Case 1
- Inner diameter
- 17.35mm
7.04
Open with these valuesRing Size Converter
7.04
Result: 7.04Measure the inside of a ring that already fits, across its widest point, in millimetres. That diameter times π is the inner circumference, which is the EU/ISO size outright; the US size is a linear scale on the same circumference. A 17.35 mm ring is EU 54.5 and US 7.
The numbers at a glance
| Inner diameter (mm) | Result |
|---|---|
| 14.00 | 2.92 |
| 16.00 | 5.38 |
| 17.35Your value | 7.04 |
| 18.00 | 7.84 |
| 20.00 | 10.31 |
| 22.00 | 12.77 |
Worked examples
Case 2
- Inner diameter
- 15.7mm
5.01
Open with these valuesCase 3
- Inner diameter
- 19.41mm
9.58
Open with these valuesHow it's calculated
US size = (diameter × π − 36.537) ÷ 2.551
- StepTake a ring that already fits the right finger.
- StepMeasure its inside across the widest point, in millimetres.
- ResultRead the US size; the EU/ISO size is the circumference beside it.
Reference table
| Inner diameter | EU/ISO (circumference) | US size |
|---|---|---|
| 15.7 | 49.32 mm | 5.01 |
| 16.51 | 51.87 mm | 6.01 |
| 17.35 | 54.51 mm | 7.04 |
| 18.19 | 57.15 mm | 8.08 |
| 19.41 | 60.98 mm | 9.58 |
Questions
How do I measure my ring size at home?
Take a ring that already fits the right finger and measure the inside diameter across its widest point with a ruler or calliper, in millimetres. Measuring an existing ring is far more reliable than wrapping string around your finger.
What does the EU or ISO ring size mean?
Under ISO 8653 the ring size is the inner circumference of the band in millimetres. A ring with a 54.5 mm inner circumference is EU/ISO size 54.5, which makes it the most direct of the common scales.
What is a US size 7 in EU?
US 7 corresponds to an inner circumference of about 54.5 mm, so US 7 is roughly EU 54.5. The middle column of the table above gives that circumference for every diameter listed.
Why is there no UK size here?
The British and Australian scale uses letters, not numbers, so it cannot be the numeric answer of a calculator. As a guide, one letter step is 1.25 mm of circumference and size C sits at 40 mm.
What is the difference between diameter and circumference?
The diameter is the straight line across the inside of the ring; the circumference is the distance all the way around it. They are linked by circumference = diameter × π, so 17.35 mm across gives about 54.5 mm around.
Is the converted size exact?
It is close, not exact. Sizing scales differ slightly between jewellers and countries, and band width and finger temperature both affect fit. Use it as a starting point and confirm with a jeweller before ordering something that cannot be resized.
Sources and last check
Information, not professional advice.