How do I convert centimetres to millimetres for ring sizing?

Multiply your finger circumference in centimetres by 10 to get millimetres. For example, 5.0 cm equals 50 mm. The EU/ISO ring size system uses circumference in millimetres directly, so a 50 mm circumference corresponds to EU ring size 50. For US, UK/NZ/AU, and Japanese sizes you then look up the mm value in the relevant conversion chart, as each system uses a different scale and starting point.

What ring size is 5 cm circumference?

A finger circumference of 5.0 cm (50 mm) gives: EU/ISO size 50, US/Canada size approximately 5.5, UK/NZ/AU size approximately K to K1/2, and Japanese size approximately 10. The diameter at 50 mm circumference is approximately 15.9 mm. If you are between sizes, it is generally better to choose the larger size for comfort, especially in warmer weather when fingers swell slightly.

Which ring sizing system does New Zealand use?

New Zealand uses the UK/AU/NZ/SA system, which sizes rings with alphabet letters from A to Z and then Z1, Z2, and so on. Each full letter represents a circumference increment of approximately 1.25 mm and a diameter increment of about 0.4 mm. Many NZ jewellers also refer to EU sizes (circumference in mm) as a universal reference. When ordering rings online from international retailers, always confirm which sizing system the retailer uses and convert accordingly.

cm to mm Ring Size Converter

Enter your finger circumference in centimetres to instantly convert to millimetres and find your ring size across all major international sizing systems: US/Canada, UK/NZ/AU, EU/ISO, and Japanese.

To measure your finger, wrap a strip of paper or thin string snugly around the base of your finger, mark where it overlaps, and measure the length in centimetres.

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Standard sizing Based on ISO 8653:2016 and widely published international ring size charts.

1. Your Measurement

Finger circumference (cm)

Or enter in millimetres (mm)

Please enter a valid circumference between 30 mm and 90 mm.

2. Measurement Tips

Measure at the end of the day when fingers are slightly larger
Avoid measuring in cold weather (fingers shrink)
Wrap paper or string snugly, not tightly
Measure the widest part of the knuckle if the knuckle is wider than the base
Measure two or three times and average the results
If between sizes, choose the larger size

Your Ring Measurements

Circumference
-
millimetres

Diameter

millimetres (mm / pi)

EU / ISO Size

Circumference in mm

Ring Size by Standard

US / Canada

US ring size

UK / NZ / AU / SA

Letter size

EU / ISO

Circumference (mm)

Japan

Japanese ring size

Conversion Breakdown

Input (cm)-

Circumference (mm)-

Diameter (mm)-

EU / ISO size-

US / Canada size-

UK / NZ / AU size-

Japanese size-

Worked Example

Input circumference5.0 cm

Step 1: cm to mm5.0 x 10 = 50.0 mm

Step 2: diameter50.0 / pi = 15.92 mm

EU / ISO size50

US / Canada size5.5

UK / NZ / AU sizeK

Japanese size10

Reference Chart (Common Sizes)

Circ. (mm)	Diam. (mm)	EU/ISO	US/CA	UK/NZ/AU	Japan

How to Convert cm to mm for Ring Sizing

The conversion from centimetres to millimetres is straightforward: multiply by 10. A finger circumference of 5.0 cm equals 50 mm. The EU/ISO ring size system uses circumference in millimetres as the size number directly, making it the simplest standard to work with. Other systems (US, UK, Japanese) each use their own numbering or lettering schemes derived from the circumference or diameter.

International Ring Size Standards

EU / ISO (Europe, Scandinavia, Asia)

The ISO 8653:2016 standard defines ring size as the inner circumference of the ring in millimetres. A ring with an inner circumference of 50 mm is EU size 50. This is the most internationally consistent system and is increasingly used as a reference by jewellers worldwide.

UK / NZ / AU / SA (Alphabetical)

The UK system (also used in New Zealand, Australia, and South Africa) sizes rings with alphabet letters. Size A corresponds to an inner circumference of approximately 37.5 mm. Each half size (A, A.5, B, B.5...) represents an increment of approximately 0.625 mm in circumference (or about 0.2 mm in diameter). New Zealand jewellers typically work in this system, so when shopping locally, a size K or L is common for women and N to P for men.

US / Canada (Numeric)

The US and Canadian system uses numbers starting at size 1 (approximately 40 mm circumference) with each half size representing about 0.396 mm of diameter (approximately 1.245 mm circumference). Women's sizes typically fall between 4 and 8; men's between 8 and 14. The formula is: US size = (diameter_mm - 11.63) / 0.8128, where diameter is circumference divided by pi.

Japanese (Numeric)

The Japanese system uses whole numbers starting from 1. Size 0 corresponds to approximately 38.7 mm circumference; each full size is approximately 1.27 mm of circumference larger. Japanese sizes run from about 1 to 30 for practical purposes.

Tips for an Accurate Fit

Fingers vary in size throughout the day by 0.5 mm to 1 mm. Measure in the evening.
Temperature affects finger size. Avoid measuring in the cold.
If your knuckle is larger than the base of your finger, size for the knuckle and have the ring sized to fit at the base.
Wide band rings (over 6 mm) generally need to be one half size larger than a narrow ring.
When in doubt, order one size up. Most jewellers can size a ring down more easily than up.

Related Calculators

Ring Size Converter: convert between all ring size systems.
cm to mm Converter: general centimetres to millimetres conversion.
cm to m Converter: centimetres to metres.
Essential Calculators hub: all converter tools.

Sources and method: ISO 8653:2016 Ring sizes - Definition, measurement and designation. Standard ring size charts published by Tiffany and Co., Blue Nile, and the Gemological Institute of America (GIA). Formula: circumference (mm) = input (cm) x 10; diameter (mm) = circumference / pi; EU size = circumference rounded to nearest 0.5 mm; US size derived from diameter using standard progression of 0.8128 mm per size; UK size derived from circumference using standard increment of 0.625 mm per half-size starting from 37.5 mm at size A; Japanese size derived from circumference starting at 38.7 mm at size 1 with 1.27 mm per full size.

This converter provides standard size estimates. Actual ring sizing may vary by manufacturer, band width, and individual comfort preference. For an accurate fit, visit a jeweller for a professional sizing.