Ring Size Calculator
Ring size from finger circumference or diameter, in US, UK, EU and Japanese sizes.
About
Ring Size Calculator
Ring sizing is one measurement expressed four different ways, and the four systems have nothing in common. Getting the number right matters more than it sounds: half a size is about a millimetre of circumference, and a ring an size too large is a ring you eventually lose.
What is actually being measured
Every system measures the inside of the ring. The US and Japanese scales use arbitrary numbers, the UK uses letters, and the European system, defined by ISO 8653, uses the inside circumference in millimetres, which makes it the only one that is self-explanatory.
The US scale is linear in circumference:
circumference in mm = 36.537 + 2.5535 × US size
So each full US size adds 2.55 mm of circumference, or 0.81 mm of diameter. Rearranged:
US size = (circumference - 36.537) ÷ 2.5535
A worked example
A finger measuring 54.4 mm around:
- US size: (54.4 - 36.537) ÷ 2.5535 = 6.99, so size 7
- Inside diameter: 54.4 ÷ π = 17.32 mm
- UK: N½ Europe: 54 Japan: 13
Measuring at home
Two methods, both workable and both easy to get slightly wrong.
Measuring the finger. Cut a strip of paper about 6 mm wide, wrap it snugly around the base of the finger, mark where it overlaps, and measure the length in millimetres. Snug means it should not slide loosely and should not indent the skin. Use a paper strip rather than string, which stretches.
Measuring an existing ring. Measure the inside diameter across the widest point with a ruler or callipers, then multiply by π to get the circumference. This is often more accurate, since a ring that already fits has resolved the knuckle question for you.
Printable ring sizers exist and work, provided the page is printed at exactly 100% scale with no fit-to-page scaling. Check by measuring the ruler printed on the sheet before trusting the result.
When to measure
Fingers change size through the day and through the year, by as much as half a size.
| Condition | Effect |
|---|---|
| End of the day | Largest; this is the size to use |
| Cold hands | Noticeably smaller |
| After exercise or salty food | Larger |
| Pregnancy | Often a full size larger or more |
| Warm weather | Larger than in winter |
Measure two or three times on different days and take the average. A single measurement on a cold morning gives a ring that will not come off in August.
Band width changes the fit
A wide band sits against more of the finger and feels tighter at the same nominal size. The usual allowance is a quarter to a half size larger for bands over about 6 mm wide, and a full size for very wide bands past 10 mm.
Comfort-fit bands, which are domed on the inside rather than flat, slide over the knuckle more easily and usually need a quarter to a half size smaller than the equivalent flat band.
The knuckle problem
Where the knuckle is much larger than the base of the finger, no single size is right: one that passes the knuckle spins at the base. Size to the knuckle, since a ring that cannot go on is useless, and then have a jeweller fit sizing beads or a small internal bar to stop it rotating.
Getting the fit right
A correctly sized ring slides on with light pressure and needs a small deliberate tug to come back over the knuckle. It should not spin freely, and it should not leave a deep mark.
Err slightly large rather than slightly small if you must choose. A tight ring cannot be removed in a medical emergency and restricts circulation; a slightly loose one can be padded or resized.
Resizing
Most plain gold and silver bands resize up or down by a couple of sizes without trouble. It gets harder or impossible with eternity rings set all the way round, tension settings, and rings in titanium, tungsten or ceramic, which cannot be cut and soldered. Anything with an inscription running the full circumference will lose part of it.
If you are buying a surprise gift in one of those materials, the size matters far more than usual, since the fallback of adjusting later does not exist.
Buying a ring as a surprise
The reliable method is to borrow a ring they already wear on the correct finger and measure its inside diameter. Note which finger it came from, since fingers differ by half a size or more even on the same hand, and a ring worn on the right middle finger tells you little about the left ring finger.
Failing that, press the ring into a bar of soap or a piece of plasticine to take an impression, then measure across it. Tracing round the inside on paper works less well, because the pencil line sits outside the true edge and the result reads large.
If nothing is available, US 6 for women and US 9 for men are the most commonly sold sizes and a reasonable gamble. Buy from somewhere with free resizing, and choose a plain band rather than an eternity ring, which cannot be resized at all.
Metal and lifestyle
Harder metals are less forgiving of a marginal fit. Platinum and gold can be stretched or cut and soldered by any jeweller. Titanium, tungsten carbide and ceramic cannot be resized at all, so the only remedy for a poor fit is a replacement.
Work matters too. Hands that swell from manual work, heat or exercise need the larger end of the range, and anyone whose fingers change noticeably through the day should size to the largest state rather than the average. A ring that is comfortable at 9am and painful at 6pm is the wrong size, even though it fitted when it was bought.
If the ring is stuck
Elevate the hand for a few minutes to reduce swelling, then work soap, hand cream or oil around the finger and twist rather than pull. Winding dental floss tightly around the finger from the knuckle down compresses the swelling and lets the ring be worked off over the wrapped section.
Persistent swelling with discolouration needs medical attention rather than more effort. Emergency departments have ring cutters and can remove a band in seconds, and a cut ring is repairable while a damaged finger is a more serious matter.
Sizing for a stacked set
Wedding and engagement rings worn together behave like one wider band, so the pair can feel tighter than either did alone. Where both are bought at once, jewellers usually size the set together rather than each ring on its own.
Adding a ring later to sit alongside an existing one is the case that catches people out, since the new ring measured on its own may be right and the pair uncomfortable. Trying both on together before committing is the only reliable check, and it is worth doing at the end of the day when the finger is at its largest.
Common mistakes
Measuring cold hands. Fingers shrink in the cold and the ring comes out too small.
Using string. It stretches, and the result reads large. Paper does not.
Measuring the wrong finger. Left and right hands differ, often by half a size, and the dominant hand is usually larger.
Ignoring band width. A wide band at your measured size will feel a size too tight.
Common questions
Frequently asked questions
Wrap a narrow strip of paper snugly around the base of the finger, mark the overlap, and measure the length in millimetres. That length is the inside circumference. Use paper rather than string, which stretches and gives a size too large.
For women the most commonly sold sizes are US 6 to 7, and for men US 9 to 10. These are only starting points, since hand size varies widely and buying to an average rather than a measurement is how a gift ends up needing resizing.
US sizes are numbers and UK sizes are letters, with no simple formula between them. US 6 is UK L and a half, US 7 is UK N and a half, and US 9 is UK R and a half. Roughly, one US size covers two UK letter steps.
Under ISO 8653 the European size is simply the inside circumference in millimetres. A ring measuring 54 mm around is a European size 54. It is the most transparent of the systems, since the number is the measurement.
Yes. A wide band contacts more of the finger and feels tighter at the same nominal size. Go a quarter to a half size up for bands over about 6 mm, and a full size for anything past 10 mm.
At the end of the day, at normal room temperature, when fingers are at their largest. Measuring on a cold morning gives a size that will be uncomfortably tight in warm weather. Measure two or three times on different days and average the results.
Most plain gold and silver bands can move a couple of sizes. Eternity rings set all the way round, tension settings, and rings made of titanium, tungsten or ceramic generally cannot be resized at all, because those materials cannot be cut and soldered.
About 1.28 mm of circumference, or 0.4 mm of diameter. That is small enough that a careless measurement can put you a size out, which is why measuring more than once is worth the effort.