International ring size conversion: US, UK, EU, Japan and Korea

International ring size conversion is not one universal table. Different systems name the same physical ring by its inner diameter, its inner circumference, or a letter position. The reference conversion used by Mesurix is 17.32 mm inner diameter = 54.4 mm circumference = US 7 = UK N 1/2 = EU/ISO 54 = JP/KR 14 = FR 14.

That row is useful because it keeps the physical measurement beside every label. A number without its system can mislead you. French 14 and Japanese 14 look identical but come from different calculations. Korean 14호 can also require a second check because some shops use the number for a Japanese-style size and others use it for circumference in millimetres.

An inner diameter is the straight distance across the inside of the band through its centre. An inner circumference is the distance around the inside of the band. The systems below either state one of those measurements directly or encode it in a number or letter.

Which ring-size scale does the seller mean?

The four systems you are most likely to meet are the US/Canada number scale, the UK-style letter scale, the European millimetre scale and the Japanese 号 scale. France and Switzerland use a related circumference calculation. Korean listings can use either the Japanese-style 号 number or a circumference in millimetres.

The country shown on a shop page is not enough to identify the scale. An international marketplace may show the seller’s local size, a platform conversion, or a label copied from another product. The physical measurement in millimetres is the reliable anchor. Ask for that measurement when the listing says only “Asian size”, “European size” or “Korean size”.

Use the international ring size chart when the seller has supplied a recognised size. Use a millimetre measurement instead of guessing when the listing does not name its system. A seller who gives the inner diameter or inner circumference lets you check the conversion rather than trusting a second rounded label.

Mesurix can also help establish the physical measurement on a phone. ISO/IEC 7810 ID-1 cards measure 85.60 mm x 53.98 mm. That standard covers bank cards, many national ID cards, driving licences and transport cards, making a card a practical calibration reference. It is the most widely distributed physical length standard in the world.

A browser cannot know the physical size of its own screen pixels. A CSS pixel is defined against a 96 dpi reference display, not against the actual panel in your phone. Mesurix’s ring sizer fixes that missing scale with a one-off calibration against a bank card. The browser tool is free, needs no account, runs entirely in the page, uploads nothing and stores the calibration as one number in localStorage.

How do US and Canadian ring sizes convert?

The US and Canadian scale uses a number calculated from the ring’s inner diameter. Mesurix publishes the formula:

diameter (mm) = 11.63 + 0.8128 x size

For US 7, the calculation is 11.63 + 0.8128 x 7 = 17.32 mm. That is the inner diameter, not the diameter across the outside of the band and not a 7 mm measurement.

One whole US size changes the inner diameter by 0.8128 mm. The corresponding change in circumference is about 2.55 mm. A small-looking change on the screen therefore changes the fit around the finger enough to matter, especially when the result is close to a size boundary.

A US 7 is therefore 17.32 mm across the inside and 54.4 mm around the inside in the reference row. The matching labels are EU/ISO 54, UK N 1/2, JP/KR 14 and FR 14. Keep the millimetres with the converted size when the order crosses a border.

The ring size calculator is useful when a seller gives a diameter or circumference instead of a US number. Enter the original physical measurement first. Converting a rounded US label into another rounded label can hide the difference that decides whether the ring clears the knuckle.

Your finger does not have one fixed size throughout the day. Fingers are largest late in the day and after heat, exercise or salty food. They are smallest on a cold morning. The spread across a day is easily a quarter of a US size. Measure at a normal, warm time and repeat the reading before ordering.

How do UK, Irish, Australian and New Zealand sizes convert?

The UK-style system uses letters and half-letter steps. It is used in the United Kingdom, Ireland, Australia and New Zealand. The letter is a position in a sequence, not a millimetre measurement printed directly on the ring.

Mesurix publishes the formula:

circumference (mm) = 37.14 + 1.28 x s

Here, s counts half-letter steps from A. Under this scale, US 7 is UK N 1/2. The half step is part of the size. N 1/2 should not be silently changed to N or O when you copy the result into an order.

The word “approximate” belongs in the table because the displayed circumference is tied to the reference conversions and practical size labels. The underlying formula still uses the stated half-letter position. For an important order, ask the seller for the ring’s inner diameter or inner circumference as well as the UK letter.

A conversion can land close to a half step if the original measurement was rounded. That is a reason to preserve the original millimetre value, not a reason to remove the half step. Write “UK N 1/2, 54.4 mm circumference” rather than “UK N” and leave the seller to infer what you meant.

The how to measure ring size at home guide can help you get a starting measurement. The ring still has to pass over the knuckle. Between two UK-style sizes, choose the larger one. A loose ring can be tightened with sizing beads or a fitted bar. A ring that will not clear the knuckle cannot be worn.

Band width changes the choice even when the finger measurement is sound. The trade rule of thumb is about a quarter size up from 6 mm of band width and about a half size up from 8 mm. A wide band does not fit like a narrow band with the same nominal size. Check the seller’s guidance and the wide-band ring size guide before placing the order.

What does EU/ISO ring size 54 mean?

The EU/ISO scale expresses ring size as the inner circumference in millimetres. ISO 8653 defines that convention. EU/ISO 54 therefore means a 54 mm inner circumference in the size system, and it corresponds to US 7 in the reference conversion.

The calculated reference row also gives 17.32 mm inner diameter and 54.4 mm circumference. Those two statements serve different purposes: EU/ISO 54 is the practical ISO size label, while 54.4 mm is the circumference calculated from the reference diameter. Do not replace the physical measurement with a bare number whose system has been omitted.

“European size” can still be ambiguous on a cross-border product page. France and Switzerland use a related convention but subtract 40 from the circumference in millimetres. French 14 therefore corresponds to the same 54 mm circumference represented by EU/ISO 54. Check whether the seller means EU, ISO 8653, French or Swiss before entering a number.

A bank card gives Mesurix a physical reference for the phone screen. ISO/IEC 7810 ID-1 fixes the card at 85.60 mm x 53.98 mm. The browser’s CSS pixel definition uses a 96 dpi reference display, so it cannot reveal the real physical size of pixels on your particular panel. Card calibration supplies that scale.

After calibration, the arithmetic is exact. The measurement itself can still be wrong if the ring is not centred, if the finger is measured at an unrepresentative time, or if a strip of material has stretched. A calibrated screen is a measuring reference, not a jeweller’s steel mandrel.

For an international order, keep the original unit in your notes. “EU/ISO 54, 54 mm circumference” is safer than “European 7”, because another shop may read 7 as a US size. Compare the seller’s stated millimetre figure with the ring size chart before paying.

How do French and Swiss ring sizes work?

French and Swiss ring size equals the inner circumference in millimetres minus 40:

French or Swiss size = circumference (mm) - 40

French size 14 therefore means 54 mm - 40 = 14. It corresponds to EU/ISO 54 and US 7 in the reference row. Switzerland uses the same minus-40 convention.

The calculation works in both directions. Add 40 to a French or Swiss size to recover the circumference: FR 14 + 40 = 54 mm. Subtract 40 from a known circumference to obtain the local size: 54 mm - 40 = 14.

French 14 is not Japanese 14. French 14 comes from 54 mm circumference minus 40. Japanese-style 14 comes from the diameter formula and corresponds to 17.32 mm inner diameter. The matching labels happen to be the same number in the reference row, but their systems are not interchangeable.

Some French and Swiss product pages show an EU conversion beside the local size. Use the stated local convention as the source and the second value as a check. If the two entries disagree, ask for the ring’s inner circumference in millimetres before paying.

The complete reference row is 17.32 mm inner diameter = 54.4 mm circumference = US 7 = UK N 1/2 = EU/ISO 54 = JP/KR 14 = FR 14. Keeping both the local label and the physical measurement prevents a marketplace from changing “14” into the wrong system later.

How do Japanese and Korean ring sizes work?

Japan uses the 号 scale. The same Japanese-style scale is used in Korea and China for listings that follow this convention. Mesurix publishes the formula:

diameter (mm) = 13 + (n - 1) / 3

The reverse calculation is:

n = 3 x diameter - 38

For US 7, the reference diameter is 17.32 mm. Substituting that diameter gives JP 14. The corresponding reference labels are US 7, UK N 1/2, EU/ISO 54 and FR 14, with 54.4 mm calculated inner circumference.

A Japanese size is a number derived from inner diameter. It is not automatically a circumference in millimetres. JP 14 means the Japanese-style size in the table; it does not mean a 14 mm ring or a 14 mm circumference.

Korean ring listings need a separate check. Korean sellers often quote the same 호 numbers as the Japanese 号 scale. Korean 14호 can therefore correspond to JP 14 and US 7. Some Korean shops instead use 호 for the inner circumference in millimetres. The number alone does not resolve the listing.

Look for a millimetre figure next to the Korean number. If the page gives 14호 and a diameter near 17.32 mm, it is using the Japanese-style number. If it gives a circumference in millimetres, follow that stated measurement instead. Ask the seller directly if the page does not say which convention applies.

Product descriptions, size selectors and customer-service replies can use different labels on the same shop. Check the exact ring listing, not only the shop’s country. Request “inner diameter in millimetres” or “inner circumference in millimetres” in your message.

China can also use the Japanese-style 号 scale in the context covered here, but Chinese marketplaces may place converted values beside local product measurements. When a Chinese or Korean listing does not identify the system, ask for the inner diameter in millimetres. One physical measurement is easier to verify than a translated size name.

This check matters most when the ring is expensive or non-returnable. Confirm the converted size with the seller and, where possible, with a jeweller before ordering. A phone screen or paper measurement is not the same as a jeweller’s steel mandrel.

String and paper-strip methods have a specific failure mode: the material can stretch or compress. Depending on how hard you pull, the result can read a quarter to a half size high or low. A sheet of 80 gsm office paper is about 0.10 mm thick. A human hair is about 0.07 mm thick. One whole US size is 0.8128 mm of diameter, roughly eight sheets of paper.

That comparison explains why a small handling error matters. Use a calibrated on-screen sizer or a ring that already fits, repeat the measurement late in the day, and record the millimetre result alongside the converted label.

What should you ask an international ring seller?

Send this question before ordering:

Which ring-size system is this, and what are the ring’s inner diameter or inner circumference in millimetres?

The wording forces the seller to identify whether the listing uses US/Canada, UK-style, EU/ISO 8653, French or Swiss minus 40, Japanese-style 号, or Korean circumference 호. It also asks for the measurement that can be checked without relying on a country label.

A useful reply might say “US 7, 17.32 mm inner diameter” or “EU/ISO 54, 54 mm inner circumference”. A Korean seller should say whether 14호 is the Japanese-style number or a circumference in millimetres. A French or Swiss seller should confirm that size 14 means circumference minus 40.

Copy the reply into the order notes with both the local size and the physical measurement. For the reference row, write: “17.32 mm inner diameter, 54.4 mm circumference, US 7, UK N 1/2, EU/ISO 54, JP/KR 14, FR 14.” That record makes a later marketplace conversion easier to spot.

Measure more than once. Fingers are largest late in the day and after heat, exercise or salty food, and smallest on a cold morning. Between two sizes, choose the larger one because the ring must clear the knuckle. A loose ring can be tightened with sizing beads or a fitted bar; a ring that cannot pass the knuckle cannot be worn.

Mesurix’s free browser tools need no account and run entirely in the page. Nothing is uploaded, and the calibration is one number in localStorage. The iPhone and Android app adds saved sizes and profiles, every standard on one screen, finger-measure mode, a printable sizer, the on-screen ruler and a bubble level with a set-zero step. It works offline, and the iPhone version requires iOS 13 or later.

Use the ring sizer or ring size calculator to work from a physical measurement, then keep that measurement beside the seller’s size. The arithmetic can be exact after calibration, but the answer is only as good as the ring or finger measurement. For an expensive or non-returnable ring, confirm the result with the seller and a jeweller before ordering.