Ring Size Converter
By Mustafa Bilgic · Updated 18 August 2026
This converter translates ring sizes between the US numbered scale, UK letter scale, EU millimetre-circumference scale, and direct inner diameter in millimetres. Enter any size you know above and the tool shows all equivalents instantly. No sign-up is needed, and you can convert as many sizes as you like in one session.
Ring sizes are approximate across systems due to rounding and manufacturer tolerances. For high-value purchases, confirm your size with a professional jeweller's gauge.
How Ring Sizing Systems Differ
Ring sizes describe the same physical measurement — the inner circumference or diameter of a ring — but each region uses its own scale. The US system assigns numerical sizes with half-step increments (5, 5.5, 6, 6.5, and so on). The UK and Australia use an alphabetical scale from A through Z. The EU system, aligned with the ISO 8653 international standard, uses the inner circumference measured directly in millimetres.
Because each system was developed independently, the labels do not align intuitively. A US size 7, for example, corresponds approximately to a UK N½, an EU size of about 54, and an inner diameter of roughly 17.3 millimetres. A US 6 sits near UK L, EU 52, and a 16.5 mm diameter. A US 9 translates to roughly UK R½, EU 59, and 19.0 mm.
This converter bridges those systems. Enter the size you know from one scale, and the tool displays the closest equivalent in every other scale along with the inner diameter.
How to Use the Converter
Three steps give you your converted ring size:
- Select the sizing system you are starting from — US, UK, EU, or millimetres. This tells the converter which scale your known measurement belongs to.
- Enter your ring size in the selected system. For US sizes, type a number with or without a half increment. For UK sizes, type the letter. For EU or millimetre values, enter the numeric measurement.
- View your results. The tool instantly displays the equivalent size in every other system, along with the corresponding inner diameter in millimetres. You can copy any value directly.
If you are unsure which system a ring uses, check the retailer's country of origin. US and Canadian jewellers use the US numbered scale. British, Irish, Australian, and South African jewellers typically use the UK letter scale. Continental European jewellers and many international brands use the EU millimetre-circumference system.
No sign-up is needed, and you can convert as many sizes as you like without leaving the page.
Measuring Your Ring Size at Home
The most reliable home method uses a ring you already own that fits the finger you intend to size. Place the ring on a ruler and measure the inside diameter — the distance from one inner wall to the opposite inner wall — in millimetres. Enter that measurement into the converter using the millimetre option, and the tool shows the equivalent in every named scale.
If you do not have a well-fitting ring, wrap a thin strip of paper or a flexible measuring tape around the base of your finger. Mark where the strip overlaps, then measure the length of that section in millimetres. This gives you the circumference, which maps directly to the EU scale. Enter it as an EU size, and the converter handles the rest.
Measure at room temperature during the middle of the day. Fingers swell slightly in heat and shrink in cold, and they tend to be somewhat larger in the evening than the morning. Measuring when your hands are at a neutral, comfortable state gives the most accurate result.
When Sizes Do Not Match Perfectly
Ring sizing is not perfectly standardised across every jeweller. Two rings marked as the same size may differ by a fraction of a millimetre depending on the manufacturer's tolerances, the ring's width, and how the measurement was taken.
Wide bands — anything above about 6 mm in width — tend to fit tighter than narrow bands of the same marked size. Many jewellers recommend going up half a size for wide-band rings to compensate for the additional metal covering more of the finger's surface.
Conversion between systems introduces rounding. A US 6.5 falls between two UK letter sizes, and the converter shows the nearest match. If you are ordering a ring internationally and fall between sizes, choosing the slightly larger option is generally safer. Reducing a ring by a small amount is usually easier and less expensive than stretching it up.
When accuracy truly matters — engagement rings, for example — a professional fitting with a jeweller's mandrel and gauge set remains the most precise method available.
Frequently asked questions
Why are ring sizes different in each country?
Each country developed its sizing system independently before international standards existed. The US chose numbered increments, the UK chose letters, and Europe chose direct millimetre measurements. All three describe the same physical property — the ring's inner dimensions — but use different labels.
How do I measure my finger without a ring?
Wrap a thin strip of paper or string around the base of your finger, mark where it overlaps, and measure the length in millimetres. That measurement is your circumference, which corresponds directly to the EU size scale. Enter it in the converter to see all other equivalents.
Do ring sizes change with temperature or time of day?
Yes. Fingers swell in warm conditions and shrink in cold. They also tend to be slightly larger in the evening than in the morning. For the most accurate measurement, size your finger at room temperature during the middle of the day.
What should I do if I fall between two sizes?
Choose the larger of the two options. It is generally easier and less expensive for a jeweller to reduce a ring slightly than to stretch it. For important purchases like engagement rings, a professional fitting gives the most precise result.