Complete 2026 ring size guide with diameter and circumference in both centimetres (cm) and millimetres (mm) โ UK AโZ+4 full chart with US, EU, French & Japanese conversions, key formulas, calculator and expert measurement tips.
BS EN 28653 and ISO 8653 define all ring sizes in mm. Every jeweller, caliper and online ring sizer uses mm as the primary unit.
Every standard ruler and tape measure shows centimetres โ making home measurement easier and less prone to misreading long decimals.
๐ก Best workflow: Measure at home in cm with a standard ruler โ multiply by 10 to convert to mm โ use mm in any chart or calculator. Example: 5.12cm ร 10 = 51.2mm = UK L.
Measure in the afternoon at room temperature (18โ22ยฐC). Fingers shrink up to a full ring size in cold conditions and swell in heat. Afternoon gives the most wearable everyday fit.
Cut a strip about 8mm wide and 12cm long with clean edges. Thin printer paper works well. Avoid thick card โ it adds false width to your reading.
Wrap snugly (not tightly) around the base of your ring finger. Mark the overlap point with a pen. Keep the strip flat โ no twisting. It should move slightly when wrapped correctly.
Lay the strip flat against a standard ruler. Read to two decimal places in centimetres โ e.g. 5.12 cm. Most school rulers clearly show cm with mm sub-markings.
Multiply your cm reading by 10: 5.12cm ร 10 = 51.2mm. Find 51.2mm in the Circ. MM column of the chart below โ that is UK size L. Or enter it in the calculator above.
Measure morning, afternoon and evening. Record all three in cm or mm. Use the largest reading as your ring size โ a ring that fits when fingers are slightly swollen will always be comfortable.
| ๐ฌ๐ง UK | ๐ Diam. MM | ๐ Diam. CM | ๐ Diam. Inch | โญ Circ. MM | โญ Circ. CM | โญ Circ. Inch | ๐บ๐ธ US | ๐ฉ๐ช EU | ๐ซ๐ท French | ๐ฏ๐ต Japan | Category |
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BS EN 28653 and ISO 8653 both define ring sizes in millimetres. When ordering from any jeweller worldwide, quoting your inner circumference in mm eliminates all sizing system ambiguity.
The EU ring number is literally the inner circumference in mm โ so EU 51 = 51mm C = 5.1cm = UK L. No lookup needed: your circumference reading in mm is your EU size directly (ISO 8653:2016).
A digital caliper (under ยฃ8) measures inner ring diameter to 0.1mm โ far more precise than a paper strip. Measure an existing ring, divide by 10 for cm, then use the calculator above to find your UK size instantly.
Finger diameter changes by up to 0.5mm (0.05cm) between morning cold and afternoon warmth โ equal to one full UK letter size. Always measure at room temperature in the afternoon for the most accurate wearable reading.
If your cm or mm measurement falls between two sizes, always choose the larger size. Adjacent UK half-sizes are only ~0.2mm (~0.02cm) apart in diameter โ slightly loose is always more comfortable than tight for daily wear.
When ordering from an international jeweller, state your inner circumference in mm with the cm equivalent โ e.g. "51.2mm / 5.12cm circumference = UK L / EU 51 / US 6." This removes all risk of size letter misinterpretation.