Ring Size Calculator
Measure a strip of string or paper around your finger, or the inner diameter of a ring that already fits, and convert that millimeter measurement to US, UK and EU/ISO ring size — or enter a US size you already know to see the equivalent in the other systems.
US 7 · UK N½ · EU 54
Diameter 17.3 mm · Circumference 54.4 mm
Full ring size chart (US / UK / EU / mm)
| US | UK | EU | Diameter (mm) | Circumference (mm) |
|---|---|---|---|---|
| 3 | F | 44 | 14 | 44.2 |
| 4 | H½ | 47 | 14.9 | 46.8 |
| 5 | J½ | 49 | 15.7 | 49.3 |
| 6 | L½ | 52 | 16.5 | 51.9 |
| 7 | N½ | 54 | 17.3 | 54.4 |
| 8 | P½ | 57 | 18.2 | 57 |
| 9 | R½ | 60 | 19 | 59.5 |
| 10 | T½ | 62 | 19.8 | 62.1 |
| 11 | V½ | 65 | 20.6 | 64.6 |
| 12 | X½ | 67 | 21.4 | 67.2 |
| 13 | Z½ | 70 | 22.3 | 69.7 |
How to measure at home
- String/paper strip: wrap a strip of paper or string snugly around the base of your finger, mark where it overlaps, then measure that length in millimeters with a ruler — that's your circumference.
- Ruler on an existing ring: measure the inner diameter (edge to edge, straight across the hole) of a ring that already fits, in millimeters.
- Measure at the end of the day when fingers are largest, and avoid measuring when cold (fingers shrink).
Table follows the ISO 8653:2016 definition (EU size = inner circumference in mm), cross-checked against miniwebtool and ringsize.online. How we calculate →
Ring size systems explained: US, UK, EU/ISO and millimeters
Ring size ultimately comes down to one number: the inner circumference of the ring in millimeters. The international ISO 8653 standard is the most direct about this — an EU/ISO size is its inner circumference in mm, so a US size 9 (59.5 mm circumference) rounds to an EU size 60. The US system uses a numeric scale (3-13, with half sizes) where each step adds roughly 0.4 mm of diameter; the UK/AU system instead uses letters (A-Z, with half-letters), where a US size 6 corresponds to a UK L½.
Because millimeters are the one universal unit underneath every system, measuring your circumference or diameter directly (rather than converting a ring size you already own from memory) is the most reliable way to buy a ring in a country whose sizing system differs from your own.
How to measure your ring size at home (string or ruler method)
String/paper strip method: wrap a strip of paper or a piece of string snugly around the base of the finger you'll wear the ring on, mark where it overlaps, then lay it flat and measure the length in millimeters with a ruler — that length is your circumference. A US size 7 corresponds to a 54.4 mm circumference.
Ruler method: if you already own a ring that fits the intended finger well, measure its inner diameter (straight across the hole, edge to edge, not around the outside) in millimeters. A US size 7 has a 17.3 mm inner diameter.
Measure at the end of the day, when fingers are at their largest (they shrink in cold weather and first thing in the morning), and measure 2-3 times to confirm consistency before ordering.
Why ring sizes aren't perfectly interchangeable between systems
Because US half-sizes (0.4-0.5 mm of diameter) don't line up exactly with UK half-letter steps (about 1.3 mm of circumference per whole letter), a converted size can be off by a hair at the boundary between two sizes — which matters more for a snug fit than for a loose one. When a conversion falls between two rows in the table, sizing up rather than down is the safer default, since a ring that's very slightly loose is easier to resize down than one that's too tight.
Frequently asked questions
What is my ring size in mm?
Measure your finger's circumference (string method) or an existing ring's inner diameter (ruler method) in millimeters, then match it to the chart above. A US size 6 is 51.9 mm circumference / 16.5 mm diameter.
How do I measure my ring size without a ring sizer?
Wrap a strip of paper or string around your finger, mark the overlap point, then measure that length with a ruler in millimeters — that's your circumference, which you can match directly against the mm column of a size chart.
What is a US size 7 ring in UK sizing?
A US size 7 is a UK N½, with a 54.4 mm circumference and a 17.3 mm inner diameter.
What does EU ring size mean?
Under ISO 8653, the EU/ISO ring size number is simply the inner circumference of the ring in millimeters (rounded) — a 54.4 mm circumference is an EU/ISO size 54, no separate conversion table needed.
Should I round up or down between two ring sizes?
Round up. A slightly loose ring can be resized smaller relatively easily and cheaply; a ring that's too tight often can't be resized larger without adding metal, which is more difficult and expensive.
Does ring size change with temperature or time of day?
Yes — fingers are typically largest at the end of the day and in warm weather, and smallest in the morning or in cold temperatures. Measuring in the evening, at normal body temperature, gives the most representative everyday size.
Researched & verified by the Calcuris Data & Research Team. How we build and check our tools →