Ring Size Conversion Table
| US | UK | EU (ISO) | Inside diameter (mm) |
|---|---|---|---|
| 3 | F | 44 | 14.0 |
| 4 | H½ | 46 | 14.9 |
| 5 | J½ | 49 | 15.7 |
| 6 | M | 52 | 16.5 |
| 7 | O | 54 | 17.3 |
| 8 | Q | 57 | 18.1 |
| 9 | S | 59 | 19.0 |
| 10 | U | 62 | 19.8 |
| 11 | W | 64 | 20.6 |
| 12 | Y | 67 | 21.4 |
| 13 | Z+1 | 69 | 22.2 |
Ring sizing is four systems for one millimeter measurement: a US 7 is a UK O is an EU 54 is a ring with a 17.3 mm inside diameter - and like shoe sizes, only the millimeter measures the thing. The table below maps US 3 through 13 across all four systems, and the converter turns any one of them into the other three.
Bottom line: a 17.3 mm inside diameter (US 7) is the most-bought women's size, and the safest surprise-gift strategy is to borrow a ring she already wears on the target finger and measure its inner edge with a ruler marked in millimeters - everything else on this page exists to translate that one number.
The honest part: fingers change - knuckles swell with heat, salt and age, fingers shrink in cold and in the morning, and the dominant hand runs about a quarter size larger. Ring bands wider than 6 mm feel tighter than their number (go up a quarter), and a ring that spins but cannot pass the knuckle is the classic wrong-size tragedy: size to the knuckle and accept a little spin at the base.
How to use
- Measure the inside diameter of a ring that already fits the right finger, in millimeters, or wrap a strip of paper snug (not tight) around the finger's base and measure its length.
- Find your diameter in the table - or type your US size, UK letter or EU number into the converter for the other three.
- Between sizes, size up for wide bands (6 mm+) and knuckle issues, down for thin bands and cold-climate daily wear.
Frequently asked questions
How do I find my ring size without a ring sizer?
Two honest methods: borrow a well-fitting ring and measure its inside diameter across the widest part in millimeters (most accurate, and the method this table is built around), or wrap a thin strip of paper around the finger base, mark the overlap, and measure the strip length in millimeters - that length is the circumference, and dividing it by 3.14 gives the inside diameter. Do it at the end of the day, at room temperature, twice on different days; fingers drift a quarter size between morning and evening.
Why does the UK use letters and the US uses numbers?
History, twice over: the UK letters descend from Victorian jewellers' stick gauges stepped in whole letters (about 0.4 mm of diameter per step), while the US numeric scale steps about 0.4 mm too but counts from a different zero, with half sizes splitting the difference. The EU (ISO 8653) simply states the inside circumference in millimeters - the most literal system - and the millimeter diameter is the measurement all of them are approximating. When a jeweller says 'size between M and N', that is a 0.4 mm decision.
Should I size up or down between sizes?
Up if the band is wide (6 mm or more contacts the finger differently and feels tighter than its number), if your knuckle is larger than the finger base (the ring must pass the knuckle - size for it and accept spin), or if the ring will be worn in hot climates. Down for thin bands under 2 mm, for cold-hand daily wear, and for very heavy or top-heavy rings (a big stone flips and spins on a loose band). Resizing down is easier than up on most bands, so the jeweller's default is also down.
Do rings for different fingers need different sizes?
Yes - the middle finger is usually the largest, the pinky the smallest, and the difference between adjacent fingers is commonly a full size. A ring sized on the index will not fit the ring finger even on the same hand, and the dominant hand runs about a quarter size larger than the non-dominant. Measure the exact finger, on the exact hand, and note that knuckle-to-base ratios differ finger to finger - which is why borrowed-ring sleuthing works best when the borrowed ring comes from the same finger.