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Ring Size Converter

Converter
Note
Ring sizes vary slightly between jewellers and brands. Use these conversions as a guide and confirm sizing with your jeweller when purchasing.
Range: 0 – 20 (half sizes allowed, e.g. 7.5)

Enter a ring size above to see all international equivalents.

Quick Reference — Common Ring Sizes

Diameter (mm)Circ. (mm)USUKEUJapanSwissItalian
14.4845.487I451199
14.8846.758J46.5131111
15.2948.039K48151313
15.749.3210L49171515
16.150.5811M50.5191717
16.5151.8712N52211919
16.9253.1613O53232121
17.3254.4114P54.5252323

About This Tool

Ring Size Converter – International Ring Sizing Guide

Shopping for a ring online from an international retailer often means encountering an unfamiliar sizing system. A US size 7 becomes a UK size I, an EU size 45, or a Japanese size 11 — all describing exactly the same ring. This Ring Size Converter instantly translates between every major international ring sizing standard, so you can shop with confidence wherever the ring comes from.

Sizing Systems Explained

Ring sizes worldwide fall into three broad approaches: numeric systems tied to circumference, numeric systems tied to diameter, and letter-based systems.

  • US / Canada: A numeric scale starting near size 0. Each whole size step equals approximately 0.8128 mm of inner diameter. Half sizes (e.g., 6½, 7½) are widely available.
  • UK / Australia / Ireland: A letter-based system running from A through Z (and beyond to Z1, Z2 for very large sizes). Half sizes are denoted with a "½" suffix (e.g., N½). Each half-letter step equals approximately 1.25 mm of inner circumference.
  • EU / French: The simplest of all systems — the EU size number is exactly equal to the inner circumference in millimetres. EU 52 means the ring's inner circumference is 52 mm.
  • Japanese / Chinese: A numeric scale based on a published reference table, with each step covering a small increment of circumference. Japanese sizes typically run from 1 to 37 for standard adult rings.
  • Swiss: A numeric scale similar to the Italian system, also derived from circumference, used primarily in Switzerland by watchmakers and jewellers.
  • Italian: A numeric system widely used across Italian jewellery design, closely related to the EU system but with its own published offset table.

How the Conversion Works

All conversions use inner diameter in millimetres as the canonical intermediate value. When you enter a size in any system, the tool first converts it to a diameter:

  • US to diameter: diameter (mm) = 11.63 + (US size × 0.8128)
  • EU / circumference to diameter: diameter = circumference ÷ π
  • UK, Japanese, Swiss, Italian: looked up from a published reference table keyed to inner diameter.

From the inner diameter, the tool derives all other representations: inner circumference (C = π × d), and the size designation in every other system. Diameter in inches follows from dividing by 25.4 (the exact mm-per-inch conversion factor).

Entering a Direct Measurement

If you already have a physical measurement of your finger, you can enter it directly without knowing your size in any system:

  • Choose Inner Diameter (mm) and type the diameter of a ring that already fits, measured with calipers.
  • Choose Inner Circumference (mm) if you measured your finger with a strip of paper — the length of the strip wrapped around the base of the finger equals the circumference.
  • Choose Inner Diameter (inches) for caliper measurements taken in imperial units.

Measuring Your Finger at Home

The most reliable method for measuring finger size without a ring mandrel is the paper-strip technique:

  1. Cut a thin strip of paper about 10 cm long and 5 mm wide.
  2. Wrap it snugly around the base of the finger you want to size.
  3. Mark the point where the paper overlaps itself with a fine pen, then lay the strip flat and measure the length from the cut end to the mark in millimetres.
  4. This measurement is your inner circumference. Enter it into the "Inner Circumference (mm)" mode to find your ring sizes across all systems.

Measure in the afternoon — fingers are slightly larger later in the day and after physical activity. On very hot or very cold days, finger size can shift by half a size or more. For rings wider than 6 mm, many jewellers recommend sizing up by half a size to account for the additional pressure across the knuckle.

Accuracy and Limitations

Conversions are based on widely published international sizing charts and standard mathematical formulas. For US and EU systems, results are mathematically precise. For letter-based UK sizes and Japanese sizes, the closest standard table entry is returned. In practice, jewellers in different countries may apply small variations in their reference charts, so always verify the specific brand's size guide before finalising a purchase — especially for fitted bands such as wedding rings or eternity rings.

Frequently Asked Questions

Is the Ring Size Converter free?

Yes, Ring Size Converter is totally free :)

Can I use the Ring Size Converter offline?

Yes, you can install the webapp as PWA.

Is it safe to use Ring Size Converter?

Yes, any data related to Ring Size Converter only stored in your browser (if storage required). You can simply clear browser cache to clear all the stored data. We do not store any data on server.

How does the Ring Size Converter work?

The tool uses inner diameter (in millimetres) as the common reference value. When you enter a size in any supported system, the converter first calculates the corresponding inner diameter, then derives the equivalent sizes in every other international system from that diameter. For letter-based systems such as UK, a published reference table is used; for numeric systems such as US and EU, standard formulas are applied.

Which ring sizing systems are supported?

The converter supports US / Canada numeric sizes, UK / Australia / Ireland letter sizes (A through Z), EU / French sizes (equal to inner circumference in mm), Japanese / Chinese numeric sizes, Swiss numeric sizes, and Italian numeric sizes. You can also enter a raw measurement — inner diameter in millimetres or inches, or inner circumference in millimetres — and receive all size equivalents instantly.

How accurate are the conversions?

Conversions are based on widely published international ring size charts and standard formulas. For numeric systems, the results are mathematically precise. For letter-based UK sizes, the closest table entry is returned. In practice, ring sizes can vary slightly between jewellers and countries due to rounding conventions, so always confirm with the jeweller when purchasing.

How do I measure my ring size at home?

Wrap a thin strip of paper or a piece of string around the base of the finger you want to size. Mark where the strip meets itself, then lay it flat and measure the length in millimetres — this is your inner circumference. Divide by pi (approximately 3.14159) to get the inner diameter, or enter the circumference directly into the tool. Measure in the afternoon when fingers are at their largest, and account for swelling if measuring on a hot day.

What is the difference between EU size and circumference in mm?

The EU / French ring size number is numerically equal to the inner circumference of the ring in millimetres. So an EU size 52 ring has an inner circumference of 52 mm. This is why entering a circumference measurement gives the same EU size directly.

Why might my converted size differ from what a jeweller tells me?

Ring sizes are approximate standards, and individual jewellers may use slightly different reference charts or rounding conventions. Finger size also changes throughout the day (typically 0.5 – 1 full size between morning and evening) and with temperature. Use this tool as a close starting point, and always get professionally sized when purchasing an important ring such as an engagement ring.