📏 How to Use a Ring Size Chart in CM
A ring size chart in cm lets you compare a finger circumference or ring inside diameter measured in centimetres with a standard ring-size reference. UK jewellers usually publish these measurements in millimetres, but converting to centimetres is simple: divide millimetres by 10.
For example, Goldsmiths lists UK size M at about 52.5 mm circumference. In centimetres, that is 5.25 cm. The same size has an inside diameter of roughly 16.7 mm, or about 1.67 cm. This page shows both units so you can use whichever ruler or tape measure you have available.
If you already own a ring that fits, inside diameter is usually the easier physical measurement. If you are measuring the finger, circumference is the more direct number. In either case, online measurement is an estimate and important purchases should be checked against the retailer's own chart or confirmed by a jeweller.
Convert a CM Measurement to UK Ring Size
Enter finger circumference or inside ring diameter in centimetres.
One ring size can be expressed as circumference or diameter in centimetres
Example: a common UK M reference is about 5.25 cm circumference and 1.67 cm inside diameter.
Circumference in CM
Measure around the intended finger, or use a known inside-circumference value. A 5.25 cm circumference is about 52.5 mm.
Diameter in CM
For an existing round ring, measure straight across the empty centre from one inner edge to the other.
Convert CM to MM
Multiply centimetres by 10. Jewellery charts often use mm because small ring-size differences are easier to express.
This ring size chart in centimetres shows UK sizes with finger/ring circumference and inside diameter in both cm and mm. Whole-letter UK rows align with common UK jeweller chart values; intermediate half-letter rows are approximate midpoint references for convenience.
| UK Size | Circumference (cm) | Diameter (cm) | Circumference (mm) | Diameter (mm) | US / Canada | EU |
|---|
Choose the correct finger
Measure the exact finger and hand where the ring will be worn. Different fingers—and even the same finger on opposite hands—can differ.
Measure circumference or diameter
For finger circumference, measure around the base of the finger. For an existing ring, measure the widest inside distance from inner edge to inner edge.
Read your measurement in cm
If your ruler is marked in centimetres, record two decimal places when possible—for example 5.25 cm rather than simply 5.3 cm.
Convert to mm if needed
Multiply the cm value by 10. For example, 5.38 cm becomes 53.8 mm.
Match the nearest chart row
Use the calculator or chart, then compare the result with the specific jeweller's current size guide.
Why jewellers usually prefer millimetres
Ring sizes differ by fractions of a centimetre. Millimetres make those small changes easier to display clearly, which is why UK jewellery charts commonly publish diameter and circumference in mm even when shoppers search in cm.
What ring size is 5.0 cm circumference?
A circumference of about 5.00 cm (50.0 mm) corresponds approximately to UK K on the Goldsmiths UK ring-size chart.
What ring size is 5.25 cm circumference?
A circumference of about 5.25 cm (52.5 mm) corresponds approximately to UK M. The inside diameter reference is roughly 1.67 cm.
What ring size is 5.38 cm circumference?
A circumference of about 5.38 cm (53.8 mm) corresponds approximately to UK N, with an inside diameter around 1.71 cm.
What ring size is 5.51 cm circumference?
A circumference of about 5.51 cm (55.1 mm) corresponds approximately to UK O on a common UK jeweller reference.
What ring size is 6.02 cm circumference?
A circumference of about 6.02 cm (60.2 mm) corresponds approximately to UK S.
Circumference
The distance around the inside of a ring or around the finger. Typical adult ring measurements are several centimetres around.
Inside Diameter
The straight-line distance across a ring's empty centre. It is much smaller numerically than circumference.
They Are Related
For a circular ring, circumference is approximately diameter multiplied by π.
CM to MM
Multiply a centimetre value by 10. A 1.67 cm diameter equals 16.7 mm.
Use Inside Edges
When measuring a ring, do not include the thickness of the metal band in the diameter.
Professional Sizing
Jewellers use finger gauges and ring mandrels for more dependable fit than a home ruler alone.
The mathematical conversion between centimetres and millimetres is exact: 1 cm always equals 10 mm. The uncertainty comes from the physical measurement and the ring-size chart itself, not from the cm-to-mm conversion.
A ruler measurement can be affected by alignment, a ring that is not perfectly round or rounding too early. Finger circumference can vary with temperature and time of day. GIA professional guidance recommends finger-sizing gauges for fingers and ring sticks or mandrels for rings, and notes that an inside-ring measurement method assumes the ring is perfectly round.
Band width can also affect fit. GIA distinguishes narrow bands up to 4 mm from wider rings, for which wide-finger sizing gauges are recommended. For a wide wedding band or bridal set, do not rely on a centimetre chart alone.
If an online retailer lists ring size in millimetres, multiply your cm measurement by 10 before comparing. If the seller lists only UK, US or EU sizes, use the chart above or the site's conversion tools to identify the nearest equivalent.
Always check whether the retailer is referring to inside diameter or circumference. A value such as 5.25 cm makes sense as a ring circumference, while 5.25 cm would be far too large for a normal ring inside diameter. Confusing the two measurements is one of the easiest ways to order the wrong size.
For international shopping, use the Ring Size Conversion Chart. For American sizes, see US to UK Ring Size. If you want a measurement-first lookup, use the Ring Size Reference Chart.
- 📏Record centimetres to at least two decimal places when your measuring tool allows it.
- 🔟Multiply cm by 10 before comparing with a jeweller chart published in millimetres.
- ⭕For an existing ring, measure the empty inside opening—not the outside edge.
- 🔁Repeat the measurement rather than relying on one quick reading.
- 🌡️Avoid treating unusually cold or swollen fingers as your normal permanent size.
- ↔️Remember that wide bands can feel tighter than narrow bands at the same marked size.
- 🏪Confirm engagement rings, wedding rings and difficult-to-resize jewellery with a professional jeweller.
UK Ring Size Calculator
Convert circumference or diameter directly into a British letter size.
Open calculator →Finger Size Calculator
Use finger circumference or ring diameter to estimate a likely size.
Open finger calculator →Ring Size Reference Chart
Compare UK, US/Canada and EU sizes with mm and inch measurements.
View reference chart →Goldsmiths Ring Size Guide
UK jeweller circumference chart, international conversions and measuring guidance.
View Goldsmiths guide ↗Ernest Jones Size Chart
UK chart comparing inside diameter, circumference, UK, US/Canada and EU sizing.
View Ernest Jones guide ↗GIA Professional Sizing
Guidance on finger gauges, ring mandrels, fit and wide bands.
Read GIA guidance ↗