Jewelry is one of the places where millimeters and inches meet every day. Beads, chains and gemstones are almost always measured in millimeters, while US ring sizes and many bracelet and necklace lengths are given in inches or number sizes. This guide converts ring sizes to exact inside diameters and bead sizes to inches, and shows how to measure accurately at home.
US Ring Sizes in MM and Inches
A ring size describes the inside diameter of the ring. US and Canadian sizes follow a simple pattern: each whole size adds about 0.81 mm to the diameter (each half size about 0.41 mm).
| US ring size | Inside diameter (mm) | Inside diameter (in) | Inside circumference (mm) |
|---|---|---|---|
| 4 | 14.9 mm | 0.586″ | 46.8 mm |
| 4.5 | 15.3 mm | 0.602″ | 48.0 mm |
| 5 | 15.7 mm | 0.618″ | 49.3 mm |
| 5.5 | 16.1 mm | 0.634″ | 50.6 mm |
| 6 | 16.5 mm | 0.650″ | 51.9 mm |
| 6.5 | 16.9 mm | 0.666″ | 53.1 mm |
| 7 | 17.3 mm | 0.682″ | 54.4 mm |
| 7.5 | 17.7 mm | 0.698″ | 55.7 mm |
| 8 | 18.1 mm | 0.714″ | 57.0 mm |
| 8.5 | 18.5 mm | 0.730″ | 58.2 mm |
| 9 | 18.9 mm | 0.746″ | 59.5 mm |
| 9.5 | 19.4 mm | 0.762″ | 60.8 mm |
| 10 | 19.8 mm | 0.778″ | 62.1 mm |
| 10.5 | 20.2 mm | 0.794″ | 63.3 mm |
| 11 | 20.6 mm | 0.810″ | 64.6 mm |
| 11.5 | 21.0 mm | 0.826″ | 65.9 mm |
| 12 | 21.4 mm | 0.842″ | 67.2 mm |
| 12.5 | 21.8 mm | 0.858″ | 68.5 mm |
| 13 | 22.2 mm | 0.874″ | 69.7 mm |
Values are calculated from the standard relationship diameter (mm) ≈ 11.63 + 0.8128 × US size. Individual jewelers’ mandrels can differ slightly, so if you are between sizes, choose the larger one — especially for wide bands, which feel tighter.
How to Measure Your Ring Size at Home
- Using a ring that fits: measure the inside diameter across the middle with a ruler or digital calipers, then find the closest diameter in the table.
- Using string or paper: wrap a thin strip around the base of your finger, mark where it overlaps, and measure the length in millimeters. That is the circumference — compare it with the last column.
- Measure at the end of the day, when fingers are slightly larger, and make sure the ring will still slide over your knuckle.
Bead Sizes in Inches
Beads are sized by their diameter in millimeters. The last column shows roughly how many beads fit on a 16-inch (406.4 mm) strand — the standard strand length for many gemstone beads — ignoring gaps and knots.
| Bead size | Inches | Beads per 16″ strand |
|---|---|---|
| 2 mm | 0.079″ | about 203 |
| 3 mm | 0.118″ | about 135 |
| 4 mm | 0.157″ | about 102 |
| 6 mm | 0.236″ | about 68 |
| 8 mm | 0.315″ | about 51 |
| 10 mm | 0.394″ | about 41 |
| 12 mm | 0.472″ | about 34 |
| 14 mm | 0.551″ | about 29 |
| 16 mm | 0.630″ | about 25 |
| 20 mm | 0.787″ | about 20 |
For quick reference: a 6 mm bead is just under 1/4 inch, and a 12 mm bead is just under 1/2 inch.
Chain and Bracelet Lengths
Necklace and bracelet lengths are usually given in inches in the US and centimeters elsewhere. Convert with inches × 2.54 = cm: a 16″ necklace is 40.6 cm, 18″ is 45.7 cm, 20″ is 50.8 cm, and a 7″ bracelet is 17.8 cm. Chain thickness, however, is almost always given in millimeters (for example a 2 mm chain is 0.079″).
Frequently Asked Questions
What is a size 7 ring in mm?
A US size 7 ring has an inside diameter of about 17.3 mm (0.682″) and an inside circumference of about 54.4 mm.
How big is an 8 mm bead in inches?
8 mm is 0.315 inches — just over 5/16″.
How many 6 mm beads are on a 16 inch strand?
About 68, since 406.4 mm ÷ 6 mm ≈ 67.7.
Related: Small metric sizes in inches · MM to Inches chart (printable) · How to read a tape measure