Ring Size Converter Guide

What This Calculator Does

Compare US, UK, EU, Japanese, Swiss and millimeter inner-diameter sizes. Results update automatically from the Cartier international ring-size chart; they are a reference for cross-system comparison, not a guarantee for every product.

When to Use It

Use a known size or measured inner diameter to narrow a seller’s size range before checking that brand’s chart and trying the ring on.

Inputs Explained

US, EU, JP, CH and mm accept finite numbers. UK accepts a letter and optional half size, such as L 1/2. EU denotes circumference in millimeters; mm denotes inner diameter. JP means Japan only.

Supported ranges are US 3–13.25, EU 44–71, JP 4–31, CH 4–31 and approximately 14.01–22.60 mm diameter. The UK chart runs from F 1/2 to Z 1/2. Invalid and out-of-range inputs clear the result and show an explanation.

How the Calculation Works

The converter uses circumference as its internal unit. US and UK values between chart entries are linearly interpolated; UK output rounds to the nearest half size. EU equals circumference, the chart’s Japanese number and Swiss number equal circumference minus 40, and diameter = circumference / π. Missing UK entries at the upper end display “—”.

Example

US 6 returns EU 52, 52 / π ≈ 16.55 mm, UK L 1/2, JP 12 and CH 12. A measured 16.5 mm gives approximately US 5.96, close to 6. The Tiffany measurement guide labels US 6 as16.5 mm, illustrating rounding differences between charts.

How to Understand the Result

Numeric output uses up to two decimals. An interpolated size is a reference, not a promise that a seller stocks it. Display rounding and half-size labels introduce small round-trip differences.

Common Mistakes

• Confusing circumference with inner diameter.
• Entering a number without selecting its source system.
• Treating JP as a universal Chinese or Asian size.
• Treating a reference chart as proof of final fit.

FAQ

Does ring width affect fit?

Yes. Width, shape, knuckles and product design affect fit. Check the seller’s advice and try the product on.

Why reject sizes outside the chart?

Reliable neighboring entries are absent, so the converter gives an error instead of extrapolating a formula.

Do I need to press Calculate?

No. Valid changes update all outputs automatically.

Notes

This is a chart-based reference with interpolation. It does not model swelling, manufacturing tolerance or every brand’s product geometry. Use professional measurement and the seller’s chart before ordering or resizing.