Ring Size Converter

The ring size converter translates ring sizes between UK letter sizes, US numerical sizes, EU millimetre sizes, and other international standards. It is used by people buying rings online from international jewellers, couples planning engagements, and anyone who knows their size in one system but needs to order in another. Enter your size or finger circumference to find the equivalent in any standard.

How to Use the Ring Size Converter

1. Enter your ring size in your known system: UK (A to Z+), US (3 to 13), EU (circumference in mm), or Japanese numerical.
2. Alternatively, enter your finger's circumference in millimetres for the most precise result.
3. Click Convert to see equivalents in all major sizing systems at once.
4. If you are between sizes, the calculator will indicate this and suggest rounding up slightly for comfort.
5. Use the sizing guide to measure your finger if you do not yet know your ring size.

The Formula

Ring sizes are based on the internal circumference or diameter of the ring:

Circumference (mm) = pi * Internal Diameter (mm)

UK sizes progress in half-size increments corresponding to 1.25 mm of circumference per letter. US sizes progress in 0.4 mm increments of diameter. EU sizes are the internal circumference in millimetres:

EU size = Internal circumference in mm

Approximate conversions: UK size H = US size 3.75 = EU 47 (approximately) UK size L = US size 6 = EU 51.5 (approximately) UK size P = US size 7.75 = EU 56 (approximately)

Real-World Example

Convert a UK size M to US and EU sizes.

1. UK size M has an internal circumference of approximately 52.5 mm
2. Internal diameter = 52.5 / pi = 16.71 mm
3. EU size = 52.5 (EU size equals circumference in mm)
4. US size = approximately 6.5

A UK size M ring is EU 52.5 and approximately US size 6.5. When ordering from an American jeweller, asking for size 6.5 is correct; from a European jeweller, specify 52.5 mm or EU 52.

How to Measure Your Ring Size at Home

If you do not know your ring size, you can measure it with a strip of paper or a piece of string. Wrap it snugly around the base of the finger you want to wear the ring on, marking where it overlaps. Measure the length in millimetres. This is your circumference, which directly gives your EU ring size and can be used to look up your UK, US, or Japanese equivalent in a size chart. For best accuracy, measure in the evening when fingers are slightly larger due to warmth, and avoid measuring when hands are cold or swollen. Measure multiple times to confirm the result.

Frequently Asked Questions

Why do ring sizes vary between countries? Ring sizing evolved independently in different countries, leading to separate systems based on letters (UK), numbers (US), or direct measurements (EU/ISO). There is no single international standard, though the ISO 8653 standard provides a millimetre-based system that most conversion tables now reference.

How much does finger size change during the day? Fingers can vary by half a size or more between morning and evening, and also with temperature, hydration, and physical activity. A ring purchased to fit a cold, morning finger may feel tight after a warm afternoon. Measuring several times at different times of day gives a more reliable average.

Should I size up or down if I am between sizes? Size up slightly for comfort, particularly for rings you will wear continuously such as wedding bands. Wide rings (over 6 mm) also require a slightly larger size than narrow rings because they grip the finger more firmly.

Can a ring be resized if I order the wrong size? Most plain gold, silver, or platinum rings can be resized by a jeweller. Rings with stones set all around (eternity rings) or those made from alternative metals like titanium or tungsten cannot easily be resized and require more careful sizing before purchase.

Why every chart starts from a circumference

Every competent ring size chart is built on the same measurement, and it is a circumference rather than a letter or a number. ISO 8653, the international standard for ring sizes, defines the size of a ring as the length of the inner circumference measured in millimetres. The 1986 edition set that definition, and the current edition keeps it.

That choice of measurement is the reason conversion is possible at all. A circumference is a physical property of the ring, so it survives a change of country, a change of jeweller and a change of decade. A letter and a number are labels that different markets assigned to the same physical sizes at different times, which is why a United Kingdom letter, a United States number and a Japanese number can describe one ring without agreeing on anything else.

Measure a finger in millimetres and you already hold the ISO designation. Everything else on the chart is a translation of that one number.

Converting one measurement into every system

The table below takes a measured inner circumference and derives the diameter from it, then reads the other systems off the published relationships. The diameter is the circumference divided by pi. The European continental systems used in Italy, Spain and Switzerland take the circumference and subtract 40.

Two United States columns appear because two published linear fits are in common use, and they disagree at the second decimal. One expresses the US size in terms of the circumference, the other in terms of the diameter. Neither is wrong; they are separate regressions through the same published size chart, and the gap between them is under a tenth of a millimetre of metal.

The United Kingdom column is built the same way. Published charts put size C at 40 mm of inner circumference, and the letters step by 1.25 mm from there, so a measurement 10 steps above C is M. That is the 1.25 mm per letter the formula section above quotes, and it comes out at 1.252 mm when measured from A to Z across a full published chart.

Placing the approximate conversions above

The three conversions printed in the formula section of this page are labelled approximate, and the label is doing real work. Checked against the published fits, each one is close and none is exact.

UK P, which the page prints as US 7.75 against EU 56, comes out at 7.765 on the same fit, so that one is right to two decimal places.

None of these differences matters when the answer is used to pick a ring from a chart. All of them matter when the answer is used to order a ring made to measure, because a quarter of a size is a real gap between a band that turns on the finger and one that does not. Where a jeweller works to a tolerance tighter than half a size, give the circumference in millimetres and let the maker work from that rather than from a converted letter or number.

Reconciling the two fits

The two United States fits are worth stating explicitly, because a reader who uses both on one measurement will find they return different answers.

The circumference fit reads: circumference in millimetres equals 2.55 times the US size, plus 36.5. The diameter fit reads: diameter in millimetres equals 0.8128 times the US size, plus 11.63.

At a US size of 6.5 the circumference fit gives 53.075 mm and the diameter fit gives 16.9132 mm of diameter, which is 53.134 mm of circumference once pi is applied. The two differ by 0.0594 mm, which is under a sixteenth of a millimetre.

Both fits are straight lines fitted through a curved reality. Round a measured circumference to the nearest whole millimetre before converting, and choose one fit and stay with it, so that a finger measured twice gives one answer rather than two. The diameter of a ring is the harder measurement to take at home in any case, because it requires a ring that already fits.

Width, temperature and the size to order

The size a chart returns is a starting point, and three things move it in practice.

Width matters more than most charts admit. A band wider than about 6 mm contacts more of the finger and grips harder, so it fits tighter than a narrow band of the same internal circumference. A reader moving from a 2 mm band to an 8 mm band should expect to go up about half a size.

Temperature and time of day move a finger by half a size or more. Fingers are smallest in the morning and in the cold, and larger after exercise or in the heat. Measure in the afternoon or evening, at a normal body temperature, and measure more than once on different days.

Then there is the sizing decision itself. Size up when between sizes if the ring will be worn continuously and the metal cannot be resized. Plain gold, silver and platinum bands can usually be altered later. Eternity rings set all the way round, and rings in titanium or tungsten, generally cannot, so the measurement matters more for those than for anything else on the bench.

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