What is UK O in millimeter diameter and circumference?

UK Size O to US, Australia, mm & EU Ring Size (17.52 mm / US 7.25) corresponds to an ISO 8653 internal bore diameter of 17.52 mm (0.690 inches) and an inner circumference of 55.04 mm. Across international hallmarks, this equals US/Canada size 7, UK/Australia size O, European/Cartier size 55, Swiss size 15, Japan size 16, and India size 15.

Should I size up from US 7 (UK O / EU 55) for a wide 6mm or 8mm wedding band or Cartier Love ring?

Yes. For a 6mm 18k gold or platinum comfort-fit wedding band or classic Cartier Love ring, bench jewelers recommend ordering US 7.5 (UK P / EU 56, 17.82 mm effective bore). For an 8mm flat-interior band, size up to US 8 (UK P½ / EU 57).

INTERNATIONAL RING SIZE CONVERTER

One size. Every standard.

Compare inside diameter, circumference and international ring sizes.

Screen scale96 PPI default

Resolution±0.05 mm steps

Privacy100% on-device

StandardsUS · UK · EU · JP · IN

ANIMATED VISUAL GUIDE · HOW TO MEASURE IN 15 SECONDS

STEP 1 · CALIBRATEHold a card to screen (or pick device) Tap to open calibration

STEP 2 · PLACE & ALIGNPlace ring on circle & match INSIDE edge Tap to scroll to stage · Use slider or ← / → keys

STEP 3 · YOUR SIZERead your size on the right or save card Tap to download PNG Size Passport

Internal diameter

17.52mm

Calibration needed1:1 scale

INTERNAL EDGE

Keep your eye directly above the centre · drag reticle to resize

Quick size presets & brand equivalents

INSTANT JUMP

Effective Ø17.52 mm

Circumference55.0 mm

Cartier ISOSize 55

Tiffany & Co.US 7.25

Three small steps. One confident fit.

THE METHOD

Calibrate your screen

Use a standard bank card to set the exact scale. It takes a moment.

Match the inner edge

Place your ring on the circle or drag the circle until it touches the inside.

Find your perfect fit

Keep your eye directly above the crosshair. Read your size on the right.

Evening check: Measure when hands are warm.

Wide bands: Add +0.25–0.5 size for 6 mm+ rings.

Zero parallax: Look straight down at the crosshair.

Polished gold wedding band on dark stone

MADE FOR THE MOMENT

Small detail.
Lifelong difference.

Measure with a little more care.

International & brand ring size reference

Inside diameter	Circumference	US / Canada	UK / Australia	EU / ISO	India (approx.)	Japan / China	Swiss (CH)	Cartier	Tiffany & Co.
14.07 mm	44.20 mm	3	F	44	4	5	4	44	3
14.47 mm	45.46 mm	3.5	G½	45	5	6	5.5	45	3.5
14.88 mm	46.75 mm	4	H½	47	7	8	6.5	47	4
15.29 mm	48.03 mm	4.5	I½	48	8	9	8	48	4.5
15.69 mm	49.29 mm	5	J½	49	9	10	9.5	49	5
16.10 mm	50.58 mm	5.5	K½	51	11	11	10.5	51	5.5
16.51 mm	51.87 mm	6	L½	52	12	13	12	52	6
16.91 mm	53.12 mm	6.5	M½	53	13	14	13	53	6.5
17.32 mm	54.41 mm	7	N½	54	14	15	14.5	54	7
17.73 mm	55.70 mm	7.5	O½	56	16	16	15.5	56	7.5
18.13 mm	56.96 mm	8	P½	57	17	17	17	57	8
18.54 mm	58.25 mm	8.5	R	58	18	19	18	58	8.5
18.95 mm	59.53 mm	9	S	60	20	20	19.5	60	9
19.35 mm	60.79 mm	9.5	T	61	21	21	21	61	9.5
19.76 mm	62.08 mm	10	U	62	22	22	22	62	10
20.16 mm	63.33 mm	10.5	V	63	23	23	23.5	63	10.5
20.57 mm	64.62 mm	11	W	65	25	25	24.5	65	11
20.98 mm	65.91 mm	11.5	X	66	26	26	26	66	11.5
21.38 mm	67.17 mm	12	Y	67	27	27	27	67	12
21.79 mm	68.46 mm	12.5	Z	68	28	28	28.5	68	12.5
22.20 mm	69.74 mm	13	—	70	30	30	29.5	70	13

Direct 1:1 Size Reference Pages

US 5 → 15.69 mm / UK J½ / EU 49 US 6 → 16.51 mm / UK L½ / EU 52 US 6.5 → 16.91 mm / UK M½ / EU 53 US 7 → 17.30 mm / UK N½ / EU 54 US 7.5 → 17.73 mm / UK O½ / EU 56 US 8 → 18.13 mm / UK P½ / EU 57 US 9 → 18.95 mm / UK R½ / EU 60 US 10 → 19.76 mm / UK T½ / EU 62 16.51 mm → US 6 / UK L½ / EU 52 17.30 mm → US 7 / UK N½ / EU 54 Cartier 52 → US 6 (16.51 mm)Cartier 54 → US 7 (17.30 mm)UK L → US 5.75 (16.31 mm / EU 51)UK M → US 6.25 (16.71 mm / EU 52)UK N → US 6.75 (17.11 mm / EU 54)UK O → US 7.25 (17.52 mm / EU 55)UK P → US 7.75 (17.93 mm / EU 56)UK R → US 8.75 (18.75 mm / EU 59)UK T → US 9.75 (19.56 mm / EU 61)Cartier 50 → US 5.25 (15.90 mm / UK K)Cartier 51 → US 5.5 (16.10 mm / UK K½)Cartier 53 → US 6.5 (16.91 mm / UK M½)Cartier 55 → US 7.25 (17.52 mm / UK O)Cartier 56 → US 7.5 (17.73 mm / UK O½)Tiffany US 6 → 16.51 mm / UK L½ / Cartier 52 Tiffany US 7 → 17.30 mm / UK N½ / Cartier 54

1:1 GIA Diamond Carat & Cut Simulators (US / UK / EU)

1.00 ct (6.50 mm) vs 2.00 ct (8.19 mm) Round 1.00 ct vs 2.00 ct Oval Brilliant 1.50 ct Oval → 8.81 × 6.53 mm on US 6 2.00 ct Oval → 9.70 × 7.18 mm on US 6 1.50 ct (7.44 mm) vs 2.00 ct (8.19 mm) Round 2.00 ct Emerald Cut → 8.82 × 6.30 mm 1.00 ct Round (6.50 mm) vs 1.00 ct Oval (7.70×5.70 mm)3.00 ct Oval (11.11×8.22 mm) vs 3.00 ct Round (9.38 mm)1.50 ct Cushion Cut → 6.87 × 6.87 mm 2.00 ct Pear Brilliant → 10.84 × 7.06 mm

1:1 Luxury Watch Lug-to-Lug Wrist Comparisons

Datejust 36 (43.3 mm L2L) vs Datejust 41 (47.6 mm L2L)38 mm (45.0 mm L2L) vs 42 mm (49.5 mm L2L)Submariner 41 → 40.5 mm Case / 47.6 mm Lug-to-Lug Speedmaster 42 mm → 47.5 mm Lug-to-Lug BB54 (37 mm / 46.0 mm L2L) vs BB58 (39 mm / 47.7 mm L2L)Cartier Tank Large → 33.7 mm Lug-to-Lug Span Santos Medium (41.9 mm L2L) vs Large (47.5 mm L2L)PRX 35mm (44.9 mm Span) vs PRX 40mm (51.0 mm Span)Royal Oak 37mm (46.8 mm) vs 41mm (51.2 mm)Explorer 36 (43.3 mm L2L) vs Explorer 40 (47.6 mm L2L)

METROLOGY & SIZING REFERENCE GUIDE

International & Luxury Maison Ring Size Conversion Reference

Deterministic millimetre-to-size formulas across North American, British, European ISO, Asian, and Haute Joaillerie standards.

NORTH AMERICA & UK

US / Canada Numerical vs. UK / Australia Alphabetical

The US and Canadian system starts at Size 0 (11.63 mm inside diameter) and increases by 0.8128 mm per full size. The British and Australian Wheatsheaf gauge uses alphabetical letters (A to Z+) in 1/64-inch (~0.397 mm) diameter steps—meaning each UK half-letter step is roughly a quarter US size.

US 6 = 16.51 mm (UK L½) · US 7 = 17.35 mm (UK N½) · US 8 = 18.19 mm (UK P½)

EUROPE & HAUTE JOAILLERIE

EU ISO 8653, Cartier & Swiss Circumference Scales

Under ISO 8653, European (EU/FR/DE) and Cartier sizes equal the ring’s internal circumference in millimetres. A Cartier Size 52 has an inner circumference of 52.0 mm (16.55 mm inside diameter, ~US 6). Swiss (CH) sizing subtracts 40 from the EU circumference (so EU 54 equals Swiss 14).

Cartier / EU Size = Inside Circumference (mm) · Swiss (CH) = Circumference − 40

ASIA-PACIFIC STANDARDS

Japanese (JCS), Chinese & Indian Ring Gauges

Japanese (JIS) and Chinese standards map numerical integers to inside diameter starting at Size 1 (13.00 mm inside diameter) in 1/3 mm (~0.333 mm) increments—so JP 13 equals 17.00 mm (~US 6.5) and JP 14 equals 17.33 mm (~US 7). Indian jewellers use a 1–36 gauge tied closely to inner diameter.

Japan Size = (Diameter_mm − 12.67) × 3 · JP 14 ≈ 17.33 mm ≈ US 7 ≈ EU 54.5

FREQUENTLY ASKED QUESTIONS

Expert Answers on True-Scale Sizing & Fit

ISO 8653 · GIA · HOROLOGY STANDARDS

How do I convert Cartier ring sizes (49, 50, 52, 54, 56) to US sizes?

Cartier uses the French/ISO metric circumference system in millimetres. Cartier 49 ≈ US 5 (15.60 mm diameter), Cartier 50 ≈ US 5.25, Cartier 52 ≈ US 6 (16.55 mm), Cartier 54 ≈ US 6.75–7 (17.20 mm), and Cartier 56 ≈ US 7.5 (17.83 mm). For wide bands like the Cartier Love Ring (6.1 mm), Cartier recommends sizing up by 1 full metric size (+1 mm circumference).

Does Tiffany & Co. use standard US ring sizes?

Yes. Tiffany & Co. follows the standard US inside-diameter scale (US 3 = 14.1 mm, US 6 = 16.5 mm, US 7 = 17.3 mm, US 8 = 18.1 mm), with half-size and quarter-size precision available for engagement and wedding bands.

What is US ring size 7 in UK, EU, Japanese, and Indian sizes?

A US size 7 ring has an inside diameter of 17.35 mm and an inner circumference of 54.51 mm. That converts to UK / Australia size N½ (or O), EU / ISO size 54.5, Japan / China size 14, Swiss size 14.5, and Indian size 14.

How do I convert millimetres (diameter or circumference) to ring size?

If you measured your finger circumference with a paper strip (for example 54.4 mm), divide by π (3.14159) to get the inside diameter (17.32 mm). Our converter lets you drag the millimetre slider or type either EU circumference or diameter directly.

What should I do if my measurement falls between two ring sizes?

If your measurement sits halfway between two standard sizes, choose the larger size—or ask your jeweller for a quarter size (such as US 6.75). Narrow rings (<2.5 mm) fit slightly looser, while wide bands (6 mm+) fit tighter and always benefit from rounding up.

Why do Indian ring size charts vary slightly between jewellers?

Indian ring gauges originate from traditional wire mandrel numbering (1 to 36) where each step corresponds to roughly 0.32 mm of inside diameter. Because regional mandrels vary by ±0.15 mm, always verify the inside diameter in millimetres when ordering fine jewellery in India.