Knowing your ring size circumference is the most reliable way to ensure a perfect fit — whether you are buying online, ordering from abroad, or having a ring custom made. A ring's size is determined by the inner circumference of the band: the distance around the inside of the ring that encircles your finger.
Different countries use different sizing systems. The United States and Canada use a numerical scale, the United Kingdom and Australia use letters, Europe and Denmark use millimetre-based numbers, and Japan uses its own numeric scale. Our chart converts between all four systems so you never have to guess.
International Ring Size Circumference Chart
This table lists every standard ring size from US 3 to US 13, complete with inner diameter and circumference in both millimetres and inches, plus equivalents for UK, EU/DK, and Japanese sizing systems.
| US / CA | UK / AU / NZ | EU / DK | Japan | Diameter (mm) | Circ. (mm) | Diameter (in) | Circ. (in) |
|---|
← Scroll left/right to view all columns →
* All circumference values are calculated as π × diameter. Sizes are rounded to the nearest 0.1 mm. Minor variations exist between jewellers — if you are between sizes, order the larger size.
How to Measure Your Ring Size at Home
You do not need any specialist tools to measure your ring size. A strip of paper, a pen, and a ruler are all it takes. Follow these steps for an accurate reading.
Gather your materials
Cut a thin strip of paper (about 6 mm wide) or use a piece of string. You will also need a pen or fine marker and a ruler that measures in millimetres.
Choose the right time of day
Fingers are slightly larger in the evening and after exercise. Measure at room temperature — cold shrinks fingers, heat expands them. Mid-morning is generally the most neutral time.
Wrap the strip around your finger
Wrap the paper strip snugly around the base of the finger you intend to wear the ring on. The strip should fit comfortably — not too tight and not loose enough to slide off.
Mark and measure the circumference
With a pen, mark the exact point where the strip overlaps itself. Unroll it and measure the distance from the end to your mark in millimetres. This is your ring's inner circumference.
Find your size in the chart
Locate your circumference measurement in the chart above or enter it directly into our ring size calculator for an instant conversion to all major sizing systems.
Ring Diameter vs Ring Circumference — What's the Difference?
Diameter
The ring diameter is the straight-line measurement across the inside of the ring — from one inner wall to the other, passing through the centre. It is measured in millimetres or inches and relates to the width of your finger at the point the ring will sit.
Diameter is the most common measurement jewellers use when sizing rings on a mandrel.
Circumference
The ring circumference (also called the inner circumference) is the distance around the inside of the ring. It is directly related to diameter by the formula C = π × d. Circumference is the measurement you get when you wrap a strip of paper around your finger.
Many European and Japanese sizing systems are based on circumference in millimetres.
The conversion formula
To convert between diameter and circumference:
Circumference = π × Diameter | Diameter = Circumference ÷ π
For example: a diameter of 17.3 mm gives a circumference of 17.3 × 3.14159 = 54.4 mm.
Understanding International Ring Sizing Systems
Ring sizes are not standardised globally. Depending on where you or your jeweller are based, you may encounter any of the following four major systems.
🇺🇸 US & Canada
Numerical scale starting at size 1, increasing in increments of 0.5. The most common women's sizes are 5–7; men's sizes typically fall between 8–11. Half sizes are widely available.
🇬🇧 UK, Australia & NZ
Alphabetical system starting at letter A. Each full letter represents approximately 1.25 mm in circumference. Half sizes are indicated by a "½" suffix (e.g., J½). Used across the UK, Australia, and New Zealand.
🇪🇺 Europe & Denmark
Based directly on the inner circumference of the ring in millimetres. A European size 54 means the ring has an inner circumference of 54 mm. This is one of the most precise systems available.
🇯🇵 Japan
Uses a numeric scale starting at 1, but the numbers do not align with US or UK sizes. Japanese sizes increase in approximately 0.5–1 mm circumference increments. Popular in East Asian jewellery markets.
Tips for Getting an Accurate Ring Fit
- Measure the same finger on both hands — they are often different sizes.
- Measure at the end of the day when your finger is at its largest.
- Measure multiple times and take the average for better accuracy.
- If your knuckle is large, measure both the knuckle and the base of the finger.
- Account for wide bands — rings wider than 6 mm fit tighter and may need a half-size up.
- Consider the season: fingers can change by half a size between summer and winter.
- Avoid measuring when your hands are cold, swollen, or just exercised.
- Use our online ring sizer to visualise the actual diameter on screen.
- When in doubt between two sizes, choose the larger one — it is easier to resize down.
- Use our ring size calculator for instant conversions.
Frequently Asked Questions
Ring size circumference refers to the inner circumference of a ring — the distance around the inside of the band that encircles your finger. It is measured in millimetres and is one of the most accurate and internationally consistent ways to express ring size. The EU and Japanese sizing systems are both based directly on this measurement.
Wrap a thin strip of paper around the base of your finger, mark where it overlaps, then measure the strip from its end to the mark in millimetres. That measurement is your ring's inner circumference. You can enter it directly into our ring size calculator to convert it to any sizing system.
The average ring size for women in the United States is a size 6 to 6.5, which corresponds to an inner circumference of approximately 51.9–53.1 mm and a diameter of 16.5–16.9 mm. Read our detailed guide on average ring size for women for more information.
The easiest way is to use our ring size calculator — enter your circumference and it instantly shows your US, UK, EU, and Japanese ring sizes. Alternatively, look up your circumference in the chart above and read across to the US/CA column.
In the US sizing system, each full size represents approximately 0.8–0.9 mm in diameter (or about 2.5–3 mm in circumference). A size 7 ring has an inner diameter of roughly 17.3 mm (circumference 54.4 mm), while a size 8 ring has a diameter of approximately 18.2 mm (circumference 57.2 mm).
Yes. Place an existing ring flat on a ruler and measure the inner diameter across the centre of the opening in millimetres. Then multiply by π (3.14159) to get the circumference, or enter the diameter directly into our calculator. You can also use our online ring sizer to match a circle on screen to your existing ring.
Explore Our Ring Sizing Tools
Ring Size Calculator
Enter your diameter or circumference and instantly get your size in US, UK, EU, and Japanese systems.
Online Ring Sizer
Use our interactive tool to visually match a ring circle on your screen and find your size in real time.
Average Ring Size for Women
Discover what the most common ring sizes are for women by age, hand, and finger, with a helpful size guide.