Ring Size Calculator — Diameter, Circumference
Convert ring diameter and circumference, and match your finger to your own size chart
Runs in your browser · nothing is uploaded
Updated 2026-10-04Inner diameter and inner circumference
The 17 mm in the field is only an example. Replace it with the inner diameter of your own ring.
The widest distance across the inside of the ring. Enter 5 to 40 mm.
Inner diameter 17.00 millimetres, inner circumference 53.41 millimetres
Conversion
- Inner diameter (2 decimals)
- 17.00mm
- Inner diameter (1 decimal)
- 17.0mm
- Inner circumference (2 decimals)
- 53.41mm
- Inner circumference (1 decimal)
- 53.4mm
The formulas with your numbers
- Circumference = pi x 17 = 53.41 mm
Match with your own size chart
The finger measurement and the chart shown first are only examples, and the labels A to D are not real ring sizes. Copy in the size chart of the brand you are buying from.
Wrap a tape measure or a paper strip around the finger where the ring will sit. Enter 30 to 120 mm.
Extra room to add if you like a looser fit. 0 to 10 mm, default 0, guidance only.
Size chart
Suggested size C
Suggested size
- Circumference needed (finger + allowance)
- 55.00mm
- Suggested row
- C
- Its inner circumference
- 56.55mm
- Extra room
- 1.55mm
- Next size down
- B (circumference 53.41 mm, 1.59 mm too small)
- Next size up
- D (circumference 59.69 mm, 4.69 mm looser)
The formulas with your numbers
- Target = 55 + 0 = 55 mm
- C circumference = pi x 18 = 56.55 mm
- Extra room = 56.55 - 55.00 = 1.55 mm
Ring sizes such as US, UK or Korean ho numbers follow different standards from brand to brand, so no fixed conversion table is included. This tool only does the geometry (circumference = pi x diameter) and finds the matching row in a chart you copy in yourself. For US or UK sizes, enter the chart from your jeweller's own size guide. Measure your finger in the evening when it is slightly larger, and if your knuckle is wide, measure both knuckle and base and use the larger number. A paper strip works as well as a tape measure; lay it flat and measure its length.
What it is
A ring that does not fit is a hassle to exchange, so it pays to measure before you order. This ring size calculator does two jobs. The first converts between the inner diameter of a ring and the length around the inside, the inner circumference. The second takes your finger circumference and a size chart you copy in from the brand you are buying from, then tells you which row fits, plus the next size down and the next size up. Because US, UK and Korean sizes differ between brands and standards, the tool does not ship a fixed table. The numbers and the A to D chart shown at the start are examples, not real ring sizes. Everything is calculated in your browser.
How to use
- Pick inner diameter or inner circumference, whichever you know, and enter it in mm. The other value appears at once.
- For matching, enter your finger circumference in mm. Add a comfort allowance if you want a looser fit.
- Choose whether the chart lists diameters or circumferences, then enter each row’s name and value. You can add up to 20 rows.
- Read the suggested row, the extra room, and the next size down and up.
- Use Load example chart to bring back the practice rows A to D.
How it works
- Inner circumference = pi x inner diameter. Pi never ends, so results are rounded to two and one decimal places.
- Inner diameter is accepted from 5 to 40 mm and inner circumference from 15 to 125 mm.
- Finger circumference is 30 to 120 mm and the allowance 0 to 10 mm.
- The circumference needed = finger circumference + allowance.
- The suggested row is the smallest one whose circumference is at least the number needed. An equal value counts.
- If the chart lists diameters, each row is multiplied by pi before comparing.
- Rows with an empty value are ignored. If no row is large enough, the tool says how many millimetres short the largest one is.
Examples
Using the example chart of diameters, with a 55 mm finger and no allowance. The labels A to D are not real ring sizes.
| Row | Diameter | Circumference | Verdict |
|---|---|---|---|
| A | 16 mm | 50.27 mm | 4.73 mm too small |
| B | 17 mm | 53.41 mm | 1.59 mm too small |
| C | 18 mm | 56.55 mm | Suggested, 1.55 mm of room |
| D | 19 mm | 59.69 mm | 4.69 mm looser |
Good to know
A ring’s feel depends on more than finger size. A wide band grips more than a thin one with the same inner circumference, which is why many people go up a size for wide bands. Fingers also change through the day and with weather or exercise, so measure a few times and use the average or the larger reading. If you already own a ring that fits well, measure its inner diameter, enter it in the converter, and look for the same value in the brand’s chart. Before you decide, check the brand’s own size guide and its exchange policy.
FAQ
How do I convert ring diameter to circumference?
Circumference is pi times the inner diameter. A 17 mm inner diameter gives 53.41 mm. Going the other way, divide the circumference by pi.
Why is there no US or UK ring size table?
US, UK and Korean ho numbers follow different standards from brand to brand, so there is no single verified table to publish. Copy your jeweller's own chart into the tool instead and it finds the matching row.
Should the chart values be diameters or circumferences?
Use whichever your brand's chart lists. A toggle above the chart switches between inner diameter and inner circumference values. Diameters are multiplied by pi before being compared with your finger.
How much comfort allowance should I add?
There is no standard figure. The default is 0. If you like a looser fit or have a wide knuckle, try a small value such as 0.5 to 1 mm. It is guidance only.
How do I measure my finger?
Wrap a tape measure or a paper strip once around the finger where the ring will sit and read the length where it overlaps. Measure in the evening when fingers are slightly larger, and for a wide knuckle measure knuckle and base and use the larger number.
