💍 US / Canada Ring Size to Diameter (mm) Converter
Free online tool to convert US / Canada Ring Size (US) to Diameter (mm) (mm ∅). Enter a value and get instant, accurate results.
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US / Canada Ring Size to Diameter (mm) Conversion Table
| US / Canada Ring Size (US) | Diameter (mm) (mm ∅) |
|---|---|
| 5 US | 15.68 mm ∅ |
| 6 US | 16.512 mm ∅ |
| 7 US | 17.344 mm ∅ |
| 8 US | 18.176 mm ∅ |
| 9 US | 19.008 mm ∅ |
| 10 US | 19.84 mm ∅ |
Conversion Formula
Ring size conversion formulas (approximate): EU (mm circ.) = diameter_mm × π diameter_mm = 11.52 + US × 0.832 (±0.1 mm) JP size ≈ US × 2 − 1 (exact for integer sizes) Common size reference: US 5 = EU 49.3 = JP 9 = 15.7 mm diameter US 6 = EU 51.8 = JP 11 = 16.5 mm diameter US 7 = EU 54.4 = JP 13 = 17.3 mm diameter US 8 = EU 57.0 = JP 15 = 18.2 mm diameter US 9 = EU 59.5 = JP 17 = 18.9 mm diameter US 10 = EU 62.1 = JP 19 = 19.8 mm diameter UK ring sizes use letters (F–Z) — UK A ≈ US 2.5, adding ~0.5 US per letter. UK F = US 3 • UK J = US 5 • UK N = US 7 • UK T = US 10
About US / Canada Ring Size to Diameter (mm) Conversions
The US to mm ∅ ring size converter handles the differences between international sizing systems. Enter a size below for an instant result.
This conversion uses a non-linear (reciprocal) relationship, so a simple multiplication factor does not apply. The formula section below explains the method, and the reference table shows common converted values.
This page includes a reference table of common US / Canada Ring Size values pre-converted to Diameter (mm), the conversion formula, and frequently asked questions. All values use internationally recognized conversion standards.
Note on ring sizes: Conversions use a linear approximation (±0.1 mm). Minor variations exist between jewelers and countries. For important purchases, have your finger professionally sized using physical sizing rings.
Frequently Asked Questions
What is US ring size 8 in mm?
US size 8 has an inner diameter of about 18.2 mm (circumference about 57 mm). The conversion: 11.52 + (8 × 0.832) = 18.18 mm. Size 8 is a common boundary size — near the top of women's sizes and the bottom of men's.
How do jewelers measure a ring's size in millimeters?
With a graduated ring mandrel (a tapered metal cone the ring slides onto) for finished rings, or finger gauges — a keyring of sized bands — for fingers. Both read out in standard sizes, which map directly to mm diameter; calipers measure the inner diameter of an existing ring to a tenth of a millimeter.
Is a 20 mm ring size large?
It's a larger men's size: 20 mm inner diameter is just past US size 10 (19.8 mm), around US 10.25. For context, most men's rings fall between 19 and 21 mm diameter (US 9–11.5), and most women's between 15.7 and 17.8 mm (US 5–7.5).
Why do international jewelry listings give ring sizes in mm?
Millimeters are unambiguous. US numbers, UK letters, and Japanese sizes all describe the same physical band differently, but an inner diameter (or circumference) in mm means exactly one thing everywhere — so global marketplaces standardize on it.
Related Ring Size Conversions
Reference Charts
Related Guides
Ring Size Conversion Guide
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All conversion results are provided for general informational purposes only. Our formulas use internationally recognized conversion factors; however, rounding may affect precision. Do not rely on these results for professional, medical, legal, or engineering decisions without independent verification. Read our full disclaimer.