Size / Jewellery
Ring Size Converter: Circumference, Diameter and Printable Sizer
Convert ring measurements to ISO, US/Canada and metric BS 6820 UK/Australia sizes. Print a paper strip with a 50 mm scale check.
Ring Size Converter: Circumference, Diameter and Printable Sizer: Converts circumference and diameter using pi. ISO rounds circumference in mm. US uses one consistent linear diameter equation; UK uses the metric BS 6820 baseline C = 40 mm and 1.25 mm per letter. UK half sizes are included from A through Z. The paper strip is printed independently of screen calibration. Runs 100% locally in your browser with zero server file uploads.
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Measure the inner diameter of a ring that fits, across its centre without the metal edges. For a finger, measure around its base and check that the size passes the knuckle. Measure again at a different time of day.
| System or measurement | Result |
|---|---|
| Inner circumference | 54 mm / 5.4 cm / 2.13 in |
| Inner diameter | 17.19 mm / 1.72 cm / 0.68 in |
| ISO 8653 (nearest mm) | 54 |
| US / Canada — common convention (nearest quarter) | 6.75 |
| UK / Australia — BS 6820 (A–Z, half sizes) | N |
Approximate sizes: brands vary; measure and check the maker's chart.
Circumference = π × diameter. ISO size = circumference in mm, rounded to a whole mm. US diameter (mm) = 11.63 + 0.8128 × size. BS 6820: C = 40 mm; each letter adds 1.25 mm and each half letter adds 0.625 mm.
The BS 6820 metric letter scale can differ from older inch-based shop charts. Confirm the scale with your jeweller.
Printable paper strip
Print at 100% (actual size), with headers and footers off. Measure the check line with a physical ruler: it must be 50 mm. Cut the strip along its rectangle and at the zero line. Wrap loosely enough to slide over the knuckle, then read the overlap in mm. Paper gives only an estimate.
The screen preview is not calibrated. Printing uses physical millimetres; always check the 50 mm line.
ISO and British ring sizes
DATA_CHECKED 2026-10-04; accessed 2026-10-04. Ring size cites ISO 8653:2016 and BS 6820:1987. ISO uses inner circumference in mm; the BS metric scale has C = 40 mm and 1.25 mm per letter. The mixed equivalency table is not used. https://en.wikipedia.org/wiki/Ring_size
North American convention and Japan
The article cites US Bureau of Standards Circular 43 (1921), page 39, for the North American scale discussion. The implemented common convention is diameter = 11.63 + 0.8128 × US size in mm. https://en.wikipedia.org/wiki/Ring_size
JIS S 4700:1998 adopts ISO 8653:1986 circumference sizing. No supported traditional JP-number table was established, so that scale is omitted. Accessed 2026-10-04. https://kikakurui.com/s/S4700-1998-01.html
How to use it
- Measure a well-fitting ring's inner diameter or measure around the finger and check the knuckle.
- Choose the source measurement or size; cm and inches are available for physical measurements.
- Read approximate sizes or print the strip at 100% and verify its 50 mm check line.
Privacy & limitations
Measurements and the printable sizer stay in your browser. Nothing is uploaded.
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Frequently asked questions
Why is my UK letter different from a shop chart?
BS 6820:1987 uses a metric circumference scale. Older inch-based charts and mixed tables can disagree. Confirm the scale with your jeweller.
Can I measure from the screen?
The preview can reuse the saved online-ruler calibration. It is only valid at the same screen and zoom. Printing uses physical millimetres and requires a ruler check.
Is a paper strip accurate enough?
It is an estimate. Paper thickness, pulling too tightly, finger swelling and ring width affect fit. Brands vary; measure and check the maker's chart.
What range is supported?
Circumference 30–100 mm, US 0–25 and UK A–Z. Results outside a regional scale are shown as outside that scale.
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