What is a UK size O ring in US sizes?

US 7.5. On the GIA chart a UK O is 55.6mm around the inside, which is the same row as US 7.5.

How many millimetres is ring size O?

55.6mm around the inside and 17.7mm across. European and ISO sizes are the circumference rounded to the millimetre, so it is a 56.

What is between size O and size P?

O½, at 56.2mm. Each half size adds about 0.6mm to the circumference and each full letter about 1.3mm, which is why a ring that is close but not right is usually a half size out rather than a whole one.

Is O a small or a large ring size?

It sits in the middle of the A to Z scale. A size is a measurement rather than a judgement, and the only way to know it is to measure the finger the ring is for.

Should I size up for a wide band in size O?

Often by a half size. A wide band covers more of the finger, so it can feel snugger than a narrow ring at the same marked size. Measure with a gauge close to the width you are buying, and our size finder explains how.

Ring sizes

UK ring size O

A UK O is a US 7.5 and an EU 56: 55.6mm around the inside of the ring and 17.7mm across it.

US and Canada: 7.5
EU and ISO: 56
Circumference: 55.6mm
Diameter: 17.7mm
Japan and Asia: 15
Switzerland: 16½

Size O and the half sizes either side

UK sizes go up in half letters, each about 0.6mm around. If O is nearly right, the answer is usually the half size next to it rather than a whole letter away.

UK O with its neighbours, from the GIA conversion chart
UK	US	EU / ISO	Circumference	Diameter
N	7	54	54.3mm	17.3mm
N½	7.25	55	55mm	17.5mm
O	7.5	56	55.6mm	17.7mm
O½	7.75	56	56.2mm	17.9mm
P	8	57	56.9mm	18.1mm

What a band weighs in size O

A bigger size takes more metal, so the same ring weighs a little more in a larger size. These are the weights of a plain court band at 55.6mm, the inside of a size O, at four widths.

Estimated grams, plain court band, UK O, 1.6mm wall
Metal	2mm	4mm	6mm	8mm
Titanium	0.8g	1.5g	2.3g	3.0g
Zirconium	1.1g	2.2g	3.3g	4.3g
9ct yellow gold	1.9g	3.8g	5.7g	7.6g
18ct yellow gold	2.6g	5.2g	7.9g	10.5g
Platinum 950	3.4g	6.7g	10.1g	13.4g

Estimates from the model in our ring weight calculator, using the same reference alloys. A finished ring with a pattern, stone or different wall thickness will differ.

Rings made in size O

All 99 of our wedding bands are made to order in size O, in every width and finish they come in. Size is chosen on the ring's page, and if you are not sure yet you can order with "I do not know yet" and we confirm it with you before anything is made. Our 2 diamond solitaires are set to order in size O too, at the same price as any other size.

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Size O questions

What is a UK size O ring in US sizes?: US 7.5. On the GIA chart a UK O is 55.6mm around the inside, which is the same row as US 7.5.
How many millimetres is ring size O?: 55.6mm around the inside and 17.7mm across. European and ISO sizes are the circumference rounded to the millimetre, so it is a 56.
What is between size O and size P?: O½, at 56.2mm. Each half size adds about 0.6mm to the circumference and each full letter about 1.3mm, which is why a ring that is close but not right is usually a half size out rather than a whole one.
Is O a small or a large ring size?: It sits in the middle of the A to Z scale. A size is a measurement rather than a judgement, and the only way to know it is to measure the finger the ring is for.
Should I size up for a wide band in size O?: Often by a half size. A wide band covers more of the finger, so it can feel snugger than a narrow ring at the same marked size. Measure with a gauge close to the width you are buying, and our size finder explains how.