Online ring sizer
Find your ring size on your phone or computer screen. Calibrate the screen once with a bank card, then match a ring you already own or measure your finger with a strip of paper. You get US, UK, EU and Japanese sizes, plus the inside diameter and circumference.
Step 1 Calibrate your screen with a card
Checking this screen...
- Hold any bank, debit or ID card flat against the screen.
- Put the card's top-left corner exactly in the outline's top-left corner.
- Drag the orange handle (or use the buttons) until the outline's right edge meets the card's right edge, then save.
Any standard card works: bank, credit, debit, ID, driving licence, library or gift card. They are all 85.60 x 53.98 mm (ISO/IEC 7810 ID-1). Match the card's physical edge, not the printed design. The result is saved in this browser only and nothing is sent anywhere.
Step 2 Measure
Lay a ring that fits the right finger flat on the circle. Resize the circle until the yellow fills the hole exactly: no gap between the yellow and the metal, with the circle's outline just meeting the ring's inside edge.
Drag left or right on the shaded area, use the slider, or tap - and + for 0.1 mm steps. Tap a size in the chart below to show it here at true size.
- Cut a thin strip of paper, about 5 mm wide, or use a non-stretchy string.
- Wrap it around the base of the finger, snug but not tight, and mark where the end overlaps.
- Check that it still slides over your knuckle, then lay it flat and measure from the end to the mark.
No ruler? Lay the strip on the ruler below with its end at 0, then tap or drag to your pen mark.
Screen sizing is only as accurate as the calibration. An outline 1 mm off the card moves the result by a quarter to half a US size, so calibrate carefully and measure more than once. For an expensive ring, or one that cannot be resized, confirm the size with a jeweller.
Ring size chart
US sizes 3 to 13 in half sizes with their UK, EU and Japanese equivalents. Tap a US size to show that ring at true size in the sizer.
| US / Canada | UK / Australia | EU / ISO | Japan | Diameter |
|---|---|---|---|---|
| F½ | 44 | 4 | 14.1 mm | |
| G½ | 45 | 5 | 14.5 mm | |
| H½ | 47 | 7 | 14.9 mm | |
| I½ | 48 | 8 | 15.3 mm | |
| J½ | 49 | 9 | 15.7 mm | |
| K½ | 51 | 10 | 16.1 mm | |
| L½ | 52 | 12 | 16.5 mm | |
| M½ | 53 | 13 | 16.9 mm | |
| N½ | 54 | 14 | 17.3 mm | |
| O½ | 56 | 15 | 17.7 mm | |
| P½ | 57 | 16 | 18.1 mm | |
| Q½ | 58 | 18 | 18.5 mm | |
| R½ | 60 | 19 | 18.9 mm | |
| S½ | 61 | 20 | 19.4 mm | |
| T½ | 62 | 21 | 19.8 mm | |
| U½ | 63 | 22 | 20.2 mm | |
| V½ | 65 | 24 | 20.6 mm | |
| W½ | 66 | 25 | 21.0 mm | |
| X½ | 67 | 26 | 21.4 mm | |
| Z | 68 | 27 | 21.8 mm | |
| Z+1 | 70 | 29 | 22.2 mm |
How the chart is worked out: US and Canada sizes use the standard formula, inside diameter = 11.63 mm + 0.8128 mm (0.032 in) per size. EU sizes follow ISO 8653:2016, where the size is the inside circumference in millimetres. UK and Australian letters follow BS 6820:1987, 1.25 mm of circumference per letter with size C at 40 mm. Japanese sizes use the JCS scale, size 1 = 13.0 mm across plus 1/3 mm per size. Values are rounded. The US and UK scales step by slightly different amounts, so other published charts can be half a UK letter apart from this one, and brands and sizing tools can differ by about a quarter size.
Built by Nitin Raja ·
How to find your ring size at home
A ring size is just the inside of the ring measured one way or another: the US and UK use number and letter scales, Europe uses the inside circumference in millimetres, and Japan uses its own number scale. If you know the inside diameter or the circumference, you know the size in every system. This sizer gets one of those two numbers from your screen and converts it.
Step 1: calibrate the screen
Your browser draws in "CSS pixels", and how big a pixel is depends on the device. The same 100 pixels can be 16 mm on one phone and 26 mm on a laptop, so a sizer that skips calibration is guessing. Holding a real bank card against the screen fixes that: every bank, debit and ID card is 85.60 by 53.98 mm, so once the outline matches the card, the page knows how many pixels make a millimetre on your screen. On a phone the card goes upright (its 54 mm side across), because most phones are narrower than a card is long. The calibration is saved in your browser and also used by the online ruler.
Step 2a: match a ring you already own
This is the most reliable method if you have a ring that fits the same finger. Lay it flat on the circle, look straight down, and resize the circle until its line sits exactly on the inside edge of the metal. Measure the inside, never the outside: the band's thickness has nothing to do with the size. If the ring fits a different finger, the size will be wrong for the new finger, as fingers on the same hand often differ by one or two sizes.
Step 2b: measure your finger
Wrap a thin strip of paper around the base of your finger, snug enough that it does not spin, and mark where the end overlaps. Lay it flat and measure from the end to the mark, either with a ruler or with the true-size ruler in the sizer. That length is the circumference of your finger, which is also your EU size: a strip that measures 54.4 mm is an EU 54 and a US 7. String works too, but it stretches and gets pulled tight, so it often reads small.
Tips for getting the right size
- Measure at the end of the day. Fingers are smallest in the morning and when cold, and largest in the evening, in hot weather and after exercise.
- Check the knuckle. The ring has to slide over it. If your knuckle is much wider than the base of your finger, choose a size between the two measurements, closer to the knuckle.
- Wide bands go up half a size. A band 6 mm or wider presses on more of the finger and feels tighter. Tick the wide band box for the adjusted size.
- Measure the right finger on the right hand. Your dominant hand is usually a little larger.
- Measure three times. If the results agree within a quarter size, you can trust them. Between two sizes, pick the larger one.
Buying a ring as a surprise
Borrow a ring the person already wears on the same finger and match it on the circle, then put it back. If you cannot, a ring they wear on a different finger is still a useful starting point, and many jewellers resize plain bands. Avoid guessing from a typical size, as sizes vary widely between people.
Why we built this ring sizer
Before building CheckMyHeight, I tested a lot of height and sizing websites to see what people actually need from them and what just gets in the way. Online ring sizers share one well-known flaw: a web page cannot know how big your screen's pixels are. The same 100 pixels can be 16 mm on one phone and 26 mm on a laptop, so a sizer that draws circles without calibrating is guessing, and 1 mm of inside diameter is more than one US size. Printable sizers move the problem to the printer, which may scale the page. And ring sizes are not one system: the US and Canada use numbers, the UK and Australia letters, most of Europe the circumference in millimetres, and Japan its own scale.
So this sizer calibrates first, with something everyone has in a known size, a bank card, and then gives the size in every major system at once.
What this ring sizer measures
- Calibration: you match an outline to a card (ISO/IEC 7810 ID-1, 85.60 x 53.98 mm). That gives your screen's pixels per millimetre, saved in this browser only.
- Match a ring: the inside diameter of a ring you own, from a true-size circle you resize between 12 and 25 mm in 0.1 mm steps.
- Measure my finger: the circumference of your finger, from the length of a paper strip that you type in millimetres or inches, or read off the true-size ruler.
- Conversion: US and Canada, inside diameter = 11.63 mm + 0.8128 mm per size; UK letters per BS 6820:1987 (size C = 40 mm around, 1.25 mm per letter); EU sizes per ISO 8653:2016 (the circumference in mm); Japan's JCS scale (size 1 = 13.0 mm across, plus 1/3 mm per size). A wide band, 6 mm or more, adds half a US size.
Formulas last checked on 24 October 2026, and the conversions are covered by automated tests. It does not measure your knuckle, it cannot allow for fingers that swell in the heat or shrink in the cold, and it does not use the camera. With a careful calibration it lands within about a quarter to half a size; for an expensive ring, a jeweller's mandrel is still the final word.
Why this ring sizer is different
- Calibrated, not assumed: you match the screen to a real card first, and every circle and ruler is drawn at that scale.
- Two methods on one page: match a ring, or measure your finger.
- It starts empty: no size shows until you resize the circle to your ring or enter your finger measurement.
- Every major system at once: US, UK, EU and Japan, plus the diameter and circumference in millimetres.
- A live chart: tap any size in the ring size chart to see that ring at true size.
- Nothing to print, nothing to install, no account. The calibration stays in your browser and nothing is sent anywhere.
- Standards named, so you can check every number yourself.
How accurate is this ring sizer?
With a careful calibration and a ring that sits flat, expect to land within about a quarter to half a size, which is about the same spread you get between different jewellers' mandrels. The biggest source of error is the calibration itself, so take your time lining up the card. Once you have a size, compare it with the brand's own size guide: some brands run small or large. Prefer measuring from a photo instead? See ring size from a photo, which uses a card in the picture as the ruler.
Questions
How do I find my ring size on my phone or computer?
Calibrate the screen once by holding a bank card against it and matching the outline to the card. Then either lay a ring that fits on the circle and resize the circle to the ring's inside edge, or wrap a strip of paper around your finger, mark it and measure the strip on the on-screen ruler. The page shows the size in US, UK, EU and Japanese sizing.
Is an online ring sizer accurate?
It can be close, but only as close as the calibration. If the outline is 1 mm off the card, a ring measures a quarter to half a US size off. Calibrate carefully, measure two or three times, and if you are buying an expensive ring or one that cannot be resized, confirm with a jeweller or a plastic ring sizer before you order.
What ring size is 17.3 mm?
A ring with an inside diameter of 17.3 mm is a US and Canada size 7, a UK and Australia size N½, EU (ISO) size 54 and about a Japanese size 14. Its inside circumference is 54.4 mm.
How do I convert a US ring size to UK?
Use the chart on this page. Each US size is about two UK letters: US 7 is about UK N½ and US 9 is about UK R½. The two systems step differently (US by 0.81 mm of diameter, UK by 1.25 mm of circumference), so most conversions land on a half letter.
Can I measure my ring size with a string?
Yes, but string stretches and is easy to pull too tight, so it tends to give a size that is too small. A thin strip of paper about 5 mm wide works better. Wrap it snugly at the base of the finger, mark where it overlaps, then measure from the end to the mark.
Should a ring be loose enough to go over the knuckle?
Yes. A ring has to slide over the knuckle, then sit snugly at the base of the finger without spinning all the way round. If your knuckle is much wider than the base of your finger, measure both and pick a size between the two, closer to the knuckle size.
Do wide rings need a bigger size?
Usually yes. Bands about 6 mm wide or more touch more of the finger and feel tighter, so most jewellers suggest going up half a size. Tick the wide band box and the sizer shows the adjusted size.
Why do I need a bank card to calibrate?
Browsers do not know how big your screen physically is. A centimetre drawn by one phone can be 30% bigger or smaller on another. Bank and ID cards are the same size worldwide (85.60 x 53.98 mm, ISO/IEC 7810 ID-1), so matching one gives the page an exact scale for your screen.
Is anything I measure saved or uploaded?
No. Everything runs in your browser. The only thing kept is the screen calibration, saved in this browser so you do not have to repeat it.
Need to measure something else? Use the actual size online ruler, or find your pupillary distance for glasses.