Ring size converter: calculate every size from millimetres

A ring size converter is only useful after the number has been identified correctly. A shop’s “54”, “14” or “N 1/2” may describe an inner circumference, an inner diameter, or a position in a national scale. Put the number into the wrong formula and the result can be wrong before any rounding begins.

Start with the physical measurement in millimetres. The most useful reference is 17.32 mm inner diameter, equal to 54.4 mm inner circumference: US 7, UK N 1/2, EU/ISO 54, JP/KR 14 and FR 14. A ring sizer can give you that starting measurement from a phone screen. You can also measure a ring you already own, provided the measurement is taken across the inside opening rather than across the outside of the band.

The browser tools are free, need no account and run entirely in the page. Nothing is uploaded. Calibration is stored as 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 is free, works offline and supports iOS 13 or later.

Diameter or circumference: which number does a ring listing mean?

Diameter is the straight distance across the inside of a ring, through the centre of the opening. Circumference is the distance around that same inside edge. Both describe the opening, but they are not the same number and they are not used in the same way by every country’s sizing system.

A 17.32 mm inner diameter produces a 54.4 mm inner circumference because circumference equals π multiplied by diameter. A listing that says “54 mm” is therefore not describing a 54 mm diameter. It is likely giving an inner circumference or an EU/ISO size.

Look for “inside diameter”, “inner diameter”, “inside circumference”, “inner circumference” or “mm”. If the listing provides only a national label, identify the scale before doing arithmetic. “14” alone is not enough information. It could be a Japanese-style 号 number, a French size, or a Korean notation with a different local convention.

EU/ISO makes the convention explicit. Under ISO 8653, EU/ISO ring size is the inner circumference in millimetres. EU/ISO 54 means 54 mm around the inside of the ring. It does not mean a 54 mm diameter.

The US system works from diameter. Mesurix publishes the equation:

diameter (mm) = 11.63 + 0.8128 × size

Applying it to US 7 gives 17.32 mm across the inside of the band.

France and Switzerland use circumference minus 40. FR 14 means a 54 mm inner circumference, because 14 + 40 = 54. The French number is not a diameter.

Japan uses a scale number rather than simply naming the diameter:

diameter (mm) = 13 + (n − 1) ÷ 3
n = 3 × diameter − 38

The same 号 scale is used in Korea and China when those sellers follow the Japanese-style convention. JP 14 gives an inner diameter of 17.33 mm.

Korean listings need one additional check. Korean sellers often use the same 号 scale as Japan, so 14호 is commonly treated like JP 14. Some Korean shops instead use 호 for an inner circumference in millimetres. A millimetre figure beside the number, or the shop’s own size guide, tells you which convention is in use. Do not apply the Japanese equation to a Korean circumference value.

How do you convert diameter to circumference?

Multiply the inner diameter in millimetres by π. For the reference size:

17.32 mm × π = 54.41 mm

The displayed reference circumference is 54.4 mm. To convert an inner circumference to diameter, divide by π:

54.4 mm ÷ π = 17.32 mm

Keep the original measurement and delay rounding until the final comparison. If a value is rounded before it is converted, the result can move towards the wrong half size when it lies close to a boundary.

A browser cannot infer the physical size of its own pixels. A CSS pixel is defined against a 96 dpi reference display, but the actual panel can have a different physical pixel size. Calibration fixes that missing scale.

Mesurix uses a one-off comparison with an ISO/IEC 7810 ID-1 card, whose physical dimensions are 85.60 mm × 53.98 mm. This is the format used for bank cards, most national ID cards, driving licences and many transport cards. Its value as a calibration object comes from its standard dimensions and wide distribution: it is the most widely distributed physical length standard in the world.

The arithmetic after calibration is exact arithmetic applied to the entered measurement. The result cannot be better than the calibration and the measurement that produced the number. A screen or paper measurement is not the same as measuring on a jeweller’s steel mandrel. Confirm an expensive or non-returnable ring with a jeweller before ordering.

What formula does each ring-size scale use?

National ring-size systems describe the same physical opening with different labels. US and Canada use an inner-diameter equation. EU/ISO uses inner circumference in millimetres. France and Switzerland subtract 40 from circumference. The UK, Australia and Ireland use letters and half-letter steps. Japan, Korea and China use the same 号 scale when the Korean or Chinese seller follows that convention.

How is a US or Canadian ring size calculated?

Mesurix publishes this US and Canadian formula:

diameter (mm) = 11.63 + 0.8128 × size

For US 7:

11.63 + (0.8128 × 7) = 17.32 mm inner diameter

One whole US size changes the inner diameter by 0.8128 mm. In circumference, that is about 2.55 mm. A US 7.5 is 0.4064 mm larger in diameter than US 7 under the same equation, because a half size is half of 0.8128 mm.

To reverse the formula, subtract 11.63 from the measured diameter and divide by 0.8128:

(17.32 − 11.63) ÷ 0.8128 = 7.00

The result is US 7. This is useful when a shop gives an inside diameter but no US label.

What does EU/ISO 54 mean?

EU/ISO 54 means an inner circumference of 54 mm under ISO 8653. It is not an inner diameter of 54 mm and it is not a US-formula size with the number 54 inserted into the equation.

For a rough diameter equivalent, divide the circumference by π:

54 mm ÷ π = 17.19 mm

The reference row pairs EU/ISO 54 with US 7, whose published US-formula diameter is 17.32 mm and whose displayed circumference is 54.4 mm. Those values sit close together because national charts use their own increments and displayed rounding. They do not have identical mathematical definitions.

How are French and Swiss ring sizes calculated?

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

French or Swiss size = circumference (mm) − 40

FR 14 therefore means:

14 + 40 = 54 mm inner circumference

That is the French and Swiss entry in the reference conversion for US 7. The French number itself remains a circumference-based scale number, not a diameter measurement.

How do UK, Australian and Irish sizes work?

The UK, Australian and Irish systems use letters with half steps. Mesurix publishes the circumference equation:

circumference (mm) = 37.14 + 1.28 × s

Here, s counts half-letter steps from A. A result is matched to the available full-letter or half-letter position. It should not be reported as a made-up letter with more precision than the scale provides.

US 7 is paired with UK N 1/2 in the reference row. The UK label is a position in a letter sequence, not a millimetre measurement. To reverse the equation, subtract 37.14 from the circumference and divide by 1.28:

s = (circumference − 37.14) ÷ 1.28

What is the Japanese, Korean and Chinese 号 formula?

Japan uses this equation:

diameter (mm) = 13 + (n − 1) ÷ 3

The reverse equation is:

n = 3 × diameter − 38

For JP 14:

13 + (14 − 1) ÷ 3 = 17.33 mm inner diameter

The reference row pairs JP/KR 14 with 17.32 mm inner diameter, 54.4 mm circumference and US 7. The one-hundredth difference between 17.32 mm and 17.33 mm comes from the displayed conversion values and rounding. It does not establish a different practical ring size.

Korean 14호 is not safe to convert from the number alone. Many Korean shops mean the Japanese-style size 14. Some shops use 호 next to an inner circumference in millimetres instead. If a Korean listing says 14호 and also gives a millimetre value, use the millimetre value and its description.

Why is a formula safer than a rounded chart?

A chart places labels next to rounded values. A formula keeps the source measurement visible. That matters when one chart calls a ring US 7 and another calls the same product US 7.5, or when a marketplace copies an EU number into a column headed “diameter”.

The international ring size conversion guide is useful for a quick label lookup. The equations let you check what the label is supposed to represent. The ring size chart provides a second quick comparison, but the millimetre measurement remains the better reference.

How do US, UK, EU/ISO, JP and French sizes match?

The reference conversion is:

17.32 mm inner diameter = 54.4 mm inner circumference
US 7 = UK N 1/2 = EU/ISO 54 = JP/KR 14 = FR 14

Use the row as a reference conversion, not as permission to remove the measurement type. A product page that says “54” still needs to tell you whether it follows EU/ISO circumference notation or another shop-specific convention.

How do you convert US 7 to UK N 1/2?

Start with the US equation:

11.63 + (0.8128 × 7) = 17.32 mm inner diameter
17.32 mm × π = 54.4 mm approximately

Match that circumference to the UK half-letter progression. The reference conversion is UK N 1/2.

How do you convert US 7 to EU/ISO and France?

The reference conversion pairs US 7 with EU/ISO 54 and FR 14. EU/ISO 54 is the nearby whole-millimetre circumference label. FR 14 comes from subtracting 40 from the 54 mm French circumference value:

54 − 40 = 14

The published US reference is 17.32 mm diameter and 54.4 mm circumference. EU/ISO 54 and FR 14 use 54 mm through their own conventions, so the values are close but not identical mathematical definitions.

How do you convert US 7 to Japan and Korea?

US 7 corresponds to JP 14. It is also commonly listed as Korean 14호 when the Korean seller uses the Japanese-style 号 scale:

US 7: 11.63 + (0.8128 × 7) = 17.32 mm
JP 14: 13 + (14 − 1) ÷ 3 = 17.33 mm

A Korean shop may use 14호 for this Japanese-style scale, but another Korean shop may use 호 beside an inner circumference in millimetres. Look for the millimetre figure and the seller’s definition before converting it.

What does “ring size in mm” mean?

“Ring size in mm” is incomplete unless the listing identifies the measurement. It may mean inner diameter or inner circumference. Write the result in one of these forms:

17.32 mm inner diameter
54.4 mm inner circumference

Those descriptions cannot be confused with each other. A bare “17.32 mm” still needs the word diameter or circumference beside it.

The millimetre value remains the better reference when two sellers use different labels. If a marketplace gives only a country size, ask whether its number means diameter, circumference or a position in a letter or 号 scale.

Are half sizes and quarter sizes the same in every country?

No. Half sizes and quarter sizes are labels offered within particular sizing systems or by particular sellers. They do not represent one universal millimetre increment across US, UK, EU, French and JP scales.

UK, Australian and Irish sizes include half-letter positions. US sellers commonly offer half sizes and some offer quarter sizes. A quarter label is an intermediate catalogue choice, not a separate international formula. Compare the seller’s actual inside-diameter or inside-circumference figures.

How does band width change the ring size?

Band width affects the choice because a wide band covers more of the finger. The trade rule of thumb is about a quarter size up from 6 mm of width and about a half size up from 8 mm of width.

Check the stated width before choosing between adjacent sizes, and read the wide-band ring size guide. A wide band makes a close measurement more consequential. Choose the larger available size when the result falls between two options and the ring must pass the knuckle.

Why does finger size change during 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 one day is easily a quarter of a US size.

Measure at more than one time instead of treating a cold-morning reading as the permanent size. When two sizes are genuinely possible, choose the larger. A loose ring can be tightened with sizing beads or a fitted bar. A ring that will not clear the knuckle cannot be worn at all.

Why do string and paper strips give different answers?

String and paper strips can stretch and compress. The amount depends on how hard the strip is pulled, so these methods can read a quarter to a half size high or low. Pulling a strip tightly does not reproduce the fit of a rigid ring.

A rigid ring with a known inner diameter is a better comparison. Measure the opening through its centre, not the outside edge. The guide to measuring ring size at home explains how to use an existing ring without turning a flexible strip into a false precise result.

One whole US size is 0.81 mm of diameter, roughly eight sheets of 80 gsm office paper because each sheet is about 0.10 mm thick. A human hair is about 0.07 mm. A small-looking gap or compression can affect the final size choice.

Which measurement should you trust when two charts disagree?

Use the stated inner diameter or inner circumference in millimetres before using a rounded national label. The millimetre measurement describes the physical opening. A national label describes a scale that may use a different definition, increment or rounding convention.

For US sizes, the published equation gives 17.32 mm inner diameter for US 7. One whole US size changes diameter by 0.8128 mm. If one chart calls a product US 7 and another calls it US 7.5, the seller’s millimetre figures should reveal whether the physical openings differ by approximately 0.4064 mm of diameter.

A label mismatch is not automatically a measurement mismatch. EU/ISO 54 means 54 mm circumference. US 7 uses the US diameter equation and is paired with 54.4 mm circumference in the reference row. FR 14 means 54 mm circumference.

Should you measure the inside or outside of a ring?

Measure the inner opening. Ring size is based on the band’s inner diameter or inner circumference, so the outside edge includes metal that is not part of the finger space.

For diameter, measure across the opening through the centre. For circumference, use the inside edge, not the outside perimeter. Keep the unit in millimetres and write the type beside it.

A printable tool can help with a ring you already own, but printer scaling must be checked. The printable ring sizer provides that check. A screen-based tool also needs calibration against the ISO/IEC 7810 ID-1 card, whose dimensions are 85.60 mm × 53.98 mm, before the displayed millimetres represent physical millimetres.

How should you make the final order decision?

Choose the seller’s size whose stated inner millimetre measurement is closest to the measured ring or finger result. Account for band width, changing finger size and the need for the ring to clear the knuckle. If the result falls between two available sizes, choose the larger one.

Do not let a rounded chart label overrule a direct millimetre specification. If one shop lists EU 54 and another lists US 7, compare the measurements behind those labels. If a ring is expensive or non-returnable, have a jeweller confirm the size before payment. A phone screen or paper sizer is not equivalent to a jeweller’s steel mandrel.

Mesurix is published by VAST FLOW, TOO and written by Merey Tleugazin, who builds the app. He is not a jeweller, and the site says so.