | 0.929–0.945 | 15 | Z+5 | 75 |
Ring size from diameter in inches
If your ruler or caliper reads in inches, use this table. Values are every 0.02 inch.
| Diameter (in) | Diameter (mm) | Nearest US | Nearest UK | EU |
|---|---|---|---|---|
| 0.56 | 14.2 | 3 | F | 45 |
| 0.58 | 14.7 | 4 | H | 46 |
| 0.60 | 15.2 | 4.5 | I | 48 |
| 0.62 | 15.7 | 5 | J½ | 49 |
| 0.64 | 16.3 | 5.5 | K½ | 51 |
| 0.66 | 16.8 | 6.5 | M½ | 53 |
| 0.68 | 17.3 | 7 | N½ | 54 |
| 0.70 | 17.8 | 7.5 | O½ | 56 |
| 0.72 | 18.3 | 8 | P½ | 57 |
| 0.74 | 18.8 | 9 | R½ | 59 |
| 0.76 | 19.3 | 9.5 | S½ | 61 |
| 0.78 | 19.8 | 10 | T½ | 62 |
| 0.80 | 20.3 | 10.5 | U½ | 64 |
| 0.82 | 20.8 | 11.5 | W½ | 65 |
| 0.84 | 21.3 | 12 | X½ | 67 |
| 0.86 | 21.8 | 12.5 | Z | 69 |
| 0.88 | 22.4 | 13 | Z+1 | 70 |
| 0.90 | 22.9 | 14 | Z+3 | 72 |
| 0.92 | 23.4 | 14.5 | Z+4 | 73 |
| 0.94 | 23.9 | 15 | Z+5 | 75 |
How to measure a ring’s inner diameter
- Choose a ring that fits the finger you are sizing. A plain band is easiest; a ring with a stone can be measured if you keep the ruler clear of the setting.
- Place it on a ruler with mm marks, or open the inside jaws of a caliper inside it.
- Measure the widest distance across the inside, through the centre, not the outside edge.
- Turn the ring 90° and measure again. Use the average of the two readings.
With a ruler you can read to about 0.5 mm, which is a little more than one half size, so use the ruler result as a guide and check it with the on-screen sizer if the reading lands near the edge of a band. A caliper reads to 0.1 mm and can tell quarter sizes apart.
Converting diameter to size by hand
US sizes follow a straight line: US size = (diameter in mm − 11.63) ÷ 0.8128. For example, 18.5 mm gives (18.5 − 11.63) ÷ 0.8128 = 8.45, so US 8.5. To get the EU/ISO size, multiply the diameter by 3.1416: 18.5 × 3.1416 = 58.1, so EU 58. For inches, multiply by 25.4 first: 0.70 in × 25.4 = 17.8 mm, which is US 7.5.
Diameter versus circumference
Diameter is what you get from a ring; circumference is what you get from your finger. They describe the same size: circumference = diameter × π. If you measured around your finger with string or paper, use ring size by circumference instead. Mixing the two is the most common reason people order a ring three sizes too large.
Common diameters
- 15 mm – US 4 to 4.5, a small size for women or a pinky ring.
- 16 mm – US 5 to 5.5, petite women’s sizes.
- 17 mm – US 6.5 to 7, the most common women’s size.
- 18 mm – US 8, a large women’s or small men’s size.
- 19 mm – US 9, a common men’s size.
- 20 mm – US 10 to 10.5, a large men’s size.
- 21–22 mm – US 11.5 to 13, for large hands or thumb rings.
Every size has its own page with all conversions; start at ring size in mm.
Not sure of your ring size? Measure it on your phone screen in 30 seconds.
Open the free ring sizer →Frequently asked questions
What ring size is 18 mm diameter?
18 mm falls in the US 8 band (17.90–18.30 mm). It is UK P½ and EU 57.
What ring size is 20 mm diameter?
20 mm sits exactly between US 10 (19.8 mm, UK T½) and US 10.5 (20.2 mm, UK U½). Choose 10.5 for a wide band or a large knuckle, and 10 for a thin band.
What ring size is 0.7 inch diameter?
0.70 inch is 17.8 mm, which is US 7.5, UK O½, EU 56.
Is ring size measured by inner or outer diameter?
Always the inner diameter, the part that touches your finger. The outer diameter depends on how thick the band is.