Convert your finger measurement in centimetres (cm) to a UK ring size โ full CM circumference and CM diameter to UK letter conversion chart, calculator, and expert tips for 2026
The five most important CM-to-UK ring size reference points for UK buyers are shown above. The universal anchor point is UK N = 5.38cm circumference = 1.71cm diameter โ the most commonly purchased UK women's engagement ring size. For men, the most common reference is UK Q = 5.76cm circumference = 1.83cm diameter. If your measured circumference in cm falls between two of the values shown, always order the larger UK size โ a slightly larger ring is far easier to adjust than one that is too small.
| ๐ฌ๐ง UK Size | ๐ Circumference CM | ๐ Diameter CM | ๐ Circumference MM | ๐ Diameter MM | ๐บ๐ธ US Size | ๐ช๐บ EU / French |
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Wrap a thin strip of paper or non-stretchy string around the base of the finger the ring will be worn on. Mark where the paper overlaps itself with a fine-point pen, then remove the strip and lay it flat against a ruler. Read the measurement in centimetres (cm) โ most standard rulers show both cm and mm markings. The cm reading is your circumference. Write it down with two decimal places if possible (e.g. 5.38 cm) โ single decimal readings lose accuracy at this scale. Measure in the late afternoon or evening when fingers are at their daily largest, and repeat three times to get an average.
If you have an existing ring and want to find its UK size, place it flat on a white piece of paper and trace the inside edge. Measure the diameter of the traced circle across its widest point using a ruler in centimetres. This is your diameter in cm. For the most accurate result, use digital calipers (available for ยฃ5โยฃ15 online) rather than a standard ruler โ calipers measure to 0.01cm accuracy. Write down the diameter in cm with two decimal places (e.g. 1.71 cm). You can either use the diameter column of the conversion chart below directly, or convert to circumference using the formula: circumference = diameter ร 3.14159.
Locate your circumference in cm in the second column of the chart, or your diameter in cm in the third column. Read left to the first column to find your UK ring size letter. If your exact reading is not listed, find the two rows it falls between and read the UK size from the row with the larger circumference (always size up when between two sizes). The chart covers all UK half sizes from A to Z+3, giving you a precise result without needing to round to a whole letter.
If the jeweller or online retailer you are ordering from asks for your measurement in millimetres rather than centimetres, the conversion is simple: multiply your cm reading by 10. So 5.38cm ร 10 = 53.8mm. Alternatively, just use the mm circumference column in the chart below, which shows the mm equivalent alongside every cm reading. You never need to convert cm to mm separately โ the chart handles both simultaneously for your convenience.
If your CM circumference reading falls between two rows in the chart โ for example, 5.32cm falls between UK M (5.25cm) and UK Mยฝ (5.31cm) โ order the larger UK size. In this example, 5.32cm is above the UK Mยฝ row (5.31cm), so UK Mยฝ is the correct size if available, or UK N if the retailer does not offer half sizes. Never round down in ring sizing โ a ring that is slightly too large is always preferable to one that is slightly too small, and the free resize policy from most UK jewellers makes correcting a slightly large ring effortless.
Before placing your order, confirm that the retailer offers a free first resize and note the time window โ typically 30 to 90 days at major UK jewellers. Then order using your confirmed UK letter size. If you measured in cm and converted correctly, your ring should arrive fitting well. If there is any remaining uncertainty โ perhaps your cm reading was taken with a standard ruler rather than calipers, or you measured in the morning โ order the next half size up and use the free resize as your safety net. A perfectly measured ring and a free resize policy together give you complete coverage against any sizing uncertainty.
Wrap a strip of paper around the base of the ring finger. Mark the overlap point and measure with a ruler in centimetres. This gives you circumference in cm. Match the cm reading to the chart below to find your UK size. Measure three times and use the average reading for best accuracy.
Place an existing ring on paper, trace the inside, and measure the circle diameter in cm with a ruler or calipers. Match the cm diameter reading to the diameter column of the chart below. Example: 1.71cm diameter = UK N. Multiply by ฯ (3.14159) to cross-check via circumference.
If you have a reading in mm from a previous measurement or a jeweller's record, divide by 10 to get cm. 53.8mm รท 10 = 5.38cm = UK N. 52.5mm รท 10 = 5.25cm = UK M. 57.6mm รท 10 = 5.76cm = UK Q. Then use the cm column in the chart to confirm your UK size.
Enter your cm circumference or cm diameter measurement directly into the calculator at the top of this page and click Convert. The calculator instantly returns your UK size letter plus US, EU, Japanese, and Swiss equivalents โ no manual chart lookup needed.
Visit any UK jeweller for a free professional ring size measurement using a mandrel and gauge set. The jeweller measures in UK letters directly โ no cm conversion needed. Takes two minutes, is completely free, and is more accurate than any home measurement method.
If your cm reading falls exactly between two UK sizes, always order the larger one. If your circumference is 5.32cm (between UK M at 5.25cm and UK Mยฝ at 5.31cm), order UK Mยฝ or UK N. A larger ring can be resized โ a too-small ring cannot be worn until resized.
The single most important technique when measuring ring size in centimetres is to record your reading to two decimal places โ for example, 5.38cm rather than 5.4cm. The reason is that each UK half-size represents only 0.125cm of circumference. If you round your reading to one decimal place, you lose the precision needed to distinguish between, for example, UK M (5.25cm) and UK Mยฝ (5.31cm) โ a difference of just 0.06cm that is completely invisible when readings are rounded to 5.3cm. Always use a ruler with clear mm markings and estimate to the nearest 0.05cm (0.5mm) or finer. If your measuring tool only shows whole cm marks, use the mm scale on the other edge of the ruler and divide by 10 to get a cm reading with two decimal places. A reading of 5.4cm is ambiguous โ it could be anywhere from 5.35cm to 5.44cm, spanning two different UK half-size categories.
Many people who measure in centimetres assume that because they are using a different unit, the rules around measurement timing are different โ they are not. Finger size varies throughout the day in centimetres exactly as it does in millimetres: fingers are typically 0.05โ0.15cm smaller in the morning than in the evening, which translates to one or two UK half sizes. Always measure in the late afternoon or evening for the most representative reading. Similarly, cold temperatures shrink fingers by up to 0.1cm of circumference, and heat expands them by a similar amount. A CM measurement taken in a cold room in January may be 0.05โ0.1cm smaller than the same finger measured in a warm room in July โ a difference that spans one to two UK half sizes. Measure at normal indoor room temperature and at the correct time of day regardless of whether you are recording in cm or mm.
When measuring an existing ring, using digital calipers to measure the internal diameter in centimetres is significantly more accurate than the paper strip method for measuring circumference. This is because the diameter of a rigid metal ring is stable โ it does not change with how you wrap the paper or how tightly you pull the string. Digital calipers measure to 0.01cm (0.1mm) accuracy, whereas a standard ruler typically achieves 0.05cm (0.5mm) at best. To measure with calipers: open the jaws, insert them into the ring bore at the widest point, close until they touch the inside edge gently, and read the cm value. Measure twice in perpendicular directions and average the two readings. The average cm diameter reading can then be used directly in the diameter column of the conversion chart, or converted to circumference by multiplying by 3.14159. Digital calipers suitable for ring measurement are available online for ยฃ5โยฃ15 and are a worthwhile investment for anyone buying multiple rings.
A common misconception is that measuring in mm is more accurate or more "official" than measuring in cm for ring sizing. This is not true โ the unit of measurement makes no difference to the accuracy of the result, provided you record the reading to the appropriate number of decimal places. 5.38cm and 53.8mm are exactly the same measurement. UK ring size charts are traditionally published in mm in the UK, but using cm is equally valid and produces the same UK size letter when the correct number of decimal places is used. The only practical risk with cm is that beginners may round to one decimal place (5.4cm) and lose the precision needed to distinguish between adjacent UK half sizes โ which is why two decimal places are always recommended when working in cm. If you feel more comfortable working in mm, simply multiply your cm reading by 10 and use a standard mm chart.
Whether you measure in cm or mm, the wide band adjustment applies equally. A ring band 6mm (0.6cm) or wider sits differently on the finger than a narrow band and will feel significantly tighter at the same nominal UK size. The standard jeweller's rule is: for every 2mm (0.2cm) of band width above 4mm (0.4cm), add half a UK size to your measured size. If your cm circumference measurement gives you UK N and you are ordering a 6mm wide band, order UK Nยฝ. If ordering an 8mm band, order UK O. This adjustment is independent of whether you measured in cm or mm โ it is based on the physical reality of how a wider band interacts with the finger. Forgetting the wide band adjustment is one of the top reasons for ring returns from first-time buyers, and it applies identically regardless of which measurement unit was used to find the base size.