How to Read a Ring Size Chart in MM
A Ring Size Chart in MM compares ring sizes using the physical dimensions of the empty opening inside the ring. The two measurements you will see most often are inside diameter in millimetres and inside circumference in millimetres.
Diameter is the straight-line distance across the internal opening from one inner edge to the opposite inner edge. Circumference is the distance all the way around that same internal opening. Because both describe the same circle, one can be calculated from the other using π.
Millimetres are especially useful because ring-size differences are small. A few tenths of a millimetre can move the result toward the next UK half size, which is why a precise ruler, caliper or professional ring gauge is more useful than a rough visual estimate.
Convert Millimetres to a UK Ring Size
Enter either inside diameter or inside circumference in millimetres.
The result is the nearest value in this chart. Retailer charts can round slightly differently. For a valuable, custom, unusually wide or difficult-to-resize ring, confirm the final size with the seller or a professional finger gauge.
| UK Size | Inside Diameter (mm) | Inside Circumference (mm) | Diameter (in) | US / Canada | EU Approx. |
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Chart note: UK measurements are shown to practical consumer precision. International values are nearest equivalents and may differ slightly from a retailer's own conversion table.
Inside diameter
Best when you already have a ring that fits. Measure straight across the empty opening at its widest internal point. Do not include the metal band.
Inside circumference
Describes the distance around the ring's inner edge. It is a useful common reference for comparing ring-size systems and for ISO-style sizing.
GIA's international ring-size chart presents both inside diameter in millimetres and inside circumference in millimetres, which makes mm measurements particularly useful when converting between UK and international sizes. ISO 8653 also defines standardized methods for ring-size measurement and designation.
Two mm measurements, one internal ring opening
The diameter runs straight across the opening; the circumference travels around it.
Diameter
Measure the empty inner opening from edge to edge at the widest point.
Circumference
Measure or calculate the distance around that same internal opening.
Convert Between Them
Multiply diameter by π for circumference, or divide circumference by π for diameter.
Millimetre sizing is most useful when the measurement itself is accurate. If you are measuring an existing ring, use a ring that already fits the exact hand and finger intended for the new ring.
Choose a correct reference ring
Use a ring worn comfortably on the same finger and hand.
Measure the inside diameter
Measure from inner edge to inner edge. Do not measure the outside of the metal.
Use millimetres and decimals
Record the result precisely rather than rounding to the nearest whole millimetre.
Repeat the reading
Rotate the ring and measure again. If the opening is not perfectly round, a professional measurement is safer.
Use the nearest chart row
Compare the reading with the UK ring size chart in mm or enter it into the calculator above.
Confirm high-risk purchases
For expensive, custom, wide or difficult-to-resize rings, use a jeweller's physical sizing gauge.
Outside diameter produces the wrong ring size
The chart uses the empty internal opening only.
Correct
Measure only the empty inner space where the finger sits.
Incorrect
Outside diameter includes metal thickness and makes the ring appear too large.
Use Precision
A caliper or fine millimetre scale makes small differences easier to read.
15.9 mm Diameter
Close to UK K in this chart, with an internal circumference around 50.0 mm.
16.7 mm Diameter
Close to UK M, with an internal circumference around 52.5 mm.
17.1 mm Diameter
Close to UK N, with an internal circumference around 53.8 mm.
18.3 mm Diameter
Close to UK Q, with an internal circumference around 57.6 mm.
19.1 mm Diameter
Close to UK S, with an internal circumference around 60.2 mm.
20.0 mm Diameter
Close to UK U, with an internal circumference around 62.7 mm.
54 mm Circumference
Sits near UK N–N½ and near an EU-style size around 54.
60 mm Circumference
Sits close to UK S in this chart, depending on the exact decimal measurement.
Millimetres are useful when international size labels become confusing. Instead of comparing only UK letters and US numbers, compare the physical opening behind each label. A ring around 53.8–54.0 mm circumference, for example, sits near UK N and near EU 54, while the nearest US/Canada value is around the upper-6 to 7 range depending on the chart.
International conversions are not perfectly one-to-one because regional systems use different size increments. For a dedicated conversion tool, use the Ring Size Converter. For a full international table, use the Ring Size Table Converter.
The millimetre measurement describes the nominal inside opening, but two rings with the same internal diameter can feel different when one is much wider. A broad band covers more of the finger and may feel more snug.
GIA recommends using wide-finger sizing gauges for rings wider than 4 mm rather than relying only on a narrow sample. If you are buying a 6 mm, 8 mm or 10 mm ring, use the mm chart to identify the nominal size and then confirm the fit with a gauge close to the final width.
See the Ring Size Guide for Wide Bands for a dedicated width-focused fitting guide.
A good ring size chart in mm is useful for narrowing the correct size, but small differences between published charts are normal. One source may list a diameter to one decimal place while another uses two; another may round circumference to a whole millimetre for an EU-style size.
Measurement precision matters
A ring reading of 17.05 mm and one reading 17.35 mm are visually very similar on a ruler but can point toward different nearby UK sizes. Record decimals wherever possible.
The ring must be round
GIA notes that determining size from inside measurement works when the ring is perfectly round. A bent or distorted ring should be professionally checked rather than forced into a chart.
Standards improve consistency, but retailer charts still matter
ISO 8653:2016 specifies methods for measuring and designating ring sizes with a defined ring stick and corresponding finger gauges. In Europe, EN ISO 8653:2026 provides a current shared reference for ring-size definition, measurement and designation. For the final order, still compare the jeweller's own sizing chart and available increments.
- Measure an existing ring or finger carefully. Use the inside opening, not the outside metal.
- Keep the raw mm value. Do not round too early; write down the actual decimal measurement.
- Use this chart to find the nearest UK size. Compare both diameter and circumference where possible.
- Check the seller's own chart. Their available sizes and rounding should guide the final order.
- Account for wide bands. A correct nominal diameter can still feel snug in a broad ring.
- Review resizing and exchange policies. This reduces risk when buying custom or difficult-to-resize jewellery.
If you want a tool focused specifically on British sizing, use the UK Ring Size Calculator. Couples planning engagement or wedding rings can also use the Ring Size Guide for Couples.
The chart and guidance use a measurement-first approach. Useful professional and standards references include:
- GIA 4Cs — Ring Size Chart with Diameter & Circumference in MM
- GIA — Accurate Determination of Finger and Ring Sizes
- ISO — ISO 8653:2016 Jewellery — Ring-sizes
- CEN-CENELEC — EN ISO 8653:2026 Ring Size Reference