How do I measure my finger circumference in cm?

Wrap a thin strip of paper or a piece of string around the base of the finger you want to measure. Mark where the paper or string overlaps, then lay it flat against a ruler and read the length in centimetres. That measurement is your finger circumference. For the most accurate result, measure in the evening when fingers tend to be slightly larger, and measure the finger you plan to wear the ring on. A circumference of around 5.5 cm is a common starting point for many adults (roughly a size L in NZ/UK or a US size 6).

What ring size system does New Zealand use?

New Zealand, Australia, and the United Kingdom all use the same letter-based ring sizing system derived from British Standard BS 1927. Sizes run alphabetically from A (smallest, approximately 37.8 mm circumference) up through to Z and beyond (Z+1, Z+2, etc. for larger sizes). Each full letter size represents approximately 1.25 mm increase in inner circumference, with half sizes about 0.6 mm apart. Most jewellers in NZ carry sizes F through T as standard stock, with sizes outside that range available to order.

How does the EU/ISO ring size system work?

The EU/ISO ring size (also used in France, Germany, Spain, Italy, and other European countries) is simply the inner circumference of the ring in millimetres, rounded to the nearest half millimetre. So a finger circumference of 5.2 cm equals 52 mm, which is an EU size 52. This makes EU sizing very straightforward to work with: your finger circumference in mm equals your EU ring size. The EU system is also used in China and Japan (with a slightly different number offset for Japanese sizes).

cm to Ring Size Converter

This tool converts a simple finger measurement into an accurate ring size across every major sizing system used around the world, so you can buy, order or resize a ring with confidence. You enter your finger circumference in centimetres, or in millimetres if you prefer, measured by wrapping a strip of paper or string snugly around the base of the finger you plan to wear the ring on. The calculator instantly returns your NZ/AU/UK letter size, based on British Standard BS 1927, your US/Canada number size, your EU/ISO size, which is the circumference itself in millimetres, and the ring's inner diameter, worked out by dividing circumference by pi. A full comparison chart below the results shows how every system lines up, from the smallest letter sizes through to the largest, so you can cross-check a size quoted by an overseas retailer against the NZ standard. For the most reliable reading, measure in the evening when fingers are at their largest, measure the exact finger you intend to wear the ring on, and round up half a size if you are between two measurements or choosing a wider band, since wide bands sit tighter than thin ones. Because individual ring designs fit slightly differently, treat the result as a reliable starting point and confirm with a jeweller before buying an expensive or non-resizable ring.

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Standard Reference Based on British Standard BS 1927 (NZ/AU/UK), ASTM E2966 (US), and ISO 8653 (EU). Ring size tables are standardised and do not change year to year.

1. Your Measurement

Finger circumference (cm)

Or enter circumference in mm (optional)

2. How to Measure

Wrap a thin strip of paper tightly around the base of the finger you want to measure. Mark where it overlaps and lay it flat against a ruler. Read the length in centimetres.

Tip: measure in the evening when fingers are at their largest. If between sizes, choose the larger size for comfort.

Your Ring Sizes

NZ / AU / UK Size
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Letter system (BS 1927)

US / Canada Size

Number system

EU / ISO Size

Circumference in mm

Inner Diameter

Result: Enter your finger circumference above.

Full Ring Size Chart

NZ / AU / UK	US / Canada	EU / ISO	Circumference (mm)	Inner Diameter (mm)

How the Conversion Works

The core measurement in ring sizing is the inner circumference of the ring (which matches the outer circumference of your finger at the knuckle or base, whichever is larger). Once you know that measurement in millimetres, you can look it up in any ring size table.

EU/ISO size is simply the inner circumference in millimetres. A finger measuring 5.2 cm (52 mm) equals EU size 52. This is the most logical system as it directly represents the measurement with no lookup needed.
NZ/AU/UK size uses alphabetical letters (A through Z+6) based on British Standard BS 1927. Each letter corresponds to a specific inner circumference, with each full letter step (e.g. E to F) representing approximately 1.25 mm and each half size (e.g. E.5) about 0.6 mm.
US/Canada size uses a number scale from 1 to 13 (with half and quarter sizes). US size 6 corresponds roughly to a finger circumference of 51.9 mm and is one of the most common sizes.
Inner diameter is calculated by dividing the circumference by pi (circumference / 3.14159). A 52 mm circumference gives a diameter of approximately 16.55 mm.

Worked Example

You measure your finger and get a circumference of 5.2 cm (52 mm):

EU/ISO size: 52.0 (circumference in mm)
NZ/AU/UK size: L (the BS 1927 table entry for L is 51.9 mm, which is closest to 52.0 mm)
US size: 6 (the US equivalent for UK size L)
Inner diameter: 52 / 3.14159 = 16.55 mm

This matches what the calculator shows with the default input of 5.2 cm.

Tips for Accurate Sizing

Measure the finger you plan to wear the ring on (left ring finger and right ring finger can differ in size).
Fingers are slightly larger in warm weather and in the evening. Size up if you are between two sizes.
If your knuckle is wider than the base of your finger, measure the knuckle to ensure the ring fits over it, then have the ring sized to sit snugly at the base.
Wider bands (over 6 mm) tend to feel tighter, so consider going up half a size for a wide band ring.
Most NZ jewellers can resize a ring by one or two sizes. Rings with stones set all the way around (eternity rings) are harder to resize.

Related Calculators

Essential Calculators: all converter and utility tools.
Ring Size Converter: convert between NZ/UK, US, and EU ring size numbers directly.
cm to mm Converter: convert centimetres to millimetres for any measurement.
Circumference Calculator: calculate the circumference of a circle from its radius or diameter.

Sources and method: British Standard BS 1927 (UK/NZ/AU letter ring sizes). ISO 8653 (EU/ISO ring sizes by inner circumference in mm). ASTM E2966 (US ring sizes). Diameter calculated as circumference / pi.

This converter provides standard reference sizes. Individual ring designs vary slightly in how they fit. Always try a ring on or have a jeweller confirm your size before purchasing, especially for expensive or non-resizable rings.