Ring Size Converter
US, EU, diameter, and circumference ring conversions.
What it does
Ring sizes hide three rulers in one number: US quarters count diameter steps, EU counts circumference millimeters, and UK letters follow maker charts. Converting starts from the finger in millimeters.
The tool above converts either direction between diameter, US, and EU sizes. It returns all three numbers with the circumference shown.
The default 16.5 mm finger reads US 6 with EU 52 at 51.8 mm around. The 19.8 mm finger reads US 10 with EU 62.
ISO 8653 sets the EU side exactly: size equals inner circumference in millimeters. US numbers follow the diameter linear fit jewelers plot, rounded to halves, while UK letters stay chart bound.
Fit beats arithmetic on wide bands and swollen knuckles. Size up a touch for bands over 6 mm, measure warm afternoon fingers, and confirm surprise proposals with a borrowed ring on a mandrel.
How to use
- Pick diameter or US direction.
- Type millimeters or US size.
- Read US size first.
- Read EU size beside it.
- Read diameter and circ last.
- Size up for wide bands.
Formula
- dia
- Inner diameter in millimeters.
- US
- US size to nearest half.
- EU
- ISO size, inner circumference mm.
ISO 8653 defines ring size as inner circumference in millimeters, which is exactly the EU number shown. US sizes trace the diameter based linear fit used across jeweler charts, rounded to halves. UK letter scales lack one universal formula, so this page converts the exact systems and leaves letters to maker charts.
Worked example
Example: 16.5 mm finger
Default: diameter to US mode, 16.5 mm.
- Finger: 16.5 mm inside diameter.
- US: (16.5 minus 11.63) / 0.8128 = 6.0, so US 6.
- EU: round(3.1416 x 16.5) = round(51.8) = 52.
- Circumference: 3.1416 x 16.5 = 51.8 mm.
- Reverse: US 6 converts back to 16.5 mm.
- Tool shows US 6, EU 52, 16.5 mm dia (51.8 circ).
Reference table
| Finger | US | EU |
|---|---|---|
| 16.5 mm | 6 | 52 |
| 19.8 mm | 10 | 62 |
| 14.1 mm | 3 | 44 |
| 21.0 mm | 11.5 | 66 |
Limits
- US fit is approximate to halves.
- UK letters need maker charts.
- Wide bands shift real fit.
- Swelling beats static math.
Common mistakes
- Measuring the knuckle, not the base.
- Using string stretched tight.
- Ignoring wide band upsizing.
- Converting UK through US twice.
- Surprising with a guess, not a loaner.
Try it yourself
Quick checks. Answers sit tucked under each question.
19.8 mm finger?
US (19.8 minus 11.63)/0.8128 = 10, EU round(62.2) = 62.
US 9 to sizes?
Diameter 11.63 + 0.8128 x 9 = 18.9 mm, EU 60.
8 mm?
Below 10 mm sits outside finger ranges: remeasure the diameter.
Questions
Conversion formulas?
US = (diameter mm minus 11.63) / 0.8128, rounded to halves. EU (ISO 8653) is the circumference in mm.
Measure how?
Paper strip, string, or a mandrel at the jeweler. Measure the base of the finger, snug but sliding over the knuckle.
Wide bands?
Wide bands wear tight: size up a quarter to half from the converter for bands over 6 mm.
Fingers swell?
Yes, by a quarter to half size. Measure warm afternoon fingers for daily wear fit.
UK letters?
No universal letter table lives here: US numbers and EU millimeters convert exactly, UK letters vary by maker chart.
Between sizes?
Between sizes, pick up for comfort. Engraving and stones complicate later resizing.