If you are shopping online or planning a surprise, you need a size you trust. I measure two ways at home, inner diameter in millimeters if I have a ring that fits, or a simple string wrap for circumference if I do not. Then I convert to US, UK, and EU, and tweak for band width and brand notes.
Measure at room temperature. If you own a ring that fits, read the inner diameter in mm with a rigid ruler. If not, wrap a paper strip, mark it, measure the circumference in mm, then divide by π to get diameter. Convert using a table, not a guess. Size up a quarter to a half for very wide bands. Comfort-fit interiors often feel fine at your usual size.
Ring Size Finder
Find your perfect ring size by measuring your finger's inner diameter or using the string wrap method. Get accurate US and UK sizing with a printable sizing card.
How to measure your ring size
Method 1: Inner Diameter (Most Accurate)
- Step 1: Take a ring that fits your finger perfectly
- Step 2: Measure the inside diameter with a ruler in millimeters
- Step 3: Enter the measurement above
Method 2: String Wrap
- Step 1: Wrap a string or paper strip around your finger
- Step 2: Mark where the string meets
- Step 3: Measure the length in millimeters
- Step 4: Select "String Wrap" method and enter the length
Ring sizing tips
- • Measure your finger at the end of the day when it's largest
- • Make sure the ring can slide over your knuckle
- • Consider the width of the band - wider bands fit tighter
- • Account for seasonal temperature changes
- • If between sizes, choose the larger size
Printable bonus: Make it easy for readers to do this right. Download the Ring Size Card PDF. It includes a 50 mm scale check, circles by US size, a quick map, and string-wrap steps. Print at 100 percent.
How to use this Ring Size Finder
Inner diameter in millimeters
Why it works: Direct measurement of a ring that already fits the target finger.
Steps
- Place the ring on a rigid metric ruler.
- Measure inner edge to inner edge. Read millimeters to one decimal place.
- Match the number to the table below.
What I learned: I once read the outer rim by mistake and ended up almost half a size off. Now I line up the inner walls only.
String wrap for circumference
Why it works: You can size a finger without a ring.
Steps
- Wrap a paper strip around the base of the finger.
- Mark where the end meets. Lay it flat and measure in millimeters.
- That number is circumference. To get diameter, divide by 3.1416.
- Use the table below to convert.
String tip: Use paper or dental floss, not stretchy string. A sloppy mark can add 0.3 mm, which is roughly a quarter size.
Convert to US, UK, EU
Use a trusted table to map your diameter or circumference to US and UK.
- Many EU charts list the circumference in millimeters as the EU size.
- UK uses letters, for example O, Q, R½, not numbers.
Mini map for common sizes
| US | UK | Inner diameter, mm | Circumference, mm |
|---|---|---|---|
| 6 | L½ | 16.5 | 51.8 |
| 7 | O | 17.3 | 54.4 |
| 8 | Q | 18.1 | 56.9 |
| 9 | R½ | 18.9 | 59.5 |
| 10 | T½ | 19.8 | 62.2 |
| 11 | W | 20.6 | 64.8 |
| 12 | Y | 21.4 | 67.3 |
Example: Your paper strip reads 54.0 mm.
54.0 ÷ 3.1416 ≈ 17.2 mm inner diameter. That maps to about US 7 and UK O on most charts.
Need clothing sizes too, open our International Size Converterto map US, UK, EU, AU, and JP.
Fit factors that change the number
- Time and temperature: Measure in the afternoon at room temp. Cold shrinks, heat swells.
- Band width: Wider bands feel tighter. Many shoppers size up a quarter to a half size for very wide bands.
- Comfort-fit interior: Rounded inside edges can feel looser. Many people stay true to size even on wider comfort-fit bands.
- Knuckle vs base: The ring must pass the knuckle and sit snug at the base. A gentle twist can help it over.
Brand fit notes, runs small or large
Tiffany: Uses inner diameter in mm and printed circles. If your ring sits between two circles, match the closer diameter, then adjust for a wide band. Check the product page for any model callouts.
Pandora: Suggests rounding to the nearest millimeter and checking that the ring passes the knuckle with a small twist. Plastic sizers are available if you want one.
Cartier: If you are between sizes, choose the larger size. Always print the chart at 100 percent before you compare.
Silicone rings: Often whole sizes only. Silicone has a bit of give compared with metal. If you sit between sizes, the material may forgive a small miss, but still try to measure cleanly.
Smart rings: Sensors and thickness change fit. Brands like Oura ship a sizing kit and ask you to wear the sample for 24 hours before you order. Follow the kit rather than a generic chart.
First time in a brand, if returns are free, order two sizes the first time, return the miss. It costs a label, not a re-make. If you want to sanity check value before ordering two, run both options through our Capsule Wardrobe Cost Per Wear calculator.
Who this is for, How I tested this, Why it works
- Who this is for: Anyone buying rings online, gifting in secret, or choosing wedding bands, silicone rings, or smart rings.
- How I tested this: I measured three rings by inner diameter, then used the string method twice a day for a week and compared both to a jeweler’s metal sizer. The string method landed within a quarter size when I measured at room temperature.
- Why it works: Millimeters reduce guesswork. The circumference to diameter math is simple and reliable. A consistent table maps those numbers to US and UK. Brand pages close the last gap.
Key takeaways
- Measure at room temperature.
- Use inner diameter in mm if you own a ring that fits. Use a paper strip for circumference if you do not.
- Convert with a table, not a formula guess.
- Size up a quarter to a half for very wide bands.
- Follow the brand page for smart rings and special interiors.
FAQs
What is the most accurate at-home method?
Measure inner diameter in millimeters from a ring that already fits. If you cannot, use the paper strip method for circumference, then divide by 3.1416 to get diameter.
Do wide bands change my size?
Yes. Wide bands feel tighter. Many people go up a quarter to a half size on very wide bands.
Why do UK letters not line up with US numbers?
UK letters map to circumference steps, not the same steps as US numbers. Use a conversion table to match your letter.
Is the string method reliable?
Yes, if you use paper or floss, mark cleanly, and measure in millimeters at room temperature. Avoid stretchy string.
How do I size for smart rings?
Wear the brand’s sizing kit as directed, often for 24 hours. Sensor bumps and band thickness make a big difference.
More sizing tools
- International Size Converter, US, UK, EU, AU, JP in one place
- Shoe Size Converter with Fit Notes, quick chart plus brand fit tips
- Jeans Fit Calculator, find your waist, rise, and inseam
- Bra Size and Sister Size Finder, measure at home with a tape
- Balconette vs Plunge, compare shapes and coverage
- Capsule Wardrobe Cost Per Wear, buy fewer pieces that earn their keep
- Swimsuit Size Chart and Support Picker
Explore more in our fashion calculators and fit tools hub.
- About the Author
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Iskra Banović is our seasoned Editor-in-Chief at Blufashion. She has been steering the website’s content and editorial direction since 2018. With a rich background in fashion design, Iskra’s expertise spans across fashion, interior design, beauty, lifestyle, travel, and culture.
