OneFreePlace · Tool #107

Ring sizing that respects the millimetres.

Convert inner circumference, diameter and international ring labels in one place. Start from a real measurement, see the nearest reference sizes, and optionally calibrate your screen to preview the ring's physical inner diameter.

✓ Circumference ↔ diameter✓ US · UK · EU · Japan✓ Between-size detection✓ Calibrated ring preview

Ring Size Converter

Instant · local · measurement-first

Step 1

What do you know?

Input type

Measurement / size

For the best result, enter the inside circumference of a well-fitting ring or your measured finger circumference in millimetres.

Band style

This does not secretly alter the conversion. It changes the fit advice only, because wide rings can feel tighter on the same finger.

Nearest reference size

Measured

EU 54

reference ring size

54.0 mm inner circumference · 17.19 mm inner diameter

Exact reference row

Your measurement lands directly on a published reference size.

Measure without guessing

Finger size changes through the day, and measuring a ring is different from measuring a finger.

Existing ring: measure the insideMeasure straight across the widest inner opening, edge to edge. Do not include the metal thickness.

Finger: measure circumferenceUse a non-stretch strip or proper ring sizer. It should be snug but still pass the knuckle comfortably.

Repeat at different timesHands change with temperature and time of day. Avoid measuring only when fingers are cold or wet.

Between sizes? Check the ring styleWide bands usually feel tighter than slim bands. Stacking multiple rings can also change the comfortable fit.

Actual-size screen preview

Screen pixels are not physical millimetres. Match the box below to a real bank/ID card width (85.60 mm), then the ring opening can be drawn close to actual size on your screen.

1. Match this box to your card's widthDrag until equal

85.60 mm calibration width

324 px

2. After calibration, compare a ring you already own against the inner yellow opening. This preview is a convenience aid, not a replacement for a jeweller's ring gauge.

Nearby sizes

If you land between chart rows, compare the neighbors rather than treating the nearest label as exact.

EU / circ.	Diameter	US	UK	Japan

Why references can disagree

Jewellers publish slightly different conversion charts. OneFreePlace keeps the physical measurement primary and can cross-check a second retail chart where available.

Geometry stays exactInner diameter = inner circumference ÷ π. That part is not a brand convention.

Metric sizing is measurement-ledEuropean/ISO-style sizing is based on the physical ring measurement, making it the most transparent reference.

US and UK labels are lookup systemsThe same physical ring can be labeled slightly differently across published jeweller conversion charts.

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Fit is more than diameterKnuckle size, band width, finger shape and stacking all affect comfort even when the measurement is correct.

Reference basisThe primary international lookup table follows Cartier's published ring-size guide for circumference, Europe, USA, UK and Japan. A Pandora cross-check is shown where its published chart has a matching circumference. ISO 8653 is the international ring-size standard; the physical measurement remains the key reference.