Ring Size Conversion Chart: Canada, US and UK Explained

Canada and the US use the same numbered ring sizes, so a Canadian size 7 is a US size 7. The UK uses letters instead, and a US/Canada 7 converts to a UK N. That is about 17.3 mm across the inside of the ring and 54.3 mm around it. The chart below covers the common sizes, and the rest of this guide explains why a conversion is only as good as the measurement you start with.

The chart: US and Canada, UK, and millimetres

The diameter, circumference and UK letters below come from the ring size chart in the GIA's guide, How to Determine Ring Size. That chart labels its numbered column "U.S. and Canada", which confirms the two countries share one scale. I added the right-hand column myself. It shows the nearest ISO size, which is the system used across most of continental Europe. It is the circumference rounded to the nearest millimetre, for reasons explained in the next section.

The GIA chart also lists quarter sizes. Each one lines up with a UK half-letter: a US 6¼ is listed as an L½ and a US 7¼ as an N½. If you are between sizes, the half-letter is the honest equivalent. Don't round to the nearest whole letter.

Why the three systems don't line up neatly

Continental Europe measures circumference

ISO, the International Organization for Standardization, publishes the standard Jewellery — Ring-sizes — Definition, measurement and designation. ISO's catalogue entry for the original 1986 edition, ISO 8653:1986, says it "defines ring-size as inner circumference of the ring measured in mm". So a ring that measures 54 mm around the inside is a size 54. That makes the ISO system the easiest to work with. If you know the circumference, you already know the size.

The current edition is ISO 8653:2016. ISO describes it as specifying "a method to measure the ring-size using a ring stick with defined characteristics, which is mainly used during manufacturing steps," along with how sizes are designated. When I checked ISO's page in October 2026, it said the standard was confirmed in 2021 and is currently under systematic review. Europe has adopted it too: the Estonian Centre for Standardisation lists EVS-EN ISO 8653:2026 as valid from May 2026, replacing the older EN 28653.

The UK uses letters, and the old British Standard is withdrawn

UK sizes run alphabetically, with half-letters between them. The British Standard that once set out how to measure and name ring sizes was BS 6820:1987, Method for measuring and designating the size of rings used in jewellery. BSI lists it as withdrawn on 15 March 1993. The letters are still used every day in British and Australian shops. Just know that a letter is a shop convention matched to millimetres, not a figure you can calculate from first principles the way you can with an ISO number.

US and Canadian sizes go up in fixed steps

On the GIA chart, each half size adds about 1.2 to 1.3 mm of inside circumference, and each quarter size adds about 0.6 mm. That is roughly the thickness of a credit card spread around the whole finger. You can feel it, but it is small. It is also why a size from one system can land between two sizes in another, and why the rounding in the ISO column above sometimes jumps 2 mm and sometimes only 1 mm.

Where conversions go wrong

A conversion chart is precise. The size you put into it often isn't. The GIA guide is blunt about the usual mistakes, and here are the ones I see most:

• Measuring with string or paper. GIA warns that "string can stretch and paper can be easily damaged. Humidity and temperature can cause paper to shrink and curl." A string measurement that is 1 mm off is already close to a quarter size off.
• Measuring on a cold or hot day. GIA notes that "fingers shrink in the cold and swell in the heat". A size taken outdoors in a Winnipeg January will not match one taken in a humid July.
• Ignoring band width. "Rings with wide shanks usually need to be a bit larger, as they will fit more snugly," according to GIA. A 2 mm band and a 6 mm band in the same size feel different on the same finger.
• Forgetting the knuckle. The ring has to slide over the knuckle and still not slip off at the base of the finger. If your knuckles are large, size for the knuckle and consider a band with a flatter or slightly squared inside profile.
• Mixing up which column you read. Some online charts put UK and Australian letters in the same column as continental European numbers. A UK N and a European 54 are close, but they are not the same unit. Check what the retailer's chart actually means.

How to use the chart when buying across borders

If you are ordering from a UK or European jeweller, or converting a size someone gave you from overseas, this order works:

1. Get a real measurement first. A jeweller's finger gauge set, which GIA describes as "a series of metal bands in ½ size increments," beats any at-home method. Our guide to other ring topics on the blog covers measuring at home if you can't get to a store.
2. Write down the millimetres, not just the size. Note the inside diameter or circumference next to your US/Canada size. Millimetres are the one unit every system can be checked against.
3. Convert using the millimetres. Look up the UK letter or ISO number by the measurement, not by a size-to-size table someone else rounded.
4. When you land between two sizes, check the band. For a narrow band, the smaller of the two is usually fine. For a wide band (6 mm and up) or a heavy comfort-fit wedding band, go with the larger.
5. Confirm the seller's chart. Ask which standard their sizes follow and what inside diameter they make a given size to. A good jeweller will tell you without hesitating.

Does it matter which system your jeweller uses?

Not much, as long as everyone knows which one is on the order form. The physical ring is made to millimetres on a sizing stick either way. The label is bookkeeping. What matters is that the number on the order matches the finger.

Where it really matters is rings that are hard to resize later. GIA notes that "rings with pavé and channel settings cannot be resized easily" and that eternity rings can be "very difficult" to resize. If you are choosing a setting from our engagement ring settings with stones running down the shank, or a full eternity style among our wedding bands, get the size measured properly before ordering. Don't convert a size you half-remember. A plain solitaire band can be adjusted later if needed, so it is more forgiving.

One honest point on cost: you don't need to pay for a professional sizing kit or an appointment just to convert a size. A free finger gauge reading at any local jeweller is accurate enough. The chart above does the rest.

What to do next

Get your finger measured with metal gauges, ideally at room temperature and later in the day. Write down both the US/Canada size and the inside diameter in millimetres. Use the table above to find the UK letter or ISO number if you need one. When you are ready, you can enter your size when you finish your ring, along with metal and engraving. If you are between sizes and the band is wide, size up a quarter.

Sources

• Gemological Institute of America (GIA). How to Determine Ring Size: Tips and Ring Size Chart. GIA 4Cs blog, 2018. 4cs.gia.edu
• International Organization for Standardization. ISO 8653:1986 Jewellery — Ring-sizes — Definition, measurement and designation (withdrawn). 1986. iso.org
• International Organization for Standardization. ISO 8653:2016 Jewellery — Ring-sizes — Definition, measurement and designation, edition 2. 2016. iso.org
• Estonian Centre for Standardisation and Accreditation. EVS-EN ISO 8653:2026 Jewellery - Ring-sizes - Definition, measurement and designation (ISO 8653:2016). 2026. evs.ee
• British Standards Institution. BS 6820:1987 Method for measuring and designating the size of rings used in jewellery (withdrawn 1993). 1987. knowledge.bsigroup.com