Ring Size Chart: A US Size 7 Is 54.4 mm Around
Every US and Canadian size from 3 to 13, converted to UK and Australian letters, European sizes and millimetres. Then how to read it, and what a size means for the stone you choose.
The author is the founder of Draco Diamond, which mails a free plastic ring sizer to Canada and the United States. Millimetres are calculated from the published US size scale; UK letters follow GIA's chart. Sources are cited below. Read October 18, 2026.
Every US and Canadian ring size, converted to millimetres
US and Canadian ring sizes run on one numeric scale. A size 7 has an inside diameter of 17.3 mm and an inside circumference of 54.4 mm. Each whole size adds 0.81 mm across.
| US and Canada | UK and Australia | Inside diameter | Circumference, EU size |
|---|---|---|---|
| 3 | F | 14.07 mm | 44.2 mm, EU 44 |
| 3.5 | G | 14.47 mm | 45.5 mm, EU 45 |
| 4 | H | 14.88 mm | 46.8 mm, EU 47 |
| 4.5 | I | 15.29 mm | 48.0 mm, EU 48 |
| 5 | J | 15.69 mm | 49.3 mm, EU 49 |
| 5.5 | K | 16.10 mm | 50.6 mm, EU 51 |
| 6 | L | 16.51 mm | 51.9 mm, EU 52 |
| 6.5 | M | 16.91 mm | 53.1 mm, EU 53 |
| 7 | N | 17.32 mm | 54.4 mm, EU 54 |
| 7.5 | O | 17.73 mm | 55.7 mm, EU 56 |
| 8 | P | 18.13 mm | 57.0 mm, EU 57 |
| 8.5 | Q | 18.54 mm | 58.2 mm, EU 58 |
| 9 | R | 18.95 mm | 59.5 mm, EU 60 |
| 9.5 | S | 19.35 mm | 60.8 mm, EU 61 |
| 10 | T | 19.76 mm | 62.1 mm, EU 62 |
| 10.5 | U | 20.16 mm | 63.3 mm, EU 63 |
| 11 | V | 20.57 mm | 64.6 mm, EU 65 |
| 11.5 | W | 20.98 mm | 65.9 mm, EU 66 |
| 12 | X | 21.38 mm | 67.2 mm, EU 67 |
| 12.5 | Y | 21.79 mm | 68.5 mm, EU 68 |
| 13 | Z½ | 22.20 mm | 69.7 mm, EU 70 |
The scale is plain arithmetic. Each whole size adds 0.032 inch, or 0.8128 mm, to the inside diameter, a rule set out in a 1921 US Bureau of Standards circular.2
ISO 8653:2016 defines ring sizes by the inside circumference in millimetres. The EU column rounds that figure to the nearest whole millimetre.12
The UK and Australian letters follow GIA's chart. Other charts place some letters half a step higher, which section 03 explains.3
Diameters are shown to two decimals because the steps are small. Half a size is about 0.4 mm across, thinner than a credit card.
How do you read a ring size chart?
Start with the measurement you have. String and paper give a circumference. A ruler across a ring gives a diameter. Find that number in the matching column and read across.
Measure around the finger with paper or string, or across the inside of a ring that fits, in millimetres.
A length around the finger belongs in the circumference column. A width across a ring belongs in the diameter column.
Read down to the nearest figure. If you land between two rows, note both sizes.
The row gives the US and Canadian size, the UK and Australian letter and the European size.
Between two sizes, choose the larger, and choose a little more room for a wide band.
Seven ring sizes, drawn to the same scale
Inside diameters of US and Canadian sizes 4 to 10. Each whole size adds 0.81 mm across.
Source: the US ring size scale, 0.032 inch of inside diameter per whole size, via Wikipedia's ring size entry. Drawing: Draco Diamond.
The circles show how small each step is. Size 4 and size 10 differ by less than 5 mm across. Neighbouring sizes are hard to tell apart by eye.
If you do not have a measurement yet, start with our guide to measuring ring size at home. It ranks five methods by accuracy.
UK, Australian and European sizes follow different rules
Four main systems are in use, and only one counts in plain millimetres. The UK and Australia use letters, Japan uses numbers that do not track millimetres, and Italy subtracts 40.
UK letters step by 1.25 mm of circumference each. The baseline is size C, at 40 mm around, under British Standard 6820 of 1987.2
Italy, Spain and Switzerland number sizes as the circumference minus 40 mm. A ring 50 mm around is a size 10 there and an ISO 50 elsewhere in Europe.2
Japan and China use whole number scales that do not have a linear link to diameter or circumference. Convert those through a jeweller's own chart.2
UK charts do not agree with each other. GIA's chart sets the letters half a step lower than Wikipedia's for US sizes 5 to 12.5. A US 7 is an N on one and an N and a half on the other.32
The fix is simple. When a chart and a jeweller disagree, go by millimetres. A circumference or a diameter means the same thing in every country.
Canada uses the US scale, so a Canadian 7 is a US 7
Canada and the United States use the same ring sizes. A Canadian size 7 is a US size 7, with the same 17.3 mm inside diameter, and Mexico uses the same scale.
The shared scale covers the US, Canada and Mexico, in quarter size steps.2
So a Canadian buying from a US jeweller needs no conversion. A Canadian buying from a UK, Australian or European jeweller does, and the millimetre columns are the safe bridge.
The same holds for a ring sizer. A US and Canadian sizer reads the same numbers on either side of the border.
Will a 1 carat diamond look too big on my finger?
That depends on the finger more than the carat. A 1 carat round covers 44 percent of a size 4 ring's inside width, and 33 percent of a size 10.
Carat weight tells you the stone. It does not tell you the hand. The useful number is the stone's width against the width of the ring it sits on.
A 1 carat round covers less of a larger finger
Width of a 1 carat round, about 6.5 mm, as a share of the ring's inside diameter, by US and Canadian size.
Sources: round diameters from Draco's diamond size chart; inside diameters from the US ring size scale. Arithmetic: Draco Diamond.
A 1 carat round measures about 6.5 mm across. Set against a 14.9 mm size 4 ring, it covers 44 percent of the width. Against a 19.0 mm size 9, it covers 34 percent.4
So the same stone reads larger on a slender hand. Proportion, not carat weight, decides whether a ring looks bold or balanced.
Shape changes the picture too. A 1 carat marquise runs about 10.5 mm long, against 6.5 mm for a round of the same weight.4
"A 1 carat round covers 44 percent of a size 4 finger and 34 percent of a size 9. The finger decides how big the stone looks."
Garrett McMartin, Founder, Draco Diamond
Quarter sizes settle a fit between two rows
US and Canadian ring sizes come in quarter steps. A quarter size is about 0.2 mm across, a change you feel more than you see.
If the chart lands you between two rows, start with the larger. A jeweller can take metal out of a band more easily than put it back in.3
Width changes the answer. A wide shank grips more of the finger, so it usually needs a little more room.3
Hands differ too. The left ring finger is typically about half a size smaller than the right, so a chart reading from one hand may not suit the other.3
A quarter size also helps with a knuckle that is larger than the base of the finger. The ring has to pass the knuckle without spinning once it is on.
Order with a free sizer and a free resize behind you
A chart gets you close. A rigid sizer gets you the size. Draco sends a plastic ring sizer by mail, free, anywhere in Canada or the United States.
Every Draco ring includes one complimentary resize within 60 days of delivery, up to two sizes up or down. You cover shipping to us, and Draco covers the return trip.5
Some ring designs cannot be resized, and Draco discloses this before purchase.5
A bridal set is sized as a pair. Order the engagement ring and the band in the same size, so they sit together.
Every Draco piece carries 30 day returns, the Lifetime Authenticity Guarantee, free resizing, and free insured worldwide shipping to 25 markets, delivered duty paid with nothing collected at the door, and signature on delivery. Each piece is IGI certified, E to F colour and VS2 or better, with a report you can verify free at IGI.org.
Browse women's rings, bridal sets and lab grown diamond engagement rings.
Sizes, answered
Is ring size the same in Canada and the US?
Yes. Canada, the United States and Mexico share one numeric scale. A Canadian size 7 is a US size 7, with an inside diameter of 17.3 mm.
What ring size is 6 cm around?
A 60 mm inside circumference falls between a US and Canadian 9, at 59.5 mm, and a 9.5, at 60.8 mm. That is close to a 9 and a quarter, and an EU 60.
Is ring size universal?
No. The US, Canada and Mexico share a numeric scale, the UK and Australia use letters, much of Europe uses the circumference in millimetres, and Italy, Spain and Switzerland subtract 40 mm. Millimetres are the common language.
How do I convert a UK ring size to a US size?
Find the letter in the chart and read across. A UK N is close to a US 7. Charts differ by up to half a letter, so confirm the size in millimetres.
Does one ring size chart work for men and women?
Yes. Sizes measure the inside of the ring, not the wearer. Men usually wear larger sizes, but the scale is the same.
How accurate is a printed ring size chart?
Only as accurate as the printer. Print at 100 percent and check the printed scale against a ruler. A rigid plastic sizer avoids the problem, and Draco mails one free to Canada and the United States.
References
- International Organization for Standardization, ISO 8653:2016 Jewellery: Ring sizes: Definition, measurement and designation, 2016.
- Wikipedia, Ring size, accessed October 18, 2026.
- GIA 4Cs, How to Determine Ring Size: Tips and Ring Size Chart, January 18, 2018.
- Draco Diamond, Diamond Size Chart: Carat to Millimetres for Every Shape, accessed October 18, 2026.
- Draco Diamond, Refunds, Integrity and Ethical Standards, accessed October 18, 2026.
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