Ring Size Converter (mm ↔ US)

Convert ring diameter in millimeters (mm) to US ring sizes instantly. You can also switch conversion direction.

Ring Size Conversion Formula

US size ≈ (diameter in mm − 11.63) ÷ 0.8128

This formula provides an accurate approximation used by jewelers.

What Is Ring Size in Millimeters?

Ring size in millimeters refers to the inner diameter of a ring, measured straight across the inside. This method is commonly used worldwide because it is precise and independent of regional sizing systems.

What Is a US Ring Size?

The US ring size system uses numeric values (typically from size 3 to size 15) to represent ring circumference and diameter. It is widely used in the United States and Canada.

How to Convert Ring Size (mm to US)

1. Measure the inner diameter of your ring in millimeters.
2. Apply the conversion formula or use the calculator above.
3. Round to the nearest half US ring size.

Example: 18.1 mm ≈ US size 8

Ring Size Conversion Table (mm to US)

Examples

16.5 mm ≈ US size 6

18.1 mm ≈ US size 8

20.6 mm ≈ US size 11

Frequently Asked Questions

Is ring diameter or circumference better?

Diameter is easier to measure accurately using a ruler or caliper.

Should I size up or down?

If between sizes, it is usually safer to size up for comfort.

Do ring sizes differ for men and women?

The sizing system is the same, but average sizes differ.

Real-Life Ring Sizing Tips

• Measure rings you already own
• Measure fingers at room temperature
• Avoid measuring after exercise
• Wider bands may need a larger size

Ring Size Visual Guide

Small diameter changes can significantly affect comfort.

Quick Size Comparison