cm to mm Ring Size Converter
This calculator converts a finger circumference measurement from centimetres to millimetres and turns that figure into a ring size across the four sizing systems in common use worldwide: US/Canada, UK/NZ/AU/SA, EU/ISO, and Japan. It matters because jewellers, especially overseas online retailers, quote sizes using different scales, so a measurement taken in New Zealand needs converting before you order from an overseas store. You enter your finger circumference in centimetres, measured by wrapping a strip of paper or thin string snugly around the base of the finger, marking the overlap, and measuring the length; you can also enter the figure directly in millimetres. The calculator instantly returns the circumference in millimetres, the equivalent diameter, and your size in each of the four systems, alongside a step by step conversion breakdown and a reference chart covering common sizes from roughly 38 mm to 73 mm. New Zealand uses the UK/NZ/AU/SA letter system, so that is the figure to check first when buying locally. For an accurate result, measure at the end of the day when your fingers are largest, avoid measuring in cold weather, and take two or three readings and average them. If your measurement falls between two sizes, choosing the larger size is generally safer, and a jeweller can size a ring down more easily than up.
1. Your Measurement
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
Ring Size by Standard
US / Canada
UK / NZ / AU / SA
EU / ISO
Japan
Conversion Breakdown
Worked Example
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.