Ring Size by Diameter Calculator
Use this ring size by diameter calculator to find your size at home with just a ruler. Measure the inside diameter of a ring that already fits — or wrap a strip of paper around your finger to get the circumference — enter the millimetres, and the calculator returns your US, UK, EU and Japanese ring sizes.
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Find your ring size from millimetres
Enter values above and press Calculate to see your result.
Formula used
If you measured the inside diameter, the calculator uses it directly. If you measured the circumference, it converts first:
Diameter = Circumference ÷ π
It then maps the diameter to a US size and the other systems:
US size ≈ (Diameter − 11.63) ÷ 0.8128EU size = Circumference (mm)
Worked examples
17.3 mm diameter ≈ US 7, EU 54.
54 mm circumference ÷ π ≈ 17.2 mm diameter ≈ US 7.
15.7 mm diameter ≈ US 5.
How to use this calculator
- Pick whether you measured the inside diameter or the circumference.
- For diameter: lay a well-fitting ring flat and measure straight across the inside in mm.
- For circumference: wrap a thin strip of paper around the finger, mark where it overlaps, and measure its length in mm.
- Enter the value and press Calculate for your sizes.
Inside diameter to ring size
| Diameter (mm) | Circumference (mm) | US | UK | EU |
|---|---|---|---|---|
| 15.7 | 49.3 | 5 | J | 49 |
| 16.5 | 51.9 | 6 | L | 52 |
| 17.3 | 54.4 | 7 | N | 54 |
| 18.1 | 57.0 | 8 | P | 57 |
| 19.0 | 59.5 | 9 | R | 59 |
| 19.8 | 62.1 | 10 | T | 62 |
Measure to 0.1 mm if you can — half a millimetre is roughly half a ring size.
Who should use this calculator
Anyone sizing a ring at home without a jeweller — for a surprise gift, an online order, or to check a ring you already own. The diameter method is the most reliable DIY approach because it measures a physical ring you know fits.
How to measure accurately
- Diameter method: place a ring on a ruler and measure the inside edge to inside edge. This is the most accurate home method.
- Paper/string method: wrap snugly (not tight) around the base of the finger, mark the overlap, and measure the length — that's the circumference.
- Measure two or three times and take the average.
Avoiding common measuring errors
- Don't pull string tight — it should sit comfortably, as a ring would.
- Measure the right finger and hand; dominant hands are often slightly larger.
- Account for the knuckle — if your knuckle is much larger than the base, size between the two.
- Avoid cold fingers, which read small.
Limitations of this calculator
Home measurements have a margin of error, and a fraction of a millimetre shifts the size. Wide bands fit tighter, so consider sizing up. For an engagement ring or other important purchase, confirm with a jeweller's sizer before buying.
Frequently asked questions
What ring size is 17 mm diameter?
About US 6.5–7 (EU 53–54). A 17.3 mm inside diameter is approximately US 7.
How do I measure ring size with a ruler?
Lay a ring that fits flat on a ruler and measure straight across the inside in millimetres — that's the diameter. Enter it here for your size.
Can I measure with string or paper?
Yes. Wrap a thin strip around the finger, mark the overlap, and measure its length — that's the circumference. Choose the circumference option and enter the mm.
How accurate is measuring at home?
Good to within about half a size if you measure carefully and average a few attempts. For costly rings, confirm with a jeweller.
Should I size up for the knuckle?
If your knuckle is noticeably wider than the base of your finger, choose a size between the two so the ring passes the knuckle but doesn't spin.