Ring Size Calculator: Finger Circumference to US and EU Size
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A finger circumference of 54 mm is a US ring size of about 6.75. The size formula is (diameter in mm − 11.63) ÷ 0.8128, and diameter is circumference ÷ π.
A finger circumference of 54 mm gives an inside diameter of 17.2 mm, which is a US ring size of about 6.84. Rounded to the nearest quarter size, that is 6.75.
Ring size chart
| US size | Diameter | Circumference | ISO / EU |
|---|---|---|---|
| 4.0 | 14.9 mm | 46.8 mm | 47 |
| 4.5 | 15.3 mm | 48.0 mm | 48 |
| 5.0 | 15.7 mm | 49.3 mm | 49 |
| 5.5 | 16.1 mm | 50.6 mm | 51 |
| 6.0 | 16.5 mm | 51.9 mm | 52 |
| 6.5 | 16.9 mm | 53.1 mm | 53 |
| 7.0 | 17.3 mm | 54.4 mm | 54 |
| 7.5 | 17.7 mm | 55.7 mm | 56 |
| 8.0 | 18.1 mm | 57.0 mm | 57 |
Show the steps
- Diameter = circumference ÷ π = 54.0 ÷ 3.1416 = 17.19 mm.
- US size = (diameter − 11.63) ÷ 0.8128 = 6.839.
- Round to the nearest 1/4 size: 6.75.
US ring size
Diameter (mm) = 11.63 + 0.8128 × size
Size = (diameter − 11.63) ÷ 0.8128
Circumference = π × diameter
- Measure at the end of the day, when fingers are largest, and not when cold. Your knuckle may be bigger than the base of your finger, so the ring must slide over it.
- Sizes vary slightly between jewelers. If you are between sizes, choose the larger one.
Estimates for planning and learning, not financial, tax, legal or medical advice. Check figures with your lender, tax preparer or doctor before you act.
About this calculator
You can find your ring size at home with a strip of paper or a piece of string. Wrap it around the base of your finger, mark where it meets, and measure the length. Or measure the inside diameter of a ring that already fits.
This calculator turns either measurement into a US size and an ISO/EU size, rounds to the nearest quarter, and adds a quarter size for wide bands.
How to use it
- Wrap a strip of paper or string around the base of your finger and measure to the mark. Or measure the inside diameter of a ring that fits.
- Pick which one you measured and enter it, in millimeters or inches.
- If the band is wide, turn on the wide band option.
- Read your size and check the chart.
Measuring tips
- Measure at the end of the day, when fingers are largest.
- Do not measure when your hands are cold.
- The ring must slide over your knuckle but stay snug at the base.
- If you are between sizes, choose the larger one.
Sources
Quick check: test yourself
How do you get diameter from circumference?
Correct answer: Divide by π. Circumference = π × diameter.
When should you measure your finger?
Correct answer: At the end of the day. Fingers swell during the day, so the evening measurement avoids a ring that is too tight.
A wide band usually needs…
Correct answer: A slightly larger size. Wide bands cover more finger and fit tighter.
Pick an answer for each question.
Common questions
How do I find my ring size at home?
Wrap a strip of paper around your finger, mark where it overlaps and measure the length in millimeters. Use the circumference option here.
What size is a 54 mm circumference?
About US size 6.75, and ISO size 54.
Should I size up for a wide band?
Yes. A quarter to a half size larger is usual for bands wider than 6 mm.
Do sizes differ between countries?
Yes. The ISO and many European sizes equal the circumference in millimeters.
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