💍 UK Ring Size to MM Conversion
UK ring size to mm conversion tells you the approximate inside diameter and inside circumference that correspond to a British letter ring size. In the UK, rings are commonly labelled with letters such as K, L, M, N, O and P, while measurements in millimetres are useful when checking a ring with a ruler, caliper or printable sizing guide.
For example, UK size M is approximately 16.71 mm inside diameter and 52.5 mm inside circumference in the chart used on this page, while UK size N is approximately 17.12 mm diameter and 53.8 mm circumference. Published jeweller charts can round values slightly differently, so use millimetre conversions as a practical guide rather than assuming every manufacturer uses identical rounding.
If you do not already know your British letter size, use our UK Ring Size Calculator. If you are choosing a ring for a proposal, our Engagement Ring Calculator can help with the wider buying plan.
Convert a UK Ring Size into Millimetres
Select a British ring size, or enter a diameter/circumference to find the nearest size.
Tip: leave the MM field empty to convert the selected UK size. Entering an MM value switches the result to the nearest UK size for that measurement.
Diameter and circumference are different measurements
When converting a UK ring size to mm, check whether the chart means inside diameter or inside circumference.
Inside Diameter (mm)
Measure straight across the inner opening of a ring from one inside edge to the opposite inside edge.
Inside Circumference (mm)
This is the distance around the inside of the ring, closely related to the circumference of the finger where the ring sits.
Diameter × π
A useful geometry check is circumference ≈ diameter × 3.14159, although published sizing tables may use standardized or rounded values.
| UK Size | Diameter (mm) | Circumference (mm) | Diameter (in) | Circumference (in) | Approx. Band |
|---|
Important: Different jewellers may round the same nominal UK size slightly differently. Use the retailer's own sizing chart for the final purchase whenever one is supplied.
UK Size K in MM
UK K is approximately 15.90 mm inside diameter and 50.0 mm inside circumference. If a ring measures close to 15.9 mm across the inside opening, K is a useful size to compare.
UK Size L in MM
UK L is approximately 16.31 mm diameter and 51.2 mm circumference. Half sizes such as L½ sit between full letter sizes for a closer fit.
UK Size M in MM
UK M converts to approximately 16.71 mm diameter and 52.5 mm circumference in this guide.
UK Size N in MM
UK N is approximately 17.12 mm diameter and 53.8 mm circumference. Major UK jeweller charts show closely rounded values.
UK Size O in MM
UK O is approximately 17.53 mm diameter and 55.1 mm circumference. A few tenths of a millimetre can move the closest match to another letter or half size.
UK Size P in MM
UK P is approximately 17.93 mm diameter and 56.3 mm circumference. Retailer charts may show very similar values with their own rounding.
Choose a ring that fits the correct finger
A ring that fits another finger or the opposite hand can give a misleading result because finger sizes vary.
Measure the inside opening
Place the ring on a millimetre ruler or use a caliper. Measure the widest straight line across the empty inner circle, from inner edge to inner edge.
Do not include the metal
The outside diameter is larger than the sizing diameter. Including the band thickness can produce a ring size that is too large.
Compare the result with the chart
Use the nearest value rather than expecting a home measurement to match a chart to the hundredth of a millimetre.
Confirm important purchases professionally
For engagement rings, wedding rings, wide bands or expensive jewellery, ask a jeweller to confirm the final size with a physical sizing set.
📐 Useful Ring Measurement Formulas
One of the most common mistakes when using a UK ring size to mm chart is comparing a diameter measurement with a circumference column. A diameter of 17 mm is a normal ring measurement, but a circumference of 17 mm would be far too small for an adult ring. The labels beside the number matter.
Inside diameter is generally the simplest measurement when you already have a ring that fits. You measure across the empty circular opening. Inside circumference is more useful when measuring the finger itself with a strip of paper, flexible measuring tape or non-stretch material.
Goldsmiths' ring-size guide publishes both measurements and, for example, lists UK N at about 17.1 mm diameter and 53.8 mm circumference. H.Samuel's measuring instructions also explain the relationship between a string circumference measurement and ring diameter.
British sizing is not limited to whole letters. Many sizing systems and jewellers use half sizes such as K½, L½, M½ and N½. These sit between adjacent full sizes and can be useful when one letter feels loose but the next smaller letter feels tight.
In the chart above, UK M½ is approximately 16.92 mm inside diameter and 53.1 mm circumference, while UK N½ is approximately 17.32 mm diameter and 54.4 mm circumference. The difference between neighbouring half sizes is small, which is why inaccurate home measurement or rounding can easily move a result by half a size.
Half sizes are especially relevant for rings that cannot comfortably be worn loose, but the best fit also depends on band width, ring profile, knuckle size and normal finger swelling. A wide wedding band may feel tighter than a narrow ring even if both carry the same nominal letter size.
Band Width
Wide bands contact more skin and can feel tighter than slim rings at the same nominal size.
Temperature
Fingers can contract when cold and swell when warm, changing how a ring feels during the day.
Different Hands
The same finger on the left and right hand may not have exactly the same circumference.
Knuckle Size
A ring must pass the knuckle but still sit securely at the base of the finger.
Ring Profile
Flat, court, comfort-fit and other internal profiles can change the sensation of fit.
Measurement Error
A fraction of a millimetre matters, so ruler angle, string stretch and rounding can affect the result.
Goldsmiths ring size guide
Goldsmiths publishes a UK ring size guide with British letters alongside millimetre measurements and international conversions. Its chart is useful for checking how a major UK jeweller rounds common sizes such as M, N, O, P and Q.
H.Samuel ring size chart
H.Samuel's ring size chart and measuring guide explains a home method using string, a ruler and millimetres. It advises measuring the string in mm and dividing by 3.14 to estimate ring diameter before matching the result to its chart.
Retailer charts can differ slightly because of rounding and the specific sizing references used. When purchasing from a particular jeweller, their own chart should take priority for that ring.