Bracelet Size Calculator
Calculate bracelet size, inner circumference, bead-diameter allowance, and bead count from wrist circumference, fit ease, and bead size.
Open toolRing Size Calculator & International Converter (mm, US, UK, EU, JP)
Enter your ring's **inner diameter (`mm`)** or **finger circumference (`mm`)** and band width to calculate your exact **US / Canada**, **UK / Australia (`A–Z+`)**, **EU / ISO 8653**, and **Japan / China** ring size, plus wide-band comfort sizing.
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Enter the inputs, then select Calculate Ring Size.
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How Millimeter Diameter & Circumference Map to Global Ring Scales
All international ring standards derive from the circular geometry `C = pi * d`, where `d` is the internal bore diameter of the ring band and `C` is the internal perimeter.
C_mm = pi * d_mm; US_Size = (d_mm - 11.63) / 0.8128; EU_ISO = C_mm; UK_Index = round((C_mm - 37.8) / 1.25).
A half-size step in the US scale equals `0.406 mm` of diameter or `1.28 mm` of circumference (one full UK letter step).
International Ring Size Conversion & Millimeter Reference Table
Exact inside diameters (`mm`), inner circumferences (`mm`), and international equivalents across the most common ring sizes.
| US / Canada Size | Inner Diameter (mm / in) | Inner Circumference (EU / ISO) | UK / Australia & Japan (JP) |
|---|---|---|---|
| `US Size 5` | `15.70 mm` (`0.618 in`) | `49.3 mm` (`EU 49`) | `UK J½` • `JP 9` |
| `US Size 6` | `16.51 mm` (`0.650 in`) | `51.9 mm` (`EU 52`) | `UK L½` • `JP 11` |
| **`US Size 7` (Default)** | **`17.35 mm` (`0.683 in`)** | **`54.5 mm` (`EU 54.5 / 55`)** | **`UK N½` • `JP 14`** |
| `US Size 8` | `18.19 mm` (`0.716 in`) | `57.1 mm` (`EU 57`) | `UK P½` • `JP 16` |
| `US Size 9` | `18.89 mm` (`0.744 in`) | `59.5 mm` (`EU 59.5`) | `UK R½` • `JP 18` |
| `US Size 10` | `19.84 mm` (`0.781 in`) | `62.3 mm` (`EU 62`) | `UK T½` • `JP 20` |
How to Measure & Convert Your Ring Size in 3 Steps
Measure at room temperature in the evening when fingers are at their normal daily volume.
Choose Inner Diameter (mm) or Circumference (mm)
If measuring an existing ring, place it flat on a millimeter ruler or digital caliper and read the inside diameter excluding the metal rim.
Enter your millimeter measurement (e.g., 17.35 mm diameter)
If measuring your finger with a non-stretch sizing tape, enter the circumference in millimeters (`37.7` to `77.0 mm`) and ensure the loop slides over your knuckle.
Specify your ring band width (1.0 mm to 15.0 mm)
Enter your target band width so the calculator can compute whether a `+0.25` or `+0.50` wide-band comfort adjustment applies.
Worked Ring Sizing & International Conversion Examples
See how inner diameter, circumference, and wide cigar bands translate across countries.
17.35 mm Inner Diameter on a 2.5 mm Engagement Band (Default)
An inner diameter of `17.35 mm` (`0.6831 in`) yields an inner circumference of `54.51 mm` (`2.1459 in`), mapping to **US Size 7**, **UK N½**, **EU 54.5**, and **Japan Size 14** with no wide-band offset required.
Ordering an 8.0 mm Wide Wedding Band for a US Size 7 Finger
Keeping `measureValue = 17.35 mm` (`US 7` base) but increasing `bandWidthMm` to `8.0 mm` adds a `+0.50` US wide-band comfort allowance, recommending **US Size 7.5** (`17.75 mm` diameter / `UK O½` / `EU 55.8`).
Converting a European EU 52 Ring to US and UK Sizes
Selecting `Circumference (mm)` and entering `52.0 mm` computes an inner diameter of `16.55 mm`, matching **US Size 6**, **UK L½**, and **Japan Size 11**.
Tips for Knuckle Clearance, Temperature & Dominant Hands
Fingers fluctuate by up to half a ring size between cold mornings and warm evenings.
Always measure the exact finger on the exact hand where the ring will be worn: the dominant hand is typically `0.25` to `0.50` US sizes larger than the non-dominant hand. If your knuckle is noticeably wider than the base of your finger, size midway between the base and knuckle measurements so the ring can pass the joint without spinning loosely.
If you are also sizing wrist jewelry or custom cuffs, use our Bracelet Size Calculator (/tools/bracelet-size-calculator) for snug, comfort, and loose wrist allowances.
Measurement Boundaries & Sizing Tolerances
The calculator enforces realistic adult ring geometry boundaries to prevent unit confusion.
C_mm = pi * d_mm; US_Size = (d_mm - 11.63) / 0.8128; EU_ISO = C_mm; UK_Index = round((C_mm - 37.8) / 1.25). A half-size step in the US scale equals `0.406 mm` of diameter or `1.28 mm` of circumference (one full UK letter step).
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Open toolFrequently Asked Questions About Ring Sizes
Answers about using Ring Size Calculator & International Converter (mm, US, UK, EU, JP) and understanding its results.
How are US ring sizes calculated from millimeter inner diameter?
Under the standard North American mandrel scale, each full US ring size increments the inside diameter by exactly 0.032 inches (0.8128 mm) starting from US Size 0 at 11.63 mm: US Size = (d_mm - 11.63) / 0.8128. For example, an inside diameter of 17.35 mm gives (17.35 - 11.63) / 0.8128 = 7.037, which rounds to US Size 7 (inner circumference = pi * 17.35 = 54.51 mm).
How do EU (ISO 8653), UK letter, and Japanese ring sizes relate to millimeters?
European ISO 8653 ring sizes equal the inner circumference in millimeters directly (so a 54.5 mm circumference is EU Size 54.5 / 55). UK and Australian sizes use alphabetical letters (A to Z+) spaced every ~1.25 mm of circumference (half a US size), where US 7 equals UK N½. Japanese (JIS) sizes use integer numbers where US 7 corresponds to JP Size 14.
Why do wide rings (6 mm to 10 mm bands) need a larger ring size?
A wide flat or comfort-fit band covers more of the proximal phalanx, trapping skin tissue that would normally bulge around a thin 2 mm wire band. Jewelers recommend sizing up by +0.25 US size for 5.0–7.9 mm bands and +0.50 US size for bands 8.0 mm or wider.
Why is measuring an existing ring's inner diameter more accurate than the string method?
String and paper strips stretch or twist around the knuckle, easily adding 1.5 to 2.5 mm of false circumference (an entire full US size too large). Measuring the inside opening of a ring you already wear on that exact finger with a ruler or caliper is repeatable within 0.2 mm.
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