Ring Size Converter

How it works

There are three ways in. Finger circumference is the classic string method: wrap string or a paper strip around your finger, measure the length in millimeters (or inches), and type it in. Ring inner diameter is for when you have a ring that already fits — measure across the inside of the band and enter that. Both instantly show the closest US size, the matching Japanese/Korean size and EU (ISO) size, plus the exact diameter and circumference behind the result. If you already know a US size — say from a ring listing — use US size (reverse) to get its millimeter measurements and international equivalents, which is exactly what you need when shopping on Japanese, Korean, or European sites.

How the sizing systems compare

Ring sizes look arbitrary until you see that each system is just a different way of writing the same physical measurement — the ring's inner size in millimeters.

• US & Canada — diameter-based. Each whole size adds 0.032 in (about 0.81 mm) of inner diameter, following diameter (mm) = 11.63 + 0.8128 × size. Rings are commonly sold in half sizes. A US 7 is about 17.3 mm across and 54.4 mm around.
• EU / Continental Europe (ISO 8653) — circumference-based, and the most transparent of all: the size number is the inner circumference in millimeters. EU 52 means 52 mm around, full stop.
• Japan & South Korea — one shared numeric scale (the JIS S 4700 lineage; Korea's standard gauge follows the same system). Size 1 is a 13.0 mm inner diameter, and each size up adds exactly 1/3 mm. That works out to about 1.05 mm of circumference per size — a finer step than a US half size.
• UK & Ireland — letters from A to Z with half sizes. This converter doesn't compute UK letters, but as an anchor, a US 6 (EU 52, inner diameter 16.5 mm) corresponds to roughly a UK L½ on common conversion charts.

Because the step sizes differ — about 1.28 mm of circumference per US half size versus about 1.05 mm per Japanese/Korean size — the systems never line up perfectly. This converter works in exact millimeters internally and rounds each output to the nearest size in its own system, which is also why two adjacent Asian sizes occasionally map to the same US half size.

How to measure at home

You'll get the best result with a real ruler. We deliberately don't offer an on-screen sizer: screen scaling (DPI) varies between devices, so "hold your ring against the display" tools can be off by a full size or more. String and a ruler are boring but honest.

1. Cut a piece of string or a strip of paper about 5 mm (1/4 in) wide.
2. Wrap it around the base of the finger where the ring will sit — snug against the skin, but not squeezing.
3. Mark the exact point where it overlaps, then lay it flat and measure to the mark in millimeters.
4. Enter that number above as finger circumference. If your knuckle is noticeably wider than the base, measure the knuckle too and pick a size that clears it — usually the larger of the two readings, or something in between.
5. Measure two or three times, ideally at the end of the day when fingers are at their largest. Cold mornings can shrink a finger by a noticeable fraction of a size.

If you have a well-fitting ring instead: place it on a ruler and read the inside distance edge to edge across the widest point, excluding the metal. A caliper is ideal since one Japanese/Korean size is only 1/3 mm of diameter, but a careful ruler reading, taken straight from above and repeated for consistency, gets you within a size.

Example

Say the string measures 52 mm around your finger. That's an inner diameter of 52 ÷ π ≈ 16.6 mm, which rounds to US 6 (standard circumference 51.9 mm), Japanese/Korean size 12, and EU 52. Going the other way: a listing says US 7. Reverse mode shows 17.3 mm inner diameter and 54.4 mm circumference — about a Japanese/Korean 14 and EU 54, so that's the number to give a seller in Tokyo or Seoul.

Average ring sizes — commonly cited ranges

If you're buying a surprise gift and truly can't measure, jewelers' published averages are a last resort. US retailers commonly cite around US 6 to 6.5 for women — with most women's engagement rings reportedly sold between about 5.5 and 7 — and around US 9 to 10 for men. These are sales statistics, not physiology: hand size doesn't track height or weight closely, and the same person's fingers can differ by two or three sizes between hands and between fingers. If you must guess, guess slightly large and choose a design that can be resized: most plain gold or silver bands can be adjusted a size or two, while eternity bands (stones all the way around) and hard materials like tungsten generally can't.

US ring size conversion chart

Standard reference values computed from the formulas above — US sizes 3 to 13.5 with inner diameter, inner circumference, and the nearest Japanese/Korean and EU sizes. Rounded to the nearest size in each system; individual brands may differ slightly.

FAQ

How do I find my ring size at home?

Wrap string or a thin paper strip snugly around the base of your finger, mark the overlap, and measure that length in millimeters — that's your finger circumference. Enter it above. If you have a ring that already fits that finger, measuring its inner diameter is even more reliable. Skip printable and on-screen sizers unless you can verify their scale against a ruler.

How do I convert a Japanese or Korean ring size to US?

Japan and Korea share one system: size 1 is a 13.0 mm inner diameter and each size adds 1/3 mm. To convert, turn the size into millimeters and find the nearest US size — or just use reverse mode here with the US size, or the chart above. For example, Japanese/Korean 14 is about 17.3 mm across, which is US 7.

What does an EU ring size like 52 mean?

EU sizes follow ISO 8653, where the size number equals the ring's inner circumference in millimeters. EU 52 is simply a ring that measures 52 mm around the inside — about US 6, or Japanese/Korean 12.

I'm between two sizes — which should I pick?

Size up, as a rule. The ring has to slide over your knuckle, and fingers swell in heat, in the evening, and after salty food. Half a size of room is comfortable; a ring that's tight at fitting time will feel tighter in July. And since brands differ by up to about half a size in practice, prefer resizable designs when you're not certain.