Online Ring Sizer & Ring Size Chart
Place your ring on the calibrated on-screen circle or enter your finger circumference in mm.
Converts US, UK, EU (ISO 8653), and Japanese ring sizes with full chart.
Hold a standard credit card (or coin) against your screen and adjust the slider until the box matches the real dimensions.
Place your ring on the circle so its inside edge matches the outer boundary of the line
Measurement Results
How to measure with paper or string:
1. Wrap a thin strip of paper around the position where you will wear the ring
2. Mark where the paper overlaps with a fine pen
3. Lay flat and measure with a ruler (or use our Online Ruler) Open Online Ruler →
Measurement Results
| US / North America | UK / Australia | EU (ISO 8653) | Inside Dia. (mm) | Inside Circ. (mm) | Japan (号) |
|---|---|---|---|---|---|
| 1 3/4 | C½ | 41 | 13.00 | 40.8 | 1 |
| 2 | D½ | 42 | 13.33 | 41.9 | 2 |
| 2 1/2 | E½ | 43 | 13.67 | 42.9 | 3 |
| 3 | F | 44 | 14.00 | 44.0 | 4 |
| 3 1/4 | G | 45 | 14.33 | 45.0 | 5 |
| 3 3/4 | H | 46 | 14.67 | 46.1 | 6 |
| 4 1/4 | H½ | 47 | 15.00 | 47.1 | 7 |
| 4 1/2 | I½ | 48 | 15.33 | 48.2 | 8 |
| 5 | J½ | 49 | 15.67 | 49.2 | 9 |
| 5 1/2 | K | 50 | 16.00 | 50.3 | 10 |
| 5 3/4 | L | 51 | 16.33 | 51.3 | 11 |
| 6 1/4 | M | 52 | 16.67 | 52.4 | 12 |
| 6 1/2 | M½ | 53 | 17.00 | 53.4 | 13 |
| 7 | N½ | 54 | 17.33 | 54.5 | 14 |
| 7 1/2 | O½ | 56 | 17.67 | 55.5 | 15 |
| 7 3/4 | P | 57 | 18.00 | 56.5 | 16 |
| 8 1/4 | Q | 58 | 18.33 | 57.6 | 17 |
| 8 3/4 | R | 59 | 18.67 | 58.6 | 18 |
| 9 | S | 60 | 19.00 | 59.7 | 19 |
| 9 1/2 | S½ | 61 | 19.33 | 60.7 | 20 |
| 10 | T½ | 62 | 19.67 | 61.8 | 21 |
| 10 1/4 | U½ | 63 | 20.00 | 62.8 | 22 |
| 10 3/4 | V | 64 | 20.33 | 63.9 | 23 |
| 11 | W | 65 | 20.67 | 64.9 | 24 |
| 11 1/2 | X | 66 | 21.00 | 66.0 | 25 |
| 12 | X½ | 67 | 21.33 | 67.0 | 26 |
| 12 1/4 | Y½ | 68 | 21.67 | 68.1 | 27 |
| 12 3/4 | Z½ | 69 | 22.00 | 69.1 | 28 |
| 13 1/4 | Z+1 | 70 | 22.33 | 70.2 | 29 |
| 13 1/2 | Z+2 | 71 | 22.67 | 71.2 | 30 |
Important Measurement Notes
・Sizes are estimates. Professional in-store sizing is recommended before purchasing.
・Finger size can vary throughout the day. We recommend measuring several times at different times of day (such as morning and evening).
・In Japanese sizes, one whole size difference is only about 0.33 mm (1/3 mm) in diameter. Slight alignment differences can shift the result by one size.
About Online Ring Size Finder & Standards
This online ring size finder allows you to calibrate your screen to real physical millimeters using a standard credit card or coin, then place your ring directly onto an on-screen circle to determine its exact diameter, circumference, and corresponding ring size. If you do not have a physical ring on hand, you can measure your finger with a strip of paper or string and enter the circumference in millimeters to find your size instantly.
International ring size systems vary widely across regions. In Japan, traditional numerical sizes (1, 2, 3...) are defined by inside diameter (13.0 mm for size 1, increasing by 1/3 mm per size). In contrast, the international standard ISO 8653:2016 (and the corresponding Japanese standard JIS S 4700) defines ring size directly as the inner circumference in millimeters (displayed here as the EU (ISO) size).
Furthermore, this tool converts sizes to US sizes (public standards do not exist; provided based on our conversion formula d = 11.63 + 0.8128 × s mm as an estimate) and UK letter sizes (as the primary source for the letter table (BS 6820) has not been confirmed, our tool calculates estimates based on the formula C = 40 mm inner circumference for size C, adding 1.25 mm per whole letter size as an estimate). All calculations are executed completely inside your web browser, ensuring total privacy.
How to Measure Ring Size
Calibrate Your Screen
Hold a standard credit card (ISO/IEC 7810 ID-1) or a US Quarter against your screen and adjust the slider until the outline matches your card or coin exactly.
Align Ring or Enter Circumference
Place your ring on the circle in the "Ring on Screen" tab and align the inner edge with the outer circle line. Alternatively, measure your finger circumference and enter the millimeters in the "Finger Circumference" tab.
View Your Ring Size
View your US size, UK letter size, EU (ISO) size, and Japanese size simultaneously in the results panel or inspect the complete 1-30 reference chart.
Ring Size Terminology & Standards
- Inside Diameter (d)
- The straight-line distance across the interior circle of a ring. It is the primary geometrical measurement used to calculate circular dimensions.
- Inside Circumference (C)
- The total boundary length around the inside of the ring (equal to π × inside diameter). Measuring around your finger corresponds to this circumference measurement.
- ISO 8653 / JIS S 4700
- The international standard for ring sizes. Under JIS S 4700:1998 Section 4 (translating ISO 8653), ring size is designated as the measured inside circumference in millimeters, rounded to the nearest whole integer (sizes 41 through 76).
- US Ring Size (Estimate)
- The numerical sizing system used in the United States and Canada. Public standards do not exist; our tool provides estimated conversions based on the formula d = 11.63 + 0.8128 × s mm.
- UK Ring Size (Estimate)
- The alphabetical sizing scale used in the United Kingdom, Ireland, and Australia. As the primary source for the letter table (BS 6820) has not been confirmed, our tool calculates estimates based on the formula C = 40 mm inner circumference for size C, adding 1.25 mm per whole letter size.
- Japanese Customary Size (号)
- The traditional numerical ring size system used in Japan. It is defined with size 1 at 13.0 mm inside diameter, adding 1/3 mm (approx. 0.33 mm) inside diameter for each whole size increment.
- ISO/IEC 7810 ID-1 Standard
- The international standard defining the dimensions of payment cards and identity documents (85.60 mm wide by 53.98 mm high). This precise physical width is used to calibrate screen pixel density accurately.
- Screen Calibration
- The process of measuring how many screen pixels correspond to one physical millimeter (px/mm). Because device display resolutions and scaling settings vary, calibration is necessary to render real-size dimensions on screen.
- Device Pixel Ratio (devicePixelRatio)
- The ratio between physical device pixels and CSS logical pixels. Changes in browser zoom level or screen scaling alter this ratio, triggering a recalibration notice to maintain accuracy.
- Ring Stick & Finger Gauge Set
- Physical measuring tools used by jewelers. A ring stick is a tapered cylindrical rod used to measure the circumference of an existing ring, while a finger gauge set contains individual metal sizing rings to determine exact finger fit.
Frequently Asked Questions
- Q. What is the relationship between Japanese ring sizes and JIS/ISO standards?
- In Japan, ring sizes are traditionally designated by numerical sizes (size 1, 2, 3...) where size 1 has an inside diameter of 13.0 mm and increases by 1/3 mm (approx. 0.33 mm) per size. This is a customary sizing system rather than an official standard. The official Japanese Industrial Standard JIS S 4700:2022 (aligned with ISO 8653:2016) specifies ring size directly as the inner circumference in millimeters (e.g., ISO 8653-52). Our tool displays this international standard value under the "EU (ISO)" column.
- Q. How accurate is measuring a ring size on a phone or computer screen?
- Screen measurement accuracy depends directly on careful calibration and visual ring alignment. In jewelry dimensions, one whole Japanese size difference is only about 0.33 mm (1/3 mm) in inside diameter. A slight calibration or visual discrepancy of 0.33 mm can shift the displayed result by one size. This tool is designed as a convenient estimator. In-store measurement using physical sizing sticks or ring gauges is recommended before purchasing.
- Q. How do I calibrate the screen if I do not have a credit card handy?
- You can switch calibration mode to use a standard coin such as a US Quarter (24.26 mm diameter). If neither is available or if you do not own an existing ring, you can use the "Finger Circumference" tab directly without screen calibration. Wrap a strip of paper around the position where you will wear the ring, measure the millimeters with a ruler, and enter the number to calculate your size.
- Q. Can I measure my finger using paper or string, and what precautions should I take?
- Wrap a thin strip of paper around the position where you will wear the ring. Mark where the paper overlaps with a fine pen, lay it flat, and measure the length in millimeters with a ruler (or our Online Ruler). Finger size can vary throughout the day. Please measure several times across different times of the day to determine your size.
- Q. How are US, UK, EU, and Japanese ring sizes converted?
- Conversions are calculated using explicit mathematical formulas. US sizes have no official standard and are provided based on our conversion formula (d = 11.63 + 0.8128 × s mm) as an estimate. UK sizes are also based on our conversion formula (C = 40 mm inner circumference for size C, adding 1.25 mm per whole letter size) as an estimate. EU sizes follow the international standard ISO 8653 / JIS S 4700 (inner circumference rounded to the nearest millimeter).
- Q. Why do some size charts list Japan size 15 as EU 55 instead of 56?
- Japan size 15 has an inside diameter of 53/3 mm, resulting in an inner circumference of approx. 55.503 mm. Under JIS S 4700:1998 Section 4 rounding rules, 55.503 mm rounds up to 56. However, some retailer charts use a rule-of-thumb of adding 40 to Japanese sizes or truncate decimal places, listing size 15 as 55. Our tool follows the JIS S 4700:1998 Section 4 rounding rule and displays 56.
- Q. Why do ring size charts on different retailer websites differ slightly?
- Discrepancies between retailer charts occur primarily because some charts truncate decimal millimeters (for example, displaying 48.1 mm instead of 48.17 mm for Japan size 8), contain typographical errors, or round at different positions. Our chart is generated directly from calculation formulas rather than copied from external retailer charts.
- Q. Are European ring sizes standardized across all countries in Europe?
- ISO 8653 (inner circumference in mm) is widely used in France, Scandinavia, Austria, and Belgium. However, Italy, Spain, and Switzerland traditionally specify ring size as circumference minus 40 mm (e.g. 52 mm is size 12). In Germany and the Netherlands, sizes are often specified by inside diameter in millimeters. The "EU" column in our tool displays the ISO 8653 standard.
- Q. Do shared URL links include my calibration data or personal information?
- All calculations and visual measurements take place strictly within your web browser using client-side JavaScript. No ring dimensions or finger measurements are ever sent to or stored on any server. This tool only stores your physical screen calibration factor (px/mm) in your browser localStorage to share with our Online Ruler. When you visit a shared URL containing parameters (such as c=), that URL may be recorded in access analytics (Google Analytics).
- Q. What is the connection between this tool and the Online Ruler on this site?
- Both tools share the same calibrated pixel-per-millimeter storage key (oruler_pxpermm) in your browser. Once you calibrate your screen on either the Online Ruler or the Ring Sizer, the other tool automatically recognizes the setting, allowing you to switch seamlessly without repeating calibration.
When to Use the Online Ring Sizer
Checking Your Ring Size from an Existing Ring
When you have a ring that fits comfortably but forgot its numerical or letter size, simply place it on your screen to find its diameter and size in seconds.
Gift preparation
To secretly find someone's ring size for a surprise proposal or gift, place a ring they currently wear onto your smartphone screen without having to take it to a jewelry store.
International shopping
When ordering jewelry from international online stores, easily convert between US sizes, UK letter sizes, EU (ISO) numbers, and Japanese sizes to ensure you order the right fit.
Measuring Finger Size with Paper or String
When you do not own an existing ring, wrap a strip of paper around the position where you will wear the ring, measure the millimeters with a ruler, and enter the number to calculate your size.
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