Free Ring Size Converter for UK, US, EU and Japan Sizes
A ring size converter helps you translate a ring size from one regional system into the nearest equivalent used somewhere else. This is useful when a UK shopper buys from an American jeweller, a US customer orders from Europe, or a ring is listed only by inside diameter or circumference.
The important idea is that the size label can change while the physical ring opening stays almost the same. UK sizing uses letters, US and Canada use numbers, many European references are linked closely to circumference in millimetres, and Japan uses a separate numerical scale.
This converter uses inside circumference as the common measurement. A selected ring size, diameter or circumference is converted to an approximate physical opening, then matched to the nearest value in each sizing system.
Convert Your Ring Size Instantly
Choose the information you already have. The result shows the nearest UK, US/Canada, EU and Japan sizes.
The converter returns nearest equivalents because national systems do not share identical increments. For an expensive, custom or difficult-to-resize ring, confirm the final result with the retailer or a professional finger gauge.
The converter does not simply replace one label with another. It first reduces the input to a physical measurement. If you choose a regional size, the tool looks up its approximate inside circumference. If you enter diameter, it calculates circumference using π. If you enter circumference directly, that becomes the reference measurement.
Step 1 — Identify the inside circumference
Inside circumference describes the distance around the empty inner edge of the ring. It is useful as a common reference because size systems can be compared against the same physical opening.
Step 2 — Match each regional system
The calculated circumference is compared with the available UK, US/Canada, EU/ISO-style and Japan values. The closest measurement becomes the displayed equivalent.
Step 3 — Show diameter and inches
The same circumference is also converted to inside diameter and inch measurements. These values help when a retailer publishes a chart using physical dimensions rather than the sizing system you already know.
UK / Ireland
British sizing is letter based, commonly using sizes such as J, K, L, M, N and half sizes such as M½.
US / Canada
North American ring sizes use numbers. Full, half and sometimes quarter sizes may be offered by jewellers.
EU / ISO-Style
Many continental European references use size values closely related to the ring's inside circumference in millimetres.
Japan
Japanese ring sizing uses its own numerical scale, so the numbers should not be confused with US sizes.
Current European standardization also supports a measurement-led approach. CEN-CENELEC describes EN ISO 8653:2026 as an international and European reference for ring sizing, while ISO 8653:2016 specifies methods for measuring and designating ring size using standardized tools.
If you want a long row-by-row comparison instead of a quick conversion result, use the Ring Size Table Converter. That page is designed around the complete chart, while this page is optimized for fast interactive conversion.
Different labels can describe nearly the same physical ring
The converter works by translating every system back to a common inside measurement.
Start With What You Know
Use a regional ring size, inside diameter or inside circumference.
Create One Reference Measurement
The tool reduces the input to an approximate inner circumference in millimetres.
Find the Nearest Labels
The physical measurement is matched to the closest size in every supported system.
If you do not know any regional size, an existing ring that already fits can provide a useful starting measurement. Measure the inside diameter, not the outside of the band. The metal thickness must not be included because the converter is trying to identify the space where the finger sits.
GIA recommends professional finger gauges for accurate sizing and warns that improvised string or paper methods can introduce error because string can stretch and paper can distort. For a valuable ring, a jeweller's gauge or mandrel is the better final check.
Choose the right ring
Use a ring that fits the same finger and hand intended for the new ring.
Measure inside edge to inside edge
Measure straight across the empty centre at the widest internal point.
Use millimetres
Small ring-size changes are easier to distinguish with precise millimetre measurements.
Enter the result above
Select the diameter tab and let the ring size converter calculate the nearest international sizes.
Ring measurement formulas
Both measurements describe the same internal ring opening
Use whichever value you can measure accurately; the converter calculates the other.
Inside Diameter
Measure straight across the empty centre from one inner edge to the opposite inner edge.
Inside Circumference
This is the distance around the internal opening and acts as the converter's common reference.
Convert Between Them
Multiply diameter by π to estimate circumference, or divide circumference by π to estimate diameter.
Convert UK ring size to US
Choose UK / Ireland, select your letter size and press convert. The tool maps the British size to its approximate inner circumference and selects the nearest US/Canada number. A UK size near N, for example, sits around the mid-54 mm circumference range and therefore falls close to a North American size around 7.
Convert US ring size to UK
Select US / Canada and choose your numerical size. The converter then returns the nearest British letter or half-letter size. This is useful when a US jeweller lists a ring numerically but you normally shop using UK letters.
Convert EU ring size to UK or US
European circumference-style values can be particularly convenient because they are closely linked to a physical millimetre measurement. An EU-style size around 54 represents an inside circumference around 54 mm, which can be compared directly with the nearest UK and US sizes.
Convert ring size from 17.2 mm diameter
A 17.2 mm inside diameter gives an estimated circumference of about 54.0 mm. Enter 17.2 in the diameter mode to see the nearest UK, US, EU and Japan equivalents.
A ring size converter can be very useful for translating between systems, but it cannot remove every source of fitting uncertainty. The conversion itself may be close while the final ring still feels different because of band width, internal profile, finger shape or temporary swelling.
Regional increments are different
One sizing system may offer a step exactly where another system does not. The converter therefore chooses the nearest available measurement, which can make two legitimate conversion charts differ slightly.
Retailer charts may round differently
A jeweller may publish circumference to a whole millimetre while another uses decimal values. Use the converter for comparison, then follow the seller's own chart for the final commercial size.
Professional measurement is still the safest final check
ISO 8653:2016 describes standardized measurement and designation, and GIA recommends proper finger-sizing gauges for accurate consumer fitting. That is especially important when a ring is custom made, expensive or difficult to resize.
The nominal conversion stays the same
A converted size describes the approximate inner opening. Changing band width does not alter the mathematical conversion between regional labels.
The actual fit can still change
A broad band covers more finger area and can feel snugger than a narrow ring with the same nominal size.
If the ring is wider than a slim everyday band, do not rely on the converted number alone. Use the Ring Size Guide for Wide Bands to compare width, fit profile and practical sizing considerations before ordering.
- Confirm which sizing system the seller uses. A number without a country or system label can be misleading.
- Convert your known size. Use the interactive tool to find the nearest seller-system equivalent.
- Compare physical measurements. Diameter and circumference provide an extra cross-check when two charts disagree.
- Read the retailer's own sizing guide. It should take priority for the final order because commercial increments can differ.
- Check whether the ring is wide or comfort fit. These design details can change how the nominal size feels.
- Review resizing and returns. International orders, eternity rings and some materials can be costly or difficult to alter.
If you specifically need British sizing from a measurement, use the UK Ring Size Calculator. For a full row-by-row international chart, use the Ring Size Table Converter.
Use Inside Measurements
Outside diameter includes metal thickness and will produce an incorrect conversion.
Check More Than Once
Repeat a home measurement instead of relying on a single quick reading.
Avoid Extreme Temperature
Cold fingers can be smaller while heat and activity can increase swelling.
Measure the Correct Hand
The same finger on the opposite hand can have a different circumference.
Confirm Valuable Rings
Use a jeweller's finger gauge before buying an expensive or custom ring.
Consider Band Width
Wide rings can feel tighter even when the nominal conversion is correct.
Compare the Seller Chart
Retailer-specific rounding should guide the final size you add to the basket.
Check Exchange Rules
A clear size exchange policy reduces the risk of cross-border jewellery orders.
The page uses a measurement-first conversion approach and treats international outputs as nearest equivalents. For measurement and standardization guidance, see:
- GIA 4Cs — How to Determine Ring Size
- GIA — Accurate Determination of Finger and Ring Sizes
- ISO — ISO 8653:2016 Jewellery — Ring-sizes
- CEN-CENELEC — EN ISO 8653:2026 Ring Size Reference