Ring Size Chart: US Sizes in Millimeters

A ring size is the inside of the ring, not the finger you wish you had. In the United States, Canada and Mexico the scale is numeric, with quarter steps. Wikipedia records the relationship used against the ISO circumference: multiply the US size by 2.55 and add 36.5 to get millimeters around. Diameter follows a second line: multiply the US size by 0.8128 and add 11.63. Whole sizes are about 2.55 mm apart in circumference, which is why "each size is about 2.5 mm" is close enough for a conversation and not close enough for a saw.

ISO 8653 defines ring size as that inside circumference in millimeters. A European 52 is a ring that measures 52 mm around the inside. It is not a US 52. People mix those up when they read a number off a European site and order a US ring.

Figures below are the formula, rounded to one decimal. A printed paper sizer can be off. A metal mandrel in a shop is the check.

UK and Australian letters come from a different rule. British Standard practice puts size C at 40 mm around and adds 1.25 mm per full letter. Half letters sit halfway. The UK column above is that math lined up with the US formula, rounded to the nearest half size. If you are between letters, trust a mandrel over the table. Quarter US sizes exist: size 6.5 is 53.1 mm around and 16.9 mm across. Size 7.5 is 55.6 mm around and 17.7 mm across.

A finger measured at 54 mm around is the question people bring to this chart. Solve it backward: subtract 36.5 from 54, then divide by 2.55. That is 6.86, which falls between US 6.75 and US 7. On a set of whole and half sizes, call it a 7 and then test the ring, especially if the band is wide.

The chart assumes a narrow band, the kind that touches a thin slice of the finger. A band wider than about 6 mm covers more skin. Fingers are not perfect cylinders. They taper, and the knuckle is fatter than the base. A wide band has to clear the wider part of that taper, so it feels tight even when the diameter matches a thin ring that felt fine.

Fitting practice, not a law of metal: go up a quarter size for a band around 6 mm to 8 mm, and closer to a half size when the band is wider than that or when the ring has a heavy top that you cannot spin. Comfort-fit interiors, which are domed inside, often feel easier at the printed size than a flat interior. If you already own a wide ring that fits, measure that ring, not a thin band from another finger.

Engagement rings in the case have different profiles. A pear, a radiant and an emerald cut do not change your finger size. The head can change how the ring balances, and a low bezel can feel different from a high prong. Look at the shapes on pear lab diamond rings, radiant and emerald, then size the band, not the picture of the stone.

Shops in the US see a lot of women's rings around sizes 5 to 7 and a lot of men's rings around sizes 9 to 11. That is a crowd, not a standard. Ordering a 6 because it is "average" is how proposal rings miss. The dominant hand is often a quarter size larger than the other hand. The ring finger on the left is the one to measure for a wedding band in the US, and it is often a different size from the right.

Weight change, heat, salt and pregnancy move a finger by a quarter size, sometimes more. Measure at the end of the day, when the hand is larger. A ring that is perfect at 8 a.m. in an air-conditioned room can be a fight at 6 p.m. in Miami. The step-by-step at home is how to measure ring size. What a bench can and cannot do after the fact is ring resizing. Franky's published policies do not list resizing as a service. Do not buy the wrong size on the assumption that it will be opened up later. Call 954-504-5640 and ask about that design before you count on a change.

Use the chart to translate a measurement you already have. Do not use it to guess a finger you have not measured. Three cases need a physical sizer:

1. The knuckle is much wider than the base of the finger. The ring has to go on, and it also has to stay on. Sizing beads are a known way benches keep a ring from spinning, but that is bench work, not something to assume is included.
2. The band is wide, or the head is tall, or stones go partway down the shank.
3. You are converting a UK, ISO or Japanese size from an old ring and the numbers do not land cleanly on a quarter US size.

A set of metal ring sizers is the tool. Paper printouts lie when the printer scales the page. String stretches. If you can get to the showroom at 7900 NW 27th Ave, Unit 506, Miami FL 33147, that is the accurate version. The rings themselves are at rings, and the lab diamond engagement edit is lab diamond engagement rings.

Measure twice, at the end of the day, and write down the millimeter as well as the US size. The millimeter is the number that survives a move between countries. The US size is the number this shop speaks.