Ring size converter and how to measure your finger at home
In short
A US size 7 ring is 17.3 mm across the inside and 54.4 mm around, which is EU 54 and roughly UK N½. Each full US size adds about 0.8 mm of diameter. Type any one measurement below and the rest update; pick the wide-band option if the ring is 6 mm or wider.
- under 6 mm
- 6 mm or wider (+½ size)
| US | UK (approx.) | EU / ISO | Diameter mm | Circumference mm |
|---|---|---|---|---|
| 5.5 | K½ | 51 | 16.1 | 50.6 |
| 6 | L½ | 52 | 16.5 | 51.9 |
| 6.5 | M½ | 53 | 16.9 | 53.1 |
| 7 | N½ | 54 | 17.3 | 54.4 |
| 7.5 | O½ | 56 | 17.7 | 55.7 |
| 8 | P½ | 57 | 18.1 | 57 |
| 8.5 | Q½ | 58 | 18.5 | 58.2 |
How the sizes relate
The EU and ISO 8653 system is the honest one: the size number is the inside circumference in millimeters, so EU 54 means 54 mm around. US sizes are an arbitrary scale that happens to grow by 2.55 mm of circumference per full size, and UK sizes are letters on yet another scale. Because the scales do not line up exactly, any US-to-UK chart is rounded; two reputable charts can disagree by half a letter. The converter shows UK as approximate for that reason. If you are ordering from a UK jeweler, give them the millimeter circumference and let them pick the letter.
Three ways to measure at home
1. A ring that already fits
This is the most accurate method and the one to use whenever you can. Lay the ring on a ruler and read the inside width edge to edge in millimeters, ignoring the band; enter it in the diameter field. Or lay it over the circles below, printed at 100%: the circle that fills the inside exactly is the size. Use a ring worn on the same finger of the same hand, since the two hands differ by up to half a size.
2. A strip of paper
Cut a strip about 5 mm wide, wrap it around the base of the finger where the ring will sit, mark where it overlaps, and measure the length in millimeters. Do it three times and use the middle value; enter it in the circumference field. Paper beats string, because string stretches and rolls into a thinner line than a ring band and reads small. Measure at the end of the day, when fingers are largest, and not right after a cold walk or a hot shower.
3. A plastic sizer or a jeweler
Reusable plastic sizers cost a few dollars and are more accurate than paper because they mimic a band; several online jewelers mail them free. A jeweler with a steel sizing set is the gold standard, and the visit is worth it before any ring with stones around the whole band, which cannot be resized later.
If the methods disagree, trust them in that order: an existing ring, a plastic sizer, paper. Between sizes, choose the larger.
Finding someone's size without asking
Borrow a ring they wear on the ring finger of the left hand, even for an hour, and measure it against the circles above or on a ruler; a ring from another finger is only a rough guide, since the middle finger runs about a size larger and the little finger two sizes smaller. If you cannot borrow it, press the ring into a bar of soap or a piece of soft clay and measure the inside of the impression, or trace the inside of the ring on paper with a sharp pencil.
When nothing is possible, buy a plain band or a solitaire in a size that can be adjusted and plan on resizing: most women's rings are sold between 5 and 7 and most men's between 8 and 11, which is only a starting point, not a measurement. Choose a design without stones around the band, and keep the receipt; a jeweler can resize a plain band up or down two sizes without changing how it looks.
Fit rules that change the answer
Wide bands
Bands from 6 mm up cover more skin and feel tighter at the same size, so jewelers advise a half size up; the converter adds that for you. Comfort-fit bands with a rounded inside run a little looser than flat ones and cancel part of the effect.
Knuckle versus base
If the knuckle is noticeably bigger than the base of the finger, measure both and pick a size between them: the ring has to pass the knuckle once, then it should not spin. A spring insert or sizing beads inside the band solve the extreme cases where the two differ by more than a size.
Time of day, heat and years
Fingers swell in heat, in the evening, in pregnancy and with salt, and shrink in cold. A ring that is right in a warm shop can be loose in January, so aim for a fit that needs a slight push over the knuckle rather than one that slides freely. Sizes also drift upward with age; a ring bought at 25 often needs half a size more at 45.
Mistakes that produce the wrong size
Measuring with stretchy string or thread, which pulls tight and reads small. Measuring once, in the morning, with cold hands. Taking the diameter of a ring at its outside edge instead of its inside. Ordering the smaller of two sizes when between; the larger is the one that can be worn. Ignoring band width on a wide ring. And assuming the other hand is the same: measure the finger the ring will live on.
Resizing: what can and cannot be changed
Plain bands go up or down two sizes easily and cheaply, by adding or removing a sliver of metal at the bottom. Rings with stones on top only can usually be resized. Eternity bands with stones all the way around, tension settings, rings with continuous engraving or enamel, and rings made of tungsten or titanium cannot be resized in any practical way; they are sold in the size they will stay. For those, measure twice and buy once.
Questions people ask
What ring size is 17 mm?
If 17 mm is the inside diameter, it falls between US 6.5 (16.9 mm) and US 7 (17.3 mm); take the 7 unless the band is very thin. If 17 mm is the inside circumference, that is a small child's ring, around US 0.5, so re-measure: adult sizes start near 44 mm around.
What is ring size 54 in US?
US 6.75, which means ordering a 7 in practice: EU 54 is 54 mm of inside circumference and a US 7 is 54.4 mm. Going the other way, US 6 is EU 52 and US 8 is EU 57. Since EU sizes are plain millimeters, giving a European jeweler your circumference is more precise than any conversion chart.
How do I know my ring size without a ring?
Paper strip around the base of the finger, measured three times in millimeters, entered in the circumference field. Expect an error of about half a size, usually on the small side because paper pulls tighter than a ring sits. Between sizes, choose the larger: a ring that will not pass the knuckle is useless, and a slightly loose one can be sized down for a small charge.
Does the width of the band change the size?
From about 6 mm, yes. A wide band covers more skin and feels tighter than a thin ring of the same size, so jewelers advise half a size up, which the converter adds when you pick the wide option. Comfort-fit bands with a rounded inside run a little looser than flat ones and cancel part of that.
How much does a ring size differ in millimeters?
2.55 mm of circumference per full US size, 0.81 mm of inside diameter; a half size is 0.4 mm of diameter, about four sheets of paper. Fingers move that much with temperature alone, which is why a ring fitted in a warm shop can spin in January and stick in July.
Is it better for a ring to be loose or tight?
Slightly snug: it should need a small push to pass the knuckle and then sit without spinning. A ring that slides off with the hand pointing down is too loose; one that leaves a deep mark or will not come off with soap and cold water is too tight. When the two fingers you measure disagree, the ring finger of the dominant hand is usually the larger.