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Ring Size Converter

Convert ring sizes across major systems, or enter inside diameter/circumference in millimetres. Measure your finger for a comfortable fit before ordering precious metal.

Calculate

Wrap a paper strip around the base of your finger, then measure the strip 1 WRAP PAPER around base (below knuckle) 2 MEASURE mark length = size circumference (mm) or use a ring that already fits
How to measure ring size →

Updates as you type

Result

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The formula

All systems map to inside diameter in mm. EU sizes are typically inner circumference in mm (ISO-style). US numeric sizes use a linear diameter formula; circumference = π × diameter.

Ø_mm ≈ 11.63 + 0.8128 × US · EU ≈ circ_mm = π × Ø · circ = π × Ø

How to measure ring size

Measure at the end of the day when fingers are warm.

Wrap a paper strip around the base of your finger, then measure the strip 1 WRAP PAPER around base (below knuckle) 2 MEASURE mark length = size circumference (mm) or use a ring that already fits
  1. Wrap — Use a strip of paper or string around the finger base.
  2. Mark — Mark where it overlaps; measure the length in mm (circumference).
  3. Fit — The ring should slide over the knuckle with slight resistance.
  • Wide bands feel tighter — consider a half size up.
  • Dominant hand fingers can be slightly larger.

Worked example

  1. US size 7
  2. Diameter ≈ 11.63 + 0.8128×7 ≈ 17.32 mm
  3. Circumference ≈ 17.32 × π ≈ 54.4 mm (EU-style)

Result: ≈ Ø 17.32 mm · circ ≈ 54.4 mm

How ring sizes are converted

A ring size is an inside diameter (or circumference) label. We convert every input to diameter millimetres, then emit each regional label.

Diameter as the hub

US/Canada numeric sizes use Ø ≈ 11.63 + 0.8128 × size (mm). EU inputs are treated as circumference in mm (÷ π → diameter). UK letters map through a numeric index onto the US scale. JP uses an approximate mapping.

Measuring tips that affect size

Fingers swell in heat and shrink in cold — measure when warm at day’s end. Wide bands feel tighter; many jewellers suggest a half size up for bands over ~6 mm. Size so the ring clears the knuckle without spinning freely on the finger base.

What we do not model

No comfort-fit vs standard interiors, no temperature correction, and no alloy expansion. Confirm engraved sizes with a mandrel before irreversible resizing.

Interesting facts

ISO circumference

Many European ring sizes use inner circumference in millimetres (ISO 8653).

Knuckles first

If knuckles are large, size so the ring passes the knuckle — then check it will not spin freely.

Wide bands feel tighter

Bands over ~6 mm often need a half size larger for the same comfort.

Temperature matters

Fingers shrink in cold and swell in heat — measure when hands are warm and at rest.

US numbers, UK letters

US/Canada use numeric sizes; the UK traditionally uses letter codes (e.g. L, M, N).

Frequently asked questions

Convert US to inside diameter, then multiply by π for circumference in mm — that circumference is the usual EU number. The calculator shows both.

It is a good start; a jeweller’s mandrel or ring gauge is more precise, especially for precious metals.

Measure the finger that will wear the ring. Dominant-hand fingers can be slightly larger.

Often yes — wide bands feel tighter. Consider a half size larger than a thin band on the same finger.

Yes — choose Ø mm or Circ mm as the “From” system and enter the millimetre measurement.

References

  1. ISO 8653 — Jewellery — Ring-sizes — International Organization for Standardization International ring-size designation based on inner circumference.
  2. NIST — length units — National Institute of Standards and Technology Millimetre as the working unit for diameter and circumference.
  3. Consumer jewelry guidance — U.S. Federal Trade Commission Buying tips for jewelry consumers.