Ring size to inner diameter chart for jewellery CAD
A CAD model needs the inside diameter of the ring in millimetres, not a size name. Size systems differ, so it helps to convert early. The values below are approximate, and your caster's sizing may differ slightly.
Quick chart
| US size | Inner diameter | Circumference (EU size) |
|---|---|---|
| 5 | about 15.7 mm | about 49 |
| 6 | about 16.5 mm | about 52 |
| 7 | about 17.3 mm | about 54 |
| 8 | about 18.1 mm | about 57 |
| 9 | about 19.0 mm | about 60 |
How the numbers connect
EU ring sizes are the inside circumference in millimetres. Divide the circumference by 3.14 to get the inner diameter. For example, EU 54 is about 54 ÷ 3.14 = 17.2 mm.
Tips for accurate sizing
- Ask for the size in millimetres, or measure an existing ring that fits.
- Wide bands feel tighter, so some people go up half a size.
- Keep in mind that fingers change size with temperature and time of day.
- Tell your designer if the ring must be resizable.
Include the size in your design brief, and read about shank styles.
Frequently asked questions
Why does CAD use millimetres, not ring sizes?
Because ring sizes vary between countries, while a millimetre measurement is exact.
Is a UK size the same as a US size?
No. UK sizes use letters and differ from US sizes, so always confirm the millimetre measurement.
Related service
Have a design in mind?
Send a sketch, a photo or an idea on WhatsApp and we will tell you what it involves.
WhatsApp us