Ring sizes look simple until you cross a border: the United States uses numbers, the UK and Australia use letters, and most of Europe uses the inner circumference in millimetres. This chart maps all three to the underlying physical size of the band so you can convert between any of them, or find your size from a measurement.
How to measure your ring size accurately
Getting an accurate measurement matters more than knowing the conversion table. A ring that is a half-size too small is uncomfortable to wear; one that is a full size too large will spin and fall off. Here are the three most reliable home methods:
Method 1 — String or paper strip (recommended for most people): Wrap a narrow strip of paper or a piece of string around the base of the finger you intend to wear the ring on. Mark where it overlaps. Measure that length in millimetres — this is your finger’s circumference. Look up the circumference in the chart to find your EU size (which is essentially the circumference in mm), then convert.
Method 2 — Measuring an existing ring: Place a ring that fits well on a ruler and measure the inside diameter straight across. The diameter in mm gives you the physical size directly, and the tool converts to US, UK, and EU from there.
Method 3 — Jeweler’s sizing mandrel: A ring sizer mandrel is a tapered metal cone marked with sizes. Slide an existing ring down the cone until it stops and read the size at the contact point. Jewelers also carry sets of sizing rings you can try on.
How the conversion works
Every ring size ultimately describes one number: the inner diameter of the band. The other systems are derived from it:
circumference = diameter × π (π ≈ 3.1416)
EU size ≈ circumference (mm), rounded (ISO 8653)
US size ≈ (diameter − 11.63) / 0.8128 + 0.5 (approx.)UK sizes use a letter-plus-fraction system (A, A½, B, B½, …) that does not follow a linear formula as cleanly, so the tool uses a lookup table rather than a formula for UK conversions.
Quick reference: common conversions
| Inner diameter (mm) | Circumference (mm) | US size | UK size | EU size |
|---|---|---|---|---|
| 15.7 | 49.3 | 5 | J½ | 49 |
| 16.5 | 51.8 | 6 | L½ | 52 |
| 17.3 | 54.4 | 7 | N½ | 54 |
| 18.2 | 57.2 | 8½ | Q | 57 |
| 19.0 | 59.7 | 9½ | S | 60 |
For example, an inner diameter of 17.3 mm gives a circumference of about 54.3 mm, matching EU 54, roughly US 7, and around UK N½.
Sizing tips
- Measure at the end of the day, when fingers are at their largest (they swell slightly during the day and in warm weather).
- Cold fingers shrink — avoid measuring first thing in the morning or after being outdoors in winter.
- For wide bands (over 6 mm), go up a half size, because a wide band contacts more of the finger and feels tighter than a narrow ring of the same inner diameter.
- The fit test: the ring should slide onto the finger with mild resistance and require a gentle tug to come off over the knuckle.
- If you are between two sizes, choose the larger one — especially in warm climates or if you tend to have swollen fingers.