π CM to Ring Size Converter
Finger circumference to US, UK, and EU ring sizes.
Your Ring Size
How to Use This Ring Size Converter
Ring sizes are just circumference with different labels. Measure around your finger, enter the number, and the converter returns your US number size, UK letter size, EU millimeter size, and inner diameter. It works from centimeters, millimeters, or inches.
Measure your finger circumference with a flexible tape at the base of the finger, in the evening when fingers are at their largest.
Enter the number and pick the unit. Centimeters is the default; millimeters is the most precise.
Hit Convert Size. The headline shows your US size; the details list UK, EU, diameter, and circumference.
Round sensibly. The tool rounds to the nearest half size; if you fall between, size up for comfort.
The Conversion Method
US sizes are anchored so that size 3 equals 44.2 mm of circumference, with each full size adding about 2.55 mm. EU sizes skip the code entirely and just state the circumference in millimeters, which is why a US 7 is roughly EU 54. UK letters follow their own traditional scale mapped from the same circumferences. The converter rounds the US result to the nearest half size, matching how rings are actually sold.
Worked Example
A finger measuring 5.4 cm is 54 mm around. US size = 3 + (54 - 44.2) / 2.55 = 3 + 3.84 = 6.84, which rounds to US 7. EU size is 54. UK size for 54 mm is N. Inner diameter = 54 / 3.1416 = 17.2 mm. So a 5.4 cm finger takes a US 7, UK N, EU 54 ring, one of the most common women\'s sizes. If the measurement had been 5.5 cm (55 mm), the US size would compute to 7.24, still a 7, showing how half sizes absorb small measurement differences.
Measuring Tips That Actually Matter
Measure three times and average. Fingers change size through the day, so an evening measurement in a warm room is the honest one. If your knuckle is much larger than the finger base, size for the knuckle; a ring that cannot pass the knuckle is unwearable no matter how perfect it feels at the base. Wide bands of 6 mm or more fit about a half size tighter than thin bands, so size up for them. And for surprise proposals, borrow a ring they wear on the correct finger and measure its inner diameter: diameter times pi gives circumference, which converts the same way.
When to Get Sized Professionally
Home measuring is fine for fashion rings and gifts. For engagement rings, wedding bands, and anything expensive or hard to resize, visit a jeweler for a mandrel sizing. It takes two minutes and is usually free. Bring the measurement from this converter as a starting point and let them confirm. Resizing costs $50 to $150 and some designs, like eternity bands and tungsten rings, cannot be resized at all. Ten minutes at a jewelry counter beats a resize bill every time.
Worked Example: The Men's Wedding Band
A finger measuring 6.2 cm: 62 mm around. US size = 3 + (62 - 44.2) / 2.55 = 3 + 6.98 = 9.98, a clean US 10. EU size: 62. Inner diameter: 62 / 3.1416 = 19.7 mm. So the band is a US 10 / EU 62, one of the most common men's sizes. For the UK letter scale, confirm against the jeweler's chart, since letter conversions vary slightly between published tables.
Ordering online? Check the seller's return and resizing policy before paying. Many jewelers resize plain bands free within 30 days, which removes the risk from a home measurement entirely.
Measuring at Home vs. Jeweler Sizing
Home methods rank by accuracy. Printable sizers and ring mandrels you buy online are decent, usually within a quarter size. Paper strips and string work but stretch and slip, adding up to half a size of error. The jeweler's metal sizer rings, tried on the actual finger, remain the gold standard, free at any jewelry counter and worth the trip for anything expensive.
Three variables home measuring misses: knuckle size (large knuckles need a size that passes the knuckle but may spin at the base), temperature (cold fingers measure small), and band width, as the companion page explains. For engagement rings and eternity bands that cannot be resized, get the jeweler measurement. For a fashion ring, home measuring is plenty.
Eternity Bands and Wide Stones: The No-Resize Rule
Eternity bands, with stones all around, generally cannot be resized without rebuilding the ring, so the measurement must be right the first time. Get the jeweler sizing for these, and wear the sizer for a few minutes in the store: fingers compress slightly, and a band that feels snug at first often feels right after five minutes. Expensive and unresizable is exactly when free professional sizing pays.
Worked Example: 5.4 cm Finger
A finger measuring 5.4 cm around converts to 54 mm of circumference. On the US scale that lands between size 7 (54.4 mm) and size 6.75, so the practical answer is size 7 for a comfort fit or 6.75 for snug. UK scale: 54 mm is about an O. EU scale: 54 directly, since EU sizes are the circumference in millimeters. The calculator returns all three so you can order from any country's sizing chart without a second conversion.
Measure twice, at different times of day. Fingers swell in heat and after salty meals and shrink in the cold, easily shifting half a size. A 5.4 cm morning measurement can read 5.5 cm by evening. Size for the larger reading if the ring must slide over a knuckle, and remember wide bands fit tighter than narrow ones at the same nominal size, so consider going up a quarter size for bands over 6 mm wide.
Measuring Tips That Actually Matter
The string method works if you do it right: wrap a strip of paper snugly, mark the overlap, and measure the strip flat against a ruler in millimeters. Do not pull tight; the marker should slide over the paper with slight resistance, mimicking how the ring passes the knuckle. Printable ring sizers are better, and a jeweler's mandrel measurement is best, free at most shops and accurate to a quarter size.
Surprise proposals complicate measurement: borrow a ring they already wear on that finger and measure its inner diameter, then convert diameter to size (inner diameter in mm x pi = circumference). A ring from the right hand is a usable proxy but usually runs a quarter to half size smaller than the left ring finger. When in doubt between sizes, jewelers resize plain bands cheaply, while eternity bands and tension settings often cannot be resized at all, so measure those twice.
Frequently Asked Questions
How do I convert cm to ring size?
Measure your finger circumference in centimeters, multiply by 10 for millimeters, then match to a size chart. A 5.4 cm finger is 54 mm, about a US 7. Enter your measurement above for the exact US, UK, and EU size.
What is a US size 7 in mm?
A US size 7 ring has an inner circumference of about 54.4 mm and an inner diameter of about 17.3 mm. Each full US size is roughly 2.55 mm of circumference, so size 8 is about 57 mm around.
How do I measure my finger for a ring?
Wrap a flexible tape or strip of paper around the base of the finger, mark where it overlaps, and measure the length in millimeters. Measure in the evening when fingers are largest, and make sure the sizer slides over your knuckle. For the most reliable result, get sized at a jeweler.
What is the average women's ring size?
In the US, the average women's ring size is 6 to 7, about 51.9 to 54.4 mm around the finger. The average men's size is 9 to 10, about 59.5 to 62.1 mm. Averages vary by population, so always measure the actual finger.
Are US and UK ring sizes the same?
No. The US uses numbers like 7, the UK uses letters like N, and the EU uses the circumference in millimeters like 54. They measure the same physical size but label it differently. The converter above shows all three for any measurement.
What if I am between two ring sizes?
Size up, especially for wide bands, which fit tighter than thin ones. A ring should slide over the knuckle with slight resistance and sit snugly at the base without spinning freely. When in doubt between sizes, the larger one is the safer buy.
Do ring sizes change with temperature?
Yes. Fingers swell in heat and shrink in the cold, sometimes by half a size or more. Measure at room temperature in the evening for the most representative size, and expect a new ring to feel tighter on cold mornings.
What EU size is a US 8?
A US 8 is about 57 mm in circumference, which is EU 57. EU sizes are simply the inner circumference in millimeters, making them the most straightforward system. The converter above gives the EU number for any input.