ElanoryaJewelry measurement
Ring size in millimetres
ISO 8653:2016 defines a ring-size as the inside circumference in millimetres, so the calculation needs nothing but your measurement. What it will not give you is a US number or a UK letter, and the reason for that is on this page.
Result
Enter a measurement to see the size.
The five printed control points
| Ring-size | Diameter printed in Table 1 | Circumference divided by pi |
|---|---|---|
| 41 | 13.05 mm ±0.02 | 13.0507 mm |
| 51 | 16.23 mm ±0.02 | 16.2338 mm |
| 61 | 19.42 mm ±0.02 | 19.4169 mm |
| 71 | 22.60 mm ±0.02 | 22.6000 mm |
| 76 | 24.19 mm ±0.02 | 24.1916 mm |
Why there is no US, UK, EU or JP conversion here
None of the primary sources read for this page carries a conversion between the ISO measurement and the US number, the UK letter, or a national retail scale. A table like that cannot be produced from them, so it is not printed here.
What this does not tell you
A ring-size is a measurement, not a label
ISO 8653:2016 defines the ring-size as the circumference of the biggest cylinder that could be inserted in the ring. Read literally, that means the size number and the inside circumference in millimetres are the same quantity — which is why this page can calculate rather than look up. Nothing has to be matched against a chart.
The second edition, dated 15 January 2016, cancels and replaces the 1986 edition, and its own foreword records that the table of diameters and tolerances was added in this revision. The earlier edition did not carry one.
The rounding rule quoted here comes from JIS S 4700:1998, Japan's national standard, which its own text describes as a translation of ISO 8653:1986 with the technical content unchanged. The corresponding sentence in ISO could not be verified from the preview text that was read, so it is attributed to JIS rather than to ISO.
The five diameters ISO actually prints
These are control points, not a conversion table. ISO Table 1 gives a diameter and a tolerance for these five sizes only; the sizes between them do not appear in the preview text that was read. The third column is arithmetic, shown so the derivation can be checked rather than believed.
| Ring-size | Diameter printed in Table 1 | Circumference divided by pi |
|---|---|---|
| 41 | 13.05 mm ±0.02 | 13.0507 mm |
| 51 | 16.23 mm ±0.02 | 16.2338 mm |
| 61 | 19.42 mm ±0.02 | 19.4169 mm |
| 71 | 22.60 mm ±0.02 | 22.6000 mm |
| 76 | 24.19 mm ±0.02 | 24.1916 mm |
Why there is no US, UK or EU column
None of the primary sources read for this page carries a conversion between the ISO measurement and a US number, a UK letter, or a national retail scale. ISO 8653:2016 makes no reference to those systems at all, and JIS S 4700 does not define the Japanese retail scale used in shops. A conversion table cannot be produced from these documents, so none is printed.
The increments that circulate widely — a fixed number of millimetres per US size, or per UK letter — did not appear in any primary source read for this page. They are therefore not used here, in a calculation or in prose.
A 1921 Bureau of Standards paper is blunt about why the American scale resists this. It records that no printed document could be found stating what the numbers on US cone gauges correspond to, that letters from two manufacturers gave 0.485 and 0.491 inch for the same size 1, and that retail gauges measured size 1 between 0.480 and 0.491 inch and size 13 between 0.870 and 0.878 inch. It also notes that firms differed in how they used the gauge at all — some bringing the top edge of the ring to the mark, others the middle or the lower edge.
That last point is the same problem ISO's rounded-versus-flat profile rule addresses ninety-five years later. It is not a historical curiosity: it is the reason a single conversion number would be a fiction.
Questions
Why does the calculator refuse to give me a US size?
Because none of the primary sources read for this page contains that conversion. ISO 8653:2016 does not mention the US, UK or Japanese retail systems, and the 1921 Bureau of Standards paper records that no printed dimensions for the American cone gauge could be found. Printing a number anyway would mean inventing it.
My measurement gives 54.4 mm. Is the size 54 or 54.4?
The designation is 54. JIS S 4700:1998 states that the measured inside circumference is rounded to the nearest whole millimetre, and the scale steps in whole millimetres. The calculator shows you how much was lost to that rounding so you can quote the raw measurement as well if the listing needs it.
Does ISO print a formula for converting circumference to diameter?
Not in the text that was read. That is why this page does not attribute one to the standard. Instead it shows the five diameters ISO does print next to the same figures derived by dividing by pi, and reports the largest deviation between them, which is well inside the standard's own tolerance of 0.02 mm.
Does my measurement leave my device?
No. The arithmetic runs in your browser and there is no request carrying it anywhere. You can confirm that in your browser's network panel; the page makes no third-party requests at all.