Ring Size Converter
Find a ring size from the inside circumference or diameter, exactly as ISO 8653 defines it, and convert between ISO and Japanese ring sizes.
Result
ISO 8653 size: 54Estimate
The measurements are exact; a finger changes size with temperature, so check the size on a warm day.
- Japanese size
- about 14Estimate
- Inside circumference
- 54 mm (2.13 in)
- Inside diameter
- 17.19 mm (0.68 in)
- Between standard sizes
- Yes: between standard sizes on the Japanese scaleEstimate
- Neighbouring sizes
- ISO 53 or 55; Japanese 13 or 15Estimate
https://calcgallery.com/size-conversions/ring-size-converter-calculator/
Unit conversions follow from exact definitions and published notation rules. Sizes follow published standards and are approximate: makers grade their own, so check their chart. Figures are rounded only for display. Read the full disclaimer
How to use the Ring Size Converter
- Wrap a thin paper strip around the base of the finger, snug but able to slide over the knuckle, and mark where it overlaps. Or measure straight across the inside of a ring that fits.
- Enter the length of the strip as the circumference, or the width across the ring as the diameter. You can also enter an ISO or Japanese size you already know.
- Read the ISO 8653 size, the Japanese size and both measurements. A measurement between sizes shows the sizes either side.
Ring size is a measurement
ISO 8653 defines a ring’s size as the circumference of the largest cylinder that fits inside it, in millimetres. So a size is not a code to be looked up: it is the measurement itself, and the page can convert it exactly. The standard’s ring stick runs from size 41 to size 76.
ISO size = inside circumference in mminside diameter = circumference ÷ πJapanese size n = inside diameter of 13 + (n − 1) ÷ 3 mmThe Japanese scale in JIS S 4700 starts at 13.00 mm across for size 1 and adds a third of a millimetre for each size up to 35, so sizes 1, 4, 7, 10 and 13 fall on whole millimetres of diameter. Its sizes are a little over one millimetre of circumference apart, close to the ISO step but not the same, so the two scales only line up now and then.
Measure around, not across
The diameter of a ring is about 17 mm, and a millimetre of error across it becomes more than 3 mm around it, because the circumference is π times the diameter. That is three ISO sizes. A strip wrapped around the finger measures the circumference directly and needs no multiplying, so it is the more forgiving method. If you do measure an existing ring, measure the inside, at the widest point, and take the larger of two readings at right angles.
Why your finger changes size
A finger is not a fixed size. It is smaller when cold and larger when warm, after exercise and towards the end of the day. Measure more than once, at different times, and go by the reading from a warm day. The ring also has to pass over the knuckle, which on many fingers is wider than the base: a ring that fits the base but will not go over the knuckle is the wrong size.
What this page leaves out
The US and Canadian scale and the UK, Irish and Australian letter scale are the two that most people search for, and both are left out. Neither has a standard behind it: the US scale descends from a patented cone gauge whose sizes were never written down, according to the US Bureau of Standards in 1921, and the published millimetre values for the UK letters disagree. Rather than present one retailer’s table as fact, the page gives you the circumference, which any jeweller can size from.
With your numbers
- ISO 8653 size = inside circumference in mm = 54
- Inside diameter = 54 ÷ π = 17.19 mm
- Japanese size = (17.19 − 13) ÷ 0.3333 + 1 = 13.57
Quick reference
The measurements are exact; a finger changes size with temperature, so check the size on a warm day.
| Example | Result |
|---|---|
| 54 mm around | ISO 8653 size: 54 |
| 49.3 mm around | ISO 8653 size: about 49 |
| 59.5 mm around | ISO 8653 size: about 60 |
| 17 mm across | ISO 8653 size: about 53 |
| ISO 52 | Japanese size: about 12 |
| Japanese 9 | ISO 8653 size: about 49 |
Where the numbers come from
The fixed figures this calculator relies on, and the published source each one was checked against. How the calculators are built and checked
2 sources
JIS S 4700:2022, table of ring-stick diameters, size 1 = 13.00 mm, as reproduced in Japan Jewellery Association, 貴金属製品の品位等の表示 (2023 revision), section 10 (2), p. 10, https://jja.ne.jp/books/pdf/kikinzoku2023.pdf. The standard itself was not read.
Used for
- Inner diameter of Japanese ring size 1 (1号).
JIS S 4700:2022 as reproduced by the Japan Jewellery Association (2023), section 10 (2): sizes 1 to 35 at 13.00, 13.33, 13.67 … 24.33 mm, printed to 0.01 mm, which is 13 + (n − 1)/3 mm. Source as for ring.jisSize1Diameter.
Used for
- Inner diameter added by each Japanese ring size: a third of a millimetre.
Common questions about the Ring Size Converter
- How do I measure my ring size at home?
- Cut a strip of paper about 5 mm wide, wrap it around the base of the finger, and mark where it meets. Laid flat against a ruler, that length in millimetres is the inside circumference, and under ISO 8653 it is the ring size itself. Check that a ring of that size would pass over the knuckle, which is often the widest part of the finger.
- What is my US or UK ring size?
- This page does not give one, on purpose. No standard defines either scale: the US Bureau of Standards reported in 1921 that it could find no written dimensions for the US ring gauge, and the UK letter sizes in circulation give different millimetre values for the same letter. Jewellers sell and measure with physical gauges, so take the circumference in millimetres to them, or ask the seller which circumference their size means.
- Why does my ring fit differently in winter?
- Fingers shrink in the cold and swell in heat, after exercise and later in the day, so a ring that is loose on a cold morning can be tight on a warm evening. Measure when your hands are warm and at their everyday size, not straight after coming in from the cold.
- Does band width matter?
- Yes. A wide band covers more of the finger, so it grips more of it and usually feels tighter than a narrow band of the same size. If you are measuring for a wide band, measure with something of a similar width, or ask the jeweller how they allow for it.
- Is a ring size the diameter or the circumference?
- Under ISO 8653, the circumference: a size 54 ring has an inside circumference of 54 mm. The diameter follows by dividing by π, so it is about 17.19 mm. The Japanese system numbers sizes by inside diameter instead, in steps of a third of a millimetre.