MM to Ring Size: Millimeter Conversion Chart

Understanding Millimeter Ring Measurements

Whether you've measured your finger with a ruler, received a measurement from a jeweler, or simply want to convert a ring size to millimeters, understanding millimeter measurements is essential for accurate ring sizing. Millimeters provide a universal, objective way to specify ring sizes regardless of which sizing system is used in your country.

Ring measurements are expressed in two primary ways: diameter (the straight-line measurement across the inside of the ring) and circumference (the total distance around the inside of the ring). Understanding both measurements and how they convert to ring sizes across different systems ensures you can shop confidently anywhere in the world.

Diameter vs. Circumference Explained

These two measurements describe the same ring but from different perspectives:

Ring Diameter

The diameter is the straight-line distance across the inside of the ring at its widest point. For ring sizing, we measure the inner diameter. Common ring diameters range from approximately 14.86mm (US size 4) to 22.43mm (US size 13).

Example: If your ring measures 17.35mm in diameter, this corresponds to US size 7.

Ring Circumference

The circumference is the total distance if you were to measure completely around the inside of the ring. This is calculated by multiplying the diameter by π (approximately 3.14159). Common ring circumferences range from approximately 46.7mm (US size 4) to 70.5mm (US size 13).

Example: A 17.35mm diameter ring has a circumference of 17.35 × 3.14159 = 54.5mm.

When to Use Each Measurement

• Diameter: Use when measuring across an existing ring with a ruler or caliper
• Circumference: Use when measuring around your finger with a flexible ruler or string
• US System: Based on numeric sizes (1-13+), which correspond to specific diameters
• European/Argentine Systems: Based directly on circumference measurements

US Numeric System: Diameter to Ring Size

The US numeric sizing system assigns numbers based on the ring's inner diameter. This comprehensive table shows the diameter measurement for every US ring size, including half-sizes, and their circumference equivalents:

Half-Size Measurements

Half-sizes fall exactly between full sizes. For example:

• US 6.5 = 16.93mm diameter = 53.2mm circumference
• US 7.5 = 17.77mm diameter = 55.8mm circumference
• US 8.5 = 18.92mm diameter = 59.5mm circumference

European Circumference System

The European system uses circumference-based sizing, where the size number directly corresponds to the ring's circumference in millimeters. European ring sizes typically range from 36 to 72.

Converting Between EU and US Systems

To convert EU size to US size:

• Note the EU size's circumference measurement
• Divide by 3.14159 to get diameter
• Match the diameter against the US size chart

Example: EU size 54 = 54mm circumference ÷ 3.14159 = 17.2mm diameter ≈ US size 6.5-7

Converting Millimeters to Ring Sizes

If You Measured Diameter

If You Measured Circumference

Practical Examples

Example 1: You measured 17.35mm diameter

• Check the US table: 17.35mm = US size 7 ✓

Example 2: You measured 55mm around your finger (circumference)

• This is EU size 55
• Convert to diameter: 55 ÷ 3.14159 = 17.5mm
• Check US table: 17.5mm ≈ US size 7.5

Example 3: You measured 19.1mm diameter

• Using formula: (19.1 - 12.37) ÷ 0.84 + 1 = 9.1
• This is approximately US size 9 or between 9 and 9.5

International Ring Sizing Systems

While US and European systems are most common, other countries use distinct systems:

UK/Australia System

Uses letter designations (J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z), where each letter represents a specific ring size. The UK system is very similar to the European system but with different notation.

Indian System

Uses circumference-based sizing similar to the European system but with different measurement standards. Indian ring sizes typically range from 10 to 28.

Argentine System

Also circumference-based, where the size number directly corresponds to the circumference in millimeters. Argentine sizes range from approximately 45 to 75mm.

Japanese System

Uses numeric sizes but with a different scale than the US system. Japanese sizes require conversion to be compared with other systems.

Tips for Accurate Millimeter Measurements

Measuring Ring Diameter

• Use a ruler or digital caliper for best accuracy
• Place the ring flat on a surface and measure across the inside opening
• Measure at the widest point of the opening
• Take measurements multiple times and average results
• For precision, use a digital caliper rather than a standard ruler

Measuring Finger Circumference

• Use a flexible measuring tape or string
• Wrap around your finger at the widest point below the knuckle
• Keep the tape snug but not tight—a ring must slide over your knuckle
• Mark where the tape overlaps and measure with a ruler
• Measure multiple times at different times of day
• Account for finger swelling from heat, exercise, or salt intake

Measurement Timing

• Measure in the afternoon when fingers are at normal size
• Avoid measuring right after exercise or in extreme heat
• Avoid measuring in the morning when fingers may be slightly swollen
• Be consistent in timing across multiple measurements

When Millimeter Conversion Matters Most

International Ring Purchases

When buying from sellers in different countries, knowing millimeter measurements allows you to compare across sizing systems. A US size 7 (17.35mm diameter) is the same size regardless of which system names it.

Custom or Bespoke Rings

Craftspeople and artisans often prefer millimeter measurements for precision. Custom ring makers typically request measurements in millimeters for exact specifications.

Ring Resizing

When resizing an existing ring, professional jewelers often note the current and desired measurements in millimeters for accuracy.

Online Shopping

International online retailers may use different sizing systems. Converting to millimeters provides a universal reference point for accurate ordering.

Quick Reference Conversion Formulas

Frequently Asked Questions

Related Conversion Tools

• Printable Ring Size Chart
• Ring Diameter to Size Converter
• Ring Circumference to Size Converter
• How to Measure Ring Size at Home