Ring Size Calculator
Find your ring size from your finger’s circumference or a ring’s inside diameter, in US, UK, EU (ISO) and other sizes.
Results
| Item | Value |
|---|
Quick Answer
US ring size = (inside diameter in mm − 11.63) ÷ 0.8128. A finger circumference of 54 mm gives a diameter of 17.19 mm, which is about US size 6 ¾, UK size N and EU size 54. Each whole US size adds 0.8128 mm to the diameter.
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Table of Contents
- Ring Size Calculator
- Quick Answer
- Related Calculators
- How This Ring Size Calculator Works
- How to Use the Ring Size Calculator
- Core Formulas
- Key Terms
- Worked Example
- Real-Life Examples
- Circumference vs Diameter
- Ring Size Chart
- Tips for an Accurate Size
- Common Mistakes to Avoid
- Frequently Asked Questions
- References & Sources
How This Ring Size Calculator Works
Measure around your finger, or across the inside of a ring that fits, and enter the number. You can also enter a US size to convert it. The calculator finds the inside diameter and circumference, then shows the matching US, UK, EU and other sizes.
Quick explanation: each system measures the same ring in a different way.
- US and Canada: numbers in quarter sizes, based on diameter.
- UK and Australia: letters in half sizes, based on circumference.
- EU (ISO): the inside circumference in millimeters.
How to Use the Ring Size Calculator
- Choose what you know: finger circumference, ring diameter or a US size.
- Choose millimeters or inches.
- Wrap a strip of paper around your finger, mark it, and measure the length, or measure across the inside of a ring.
- Enter the number and press Calculate.
- Read your size in each system. If you are between sizes, choose the larger one.
Core Formulas
- Diameter from circumference: d = C ÷ π
- US size: (d − 11.63) ÷ 0.8128, to the nearest quarter
- UK size: size C = 40 mm circumference, plus 1.25 mm per letter
- EU / ISO size: inside circumference in mm
- Italy, Spain, Switzerland: circumference − 40
Key Terms
- Inside diameter: the distance across the inside of the ring.
- Inside circumference: the distance around the inside of the ring.
- ISO 8653: the international standard that sets ring size as inside circumference in mm.
- Ring sizer: a set of rings or a strip used to measure finger size.
- Comfort fit: a band rounded on the inside, which can feel looser than a flat band.
Worked Example
A ring that fits has an inside diameter of 18.1 mm.
- US size = (18.1 − 11.63) ÷ 0.8128 = 7.96, so size 8
- Circumference = 18.1 × π = 56.9 mm, so EU size 57
- UK size = P ½
Real-Life Examples
- Buying online: a string around your finger measures 2.1 in, or 53.3 mm, which is about US size 6 ½.
- A surprise gift: measure the inside of a ring they already wear on that finger. 17.3 mm across is about US 7.
- Buying from the UK: a US size 7 is about UK N ½.
- Buying from Europe: a US size 9 is about EU 60.
Circumference vs Diameter
- Circumference: the distance around your finger. Best measured with a strip of paper or a ring sizer.
- Diameter: the distance across the inside of a ring. Best measured with a ruler or calipers on a ring that fits.
- Which is better: a ring that fits is usually more accurate than a paper strip, which can stretch.
Ring Size Chart
Computed from the standard formulas. Brand charts may differ by a quarter size.
| US | Diameter (mm) | Circumference (mm) | UK | EU / ISO |
|---|---|---|---|---|
| 3 | 14.1 | 44.2 | F ½ | 44 |
| 4 | 14.9 | 46.8 | H ½ | 47 |
| 5 | 15.7 | 49.3 | J ½ | 49 |
| 5 ½ | 16.1 | 50.6 | K ½ | 51 |
| 6 | 16.5 | 51.9 | L ½ | 52 |
| 6 ½ | 16.9 | 53.1 | M ½ | 53 |
| 7 | 17.3 | 54.4 | N ½ | 54 |
| 7 ½ | 17.7 | 55.7 | O ½ | 56 |
| 8 | 18.1 | 57.0 | P ½ | 57 |
| 8 ½ | 18.5 | 58.2 | Q ½ | 58 |
| 9 | 18.9 | 59.5 | R ½ | 60 |
| 10 | 19.8 | 62.1 | T ½ | 62 |
| 11 | 20.6 | 64.6 | V ½ | 65 |
| 12 | 21.4 | 67.2 | X ½ | 67 |
| 13 | 22.2 | 69.7 | out of range | 70 |
Tips for an Accurate Size
- Measure warm: fingers are smaller when cold and larger in heat.
- Measure late in the day: fingers swell a little during the day.
- Check the knuckle: the ring must slide over it.
- Wide bands: bands over about 6 mm wide often need a quarter to half size larger.
Common Mistakes to Avoid
- Using string that stretches. Use paper or a plastic sizer.
- Measuring a cold finger. Your size will come out too small.
- Measuring the outside of a ring. Measure the inside edge.
- Ignoring the knuckle. If your knuckle is large, size for the knuckle.
- Mixing up hands. Your dominant hand is often a little larger.
Frequently Asked Questions
Wrap a strip of paper around the base of your finger, mark where it overlaps, and measure the length in mm. Enter it as the finger circumference.
For women in the US, sizes 6 to 7 are common. For men, sizes 9 to 10 are common.
Choose the larger size, especially for wide bands, or ask a jeweler to resize.
Choose US ring size, enter your size, and read the UK letter. For example, US 7 is about UK N ½.
The EU size is the inside circumference in mm. A US 7 is about 54.
Yes, the scale is the same. Men’s sizes are usually larger.
Yes. Measure across the inside of the ring at its widest point in mm, and enter it as the inside diameter.
It is a good start, but it can be off by up to half a size. A jeweler’s ring sizer is more accurate.
Often yes. Bands wider than about 6 mm usually need a quarter to half size larger.
Yes. Heat, cold, time of day, pregnancy and weight changes can all change it.
A 17 mm inside diameter is about US 6 ½, UK M ½ and EU 53.
2 inches is 50.8 mm of circumference, which is about US 5 ½.
References & Sources
| Source | Used For | Link |
|---|---|---|
| Wikipedia, Ring size | US, UK and ISO size systems and formulas. | https://en.wikipedia.org/wiki/Ring_size |
| ISO, ISO 8653:2016 Jewellery, ring-sizes | The international ring size standard. | https://www.iso.org/standard/65408.html |