Ring Size Converter: US, UK, EU and Japan Sizes

Is a size 7 ring the same as an N 1/2 or a ring of about 54 mm? On most charts, yes, and the Ring Size Converter shows how those labels line up. You pick a US or Canada ring size, and the tool lists the matching sizes in other systems.

It suits anyone buying a ring from an overseas jeweler or a gift shopper with a US size in hand. The results include UK and Australia letters, an EU (ISO) size, a Japan size, and inside diameter and circumference.

Table of Contents

• What the Ring Size Converter Does
• How to Use It, Step by Step
• How the Sizes Relate
• Worked Examples
• How to Read the Results
• Measuring and Size Systems
• Limitations and Assumptions
• FAQs About the Ring Size Converter

What the Ring Size Converter Does

The tool has one dropdown, one button, six result cards, and a text report.

• US / Canada Ring Size: a dropdown with 21 sizes, in half steps from 3 to 13.
• UK / Australia: the matching letter size, such as N 1/2.
• EU / ISO Size: the inside circumference rounded to whole millimeters.
• Japan Size: a whole number from a rule of thumb, explained below.
• Inside Diameter and Inside Circumference: the opening size in millimeters, with one decimal.
• Jewelry Sizing and Finger Measurement Report: a text summary with a Copy Report button.

How to Use It, Step by Step

1. Open the US / Canada Ring Size menu and choose your size.
2. The six cards and the report update at once.
3. Press Convert Ring Size if you want to refresh them.
4. Read the cards for UK, EU, Japan, diameter, and circumference.
5. Press Copy Report to copy the text. It also gives the measurements in inches.

How the Sizes Relate

Every value comes from a formula, not a lookup table. The US size sets the inside diameter, and the other sizes follow from it.

• Diameter = 11.63 mm + 0.8128 mm × US size. This is a published formula for the US scale.
• EU / ISO size = the circumference rounded to whole millimeters. ISO 8653 defines ring sizes by inside circumference in millimeters.
• Japan size = 1 + 3 × (diameter - 13), rounded to the nearest whole number.
• UK letters start with A at 37.5 mm of circumference. Each letter adds 1.25 mm, and half sizes sit between.
• Inches = millimeters ÷ 25.4, shown with three decimals in the report.

Worked Examples

Sam has a US size 7 ring. Nadia buys from a UK shop and has a US 9.5. Omar needs a small size, US 3.

For Sam, the diameter is 11.63 + 0.8128 × 7 = 17.32 mm. Multiplying by π gives 54.41 mm, so the card shows 54.4 and the ISO size is 54.

The report lists 0.681 inches for the diameter and 2.142 inches for the circumference. Sam's Japan size is 1 + 3 × (17.32 - 13) = 13.96, which rounds to 14.

How to Read the Results

The US / Canada card repeats your choice as "Size 7". The UK / Australia card gives a letter with a half step, such as N 1/2.

The EU / ISO Size card is the whole-millimeter size, and the Inside Circumference card keeps the decimal. Compare the circumference with a measured strip of paper if a shop lists sizes this way.

The Inside Diameter card helps when a seller gives the width of the opening, as with a ring sizer. The Japan Size card is an estimate, so check it against the seller's own chart.

Measuring and Size Systems

The report lists three steps for home measuring. Wrap a paper strip around the base of the finger, mark the overlap, and measure the length in millimeters. Compare that length with the Inside Circumference value.

Each country builds its scale differently. The US scale counts up by number, UK and Australia use letters, and ISO uses millimeters. Japan uses its own whole numbers.

The US diameter formula and the ISO circumference definition are published. The UK letter scale and the Japan rule are conventions. Two Japanese brand charts agree with the rule within one size, but not always with each other.

For related sizing, try the shoe size converter, the hat size converter, or the clothing size converter. The length converter changes millimeters to inches, and the unit converter handles other units. The unit and time converters hub lists more.

Limitations and Assumptions

• You can only start from a US or Canada size. You cannot enter a UK letter, a millimeter value, or a Japan number.
• The list stops at 3 and 13. Sizes below 3 or above 13 are missing. US 13 shows Z 1/2, the last letter the tool uses.
• Sizes are approximate and vary by brand and country. Published charts can differ by up to one size, especially for UK letters and Japan numbers.
• Some UK charts start letter A at a slightly different length, so a letter can differ by one step.
• The Japan rule is an approximation, and no official standard text was checked for this page.
• If the menu holds an unlisted value, the cards show N/A and a message asks for a listed size.
• The tool does not account for band width or finger changes.

FAQs About the Ring Size Converter

This calculator gives general estimates only and is not professional advice. Reviewed for general accuracy by Prof. Dr. Khalil Mudassar, PhD.