Ring Size Calculator

Three major ring-size systems are used worldwide, and they measure different things. The US system uses a numeric scale from 1 to 13 (with half sizes), where each full step equals approximately 2.55 mm of inner circumference. Size 7, one of the most common women's sizes, corresponds to a 54.0 mm circumference and a 17.2 mm inner diameter.

The UK and Australian system uses letters from A to Z, with half-letter steps. Each letter increment is roughly 1.25 mm of circumference. The European (ISO) system is the most straightforward: the size number equals the inner circumference in millimeters directly. A EU 54 ring has a 54 mm inner circumference — no conversion table needed once you know your circumference. Japan uses its own numbered system that roughly parallels the EU scale but starts at a different offset, so JP 13 aligns with US 7 and EU 54.

When buying internationally, always convert using circumference in millimeters as the universal reference, not nominal size numbers, because the same number in different systems represents completely different ring diameters.

The string method is the most accessible way to measure your finger without a jeweler's mandrel. Cut a strip of non-stretchy paper about 6 mm wide and 100 mm long — printer paper works well. Wrap it snugly around the base of the finger where the ring will sit, mark where the paper overlaps with a pen, then lay it flat and measure the marked length in millimeters with a ruler. That number is your inner circumference.

For best accuracy: measure in the evening (fingers are at their natural largest), at room temperature (cold shrinks fingers by up to a full size), and on the specific finger and hand that will wear the ring — the dominant hand is typically a quarter to half size larger. Take two measurements on different days and average them. If the readings differ by more than 1 mm, take a third and drop the outlier. A variance of 1 mm equals roughly half a US ring size, so precision matters more than it might seem when ordering online.

A standard ring chart assumes a narrow band — roughly 2–3 mm wide. As band width increases, the ring covers more skin and feels tighter at the same inner diameter. The general rule: add half a US size for every additional 3–4 mm of band width beyond the standard. A 9 mm wide wedding band typically requires a half size up from your narrow-band measurement; a 12 mm band may need a full size up.

Comfort-fit rings have a domed, rounded interior profile that makes them easier to put on and take off. Because the curved inner surface contacts less skin, the effective opening is slightly larger than a flat-interior ring at the same nominal size. Most jewelers recommend sizing a comfort-fit band a half size smaller than your standard size. If you are between sizes on any ring, always choose the larger one — resizing down is straightforward, but resizing up risks thinning the metal, especially on engraved or stone-set pieces. A ring guard or sizing beads can also be added by any jeweler to bring a slightly-too-large ring into a comfortable fit without altering the band.

Finger size is not fixed. Heat causes blood vessels to dilate and fingers to swell; cold causes them to contract. The difference between a hot summer afternoon and a cold winter morning can be a full ring size for some people. Sodium intake, alcohol, and pregnancy can all cause temporary swelling of a half to two full sizes. For this reason, measuring multiple times across different days and temperatures gives a much more reliable average than a single reading.

Long-term finger size also changes with age, significant weight gain or loss, and certain medical conditions including arthritis and hypothyroidism. If you notice your rings fitting noticeably tighter or looser than usual without a clear cause, it is worth mentioning to a doctor, as it can be an early sign of fluid retention or thyroid dysfunction. For a ring you plan to wear permanently, such as a wedding band, scheduling a professional sizing at a jeweler — ideally during the afternoon in moderate weather — is the most reliable method.