Ring Size Converter Formula

Understand the math behind the ring size converter. Each variable explained with a worked example.

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Formulas Used

Inner Diameter (mm)

diameter_mm = 11.63 + (us_size - 3) * 0.814

Circumference (mm)

circumference_mm = (11.63 + (us_size - 3) * 0.814) * pi

EU Size (Approx)

eu_size = (11.63 + (us_size - 3) * 0.814) * pi - 40

UK Size Index (Approx)

uk_approx = (us_size - 0.5) * 2

Variables

VariableDescriptionDefault
us_sizeUS Ring Size7

How It Works

Ring Size Conversion

Ring sizes vary by country. The US system uses numbers (typically 3-16), while the UK uses letters and EU uses circumference in mm minus 40.

Approximate Formula

Diameter (mm) = 11.63 + (US Size - 3) x 0.814

Circumference = Diameter x pi

EU Size = Circumference - 40

Quick Reference

USDiameter (mm)EU 515.749 717.354 919.060

Worked Example

Convert US size 7 to other systems.

us_size = 7
  1. 01Diameter = 11.63 + (7-3) x 0.814 = 14.886 mm
  2. 02Wait, let me recalculate: 11.63 + 4 x 0.814 = 14.886 mm
  3. 03Circumference = 14.886 x pi = 46.76 mm
  4. 04EU size = 46.76 - 40 = ~7

Frequently Asked Questions

How do I measure my ring size?

Wrap a thin strip of paper around your finger, mark where it overlaps, and measure the length in mm. This is the circumference. Divide by pi for diameter, then use a size chart.

Do ring sizes differ by width?

Yes. Wider bands fit tighter, so you may need a half-size to full-size larger for bands wider than 6mm compared to standard 2-4mm bands.

Ready to run the numbers?

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