Ring Size Converter
Enter what you have — US size, UK letter, millimeters, or a measurement from the paper trick. We'll handle the rest.
Try:
3/4 inch
size 7
17mm
M 1/2
2 1/4
Find your ring size in seconds. Use the converter above for any measurement, or scan the full conversion chart below for US, UK, Irish, Australian, Japanese, and Swiss sizes — with diameter and circumference in both inches and millimeters.
Jump to chart How to find your size FAQ Find her size as a surprise
Average Ring Sizes
If you're shopping for a gift and have no measurement to start from, these averages will get you close.
Ring Size Conversion Chart
Diameters and circumferences are exact for the American system and approximate for the other sizing scales, since the intervals don't correspond exactly. Quarter sizes are listed for the most accurate fit.
| Diameter (inches) |
Diameter (mm) |
Circumference (inches) |
Circumference (mm) |
US / Canada |
UK / Ireland Australia |
Japan | Swiss |
|---|---|---|---|---|---|---|---|
| 0.458 | 11.6 | 1.438 | 36.5 | 0 | — | — | — |
| 0.466 | 11.8 | 1.463 | 37.2 | 1/4 | — | — | — |
| 0.474 | 12.0 | 1.488 | 37.8 | 1/2 | A | — | — |
| 0.482 | 12.2 | 1.513 | 38.4 | 3/4 | A 1/2 | — | — |
| 0.490 | 12.4 | 1.539 | 39.1 | 1 | B | 1 | — |
| 0.498 | 12.6 | 1.564 | 39.7 | 1 1/4 | B 1/2 | — | — |
| 0.506 | 12.9 | 1.589 | 40.4 | 1 1/2 | C | — | — |
| 0.514 | 13.1 | 1.614 | 41.0 | 1 3/4 | C 1/2 | — | — |
| 0.522 | 13.3 | 1.639 | 41.6 | 2 | D | 2 | 1.50 |
| 0.530 | 13.5 | 1.664 | 42.3 | 2 1/4 | D 1/2 | — | — |
| 0.538 | 13.7 | 1.689 | 42.9 | 2 1/2 | E | 3 | 2.75 |
| 0.546 | 13.9 | 1.714 | 43.5 | 2 3/4 | E 1/2 | — | — |
| 0.554 | 14.1 | 1.740 | 44.2 | 3 | F | 4 | 4.00 |
| 0.562 | 14.3 | 1.765 | 44.8 | 3 1/4 | F 1/2 | 5 | 5.25 |
| 0.570 | 14.5 | 1.790 | 45.5 | 3 1/2 | G | — | — |
| 0.578 | 14.7 | 1.815 | 46.1 | 3 3/4 | G 1/2 | 6 | 6.50 |
| 0.586 | 14.9 | 1.840 | 46.7 | 4 | H | 7 | — |
| 0.594 | 15.1 | 1.865 | 47.4 | 4 1/4 | H 1/2 | — | 7.75 |
| 0.602 | 15.3 | 1.890 | 48.0 | 4 1/2 | I | 8 | — |
| 0.610 | 15.5 | 1.915 | 48.7 | 4 3/4 | J | — | 9.00 |
| 0.618 | 15.7 | 1.941 | 49.3 | 5 | J 1/2 | 9 | — |
| 0.626 | 15.9 | 1.966 | 49.9 | 5 1/4 | K | — | 10.00 |
| 0.634 | 16.1 | 1.991 | 50.6 | 5 1/2 | K 1/2 | 10 | — |
| 0.642 | 16.3 | 2.016 | 51.2 | 5 3/4 | L | — | 11.75 |
| 0.650 | 16.5 | 2.041 | 51.8 | 6 | L 1/2 | 11 | 12.75 |
| 0.658 | 16.7 | 2.066 | 52.5 | 6 1/4 | M | 12 | — |
| 0.666 | 16.9 | 2.091 | 53.1 | 6 1/2 | M 1/2 | 13 | 14.00 |
| 0.674 | 17.1 | 2.116 | 53.8 | 6 3/4 | N | — | — |
| 0.682 | 17.3 | 2.141 | 54.4 | 7 | N 1/2 | 14 | 15.25 |
| 0.690 | 17.5 | 2.167 | 55.0 | 7 1/4 | O | — | — |
| 0.698 | 17.7 | 2.192 | 55.7 | 7 1/2 | O 1/2 | 15 | 16.50 |
| 0.706 | 17.9 | 2.217 | 56.3 | 7 3/4 | P | — | — |
| 0.714 | 18.1 | 2.242 | 56.9 | 8 | P 1/2 | 16 | 17.75 |
| 0.722 | 18.3 | 2.267 | 57.6 | 8 1/4 | Q | — | — |
| 0.730 | 18.5 | 2.292 | 58.2 | 8 1/2 | Q 1/2 | 17 | — |
| 0.738 | 18.7 | 2.317 | 58.9 | 8 3/4 | R | — | 19 |
| 0.746 | 18.9 | 2.342 | 59.5 | 9 | R 1/2 | 18 | — |
| 0.754 | 19.2 | 2.368 | 60.1 | 9 1/4 | S | — | 20.25 |
| 0.762 | 19.4 | 2.393 | 60.8 | 9 1/2 | S 1/2 | 19 | — |
| 0.770 | 19.6 | 2.418 | 61.4 | 9 3/4 | T | — | 21.5 |
| 0.778 | 19.8 | 2.443 | 62.1 | 10 | T 1/2 | 20 | — |
| 0.786 | 20.0 | 2.468 | 62.7 | 10 1/4 | U | 21 | — |
| 0.794 | 20.2 | 2.493 | 63.3 | 10 1/2 | U 1/2 | 22 | 22.75 |
| 0.802 | 20.4 | 2.518 | 64.0 | 10 3/4 | V | — | — |
| 0.810 | 20.6 | 2.543 | 64.6 | 11 | V 1/2 | 23 | — |
| 0.818 | 20.8 | 2.569 | 65.2 | 11 1/4 | W | — | 25 |
| 0.826 | 21.0 | 2.594 | 65.9 | 11 1/2 | W 1/2 | 24 | — |
| 0.834 | 21.2 | 2.619 | 66.5 | 11 3/4 | X | — | — |
| 0.842 | 21.4 | 2.644 | 67.2 | 12 | X 1/2 | 25 | 27.50 |
| 0.850 | 21.6 | 2.669 | 67.8 | 12 1/4 | Y | — | — |
| 0.858 | 21.8 | 2.694 | 68.4 | 12 1/2 | Z | 26 | 28.75 |
| 0.866 | 22.0 | 2.719 | 69.1 | 12 3/4 | Z 1/2 | — | — |
| 0.874 | 22.2 | 2.744 | 69.7 | 13 | — | 27 | — |
| 0.882 | 22.4 | 2.769 | 70.3 | 13 1/4 | Z1 | — | — |
| 0.890 | 22.6 | 2.795 | 71.0 | 13 1/2 | — | — | — |
| 0.898 | 22.8 | 2.820 | 71.6 | 13 3/4 | Z2 | — | — |
| 0.906 | 23.0 | 2.845 | 72.3 | 14 | Z3 | — | — |
| 0.914 | 23.2 | 2.870 | 72.9 | 14 1/4 | — | — | — |
| 0.922 | 23.4 | 2.895 | 73.5 | 14 1/2 | Z4 | — | — |
| 0.930 | 23.6 | 2.920 | 74.2 | 14 3/4 | — | — | — |
| 0.938 | 23.8 | 2.945 | 74.8 | 15 | — | — | — |
| 0.946 | 24.0 | 2.970 | 75.4 | 15 1/4 | — | — | — |
| 0.954 | 24.2 | 2.996 | 76.1 | 15 1/2 | — | — | — |
| 0.962 | 24.4 | 3.021 | 76.7 | 15 3/4 | — | — | — |
| 0.970 | 24.6 | 3.046 | 77.4 | 16 | — | — | — |
Convert US Ring Size to UK
The UK, Ireland, and Australia all use the same letter-based system, often called the Wheat Sheaf system. The conversion isn't perfectly 1-to-1 — UK letters fall between US whole sizes — but here are the most common equivalents:
For exact half- and quarter-size conversions, use the converter above or scan the full chart.
Convert mm to Ring Size
If you have a millimeter measurement (most common from a digital ring sizer or a jeweler's mandrel), measure the inside diameter of the ring — the distance straight across the inside opening.
Measuring circumference instead? Use the converter above and add "mm circ" — or check the chart's circumference column.
Convert Inches to Ring Size
Two ways inches show up: as the inside diameter of an existing ring (around 0.5″ – 1.0″) or as the circumference of a finger measured with the paper trick (around 1.5″ – 3.5″).
For circumference: 2 1/4″ = US 8 · 2″ = US 5½ · 2½″ = US 11
What Ring Size is 3/4 of an Inch?
A 3/4 inch (0.75″) inside diameter equals US ring size 9, UK R½, Japanese 18, with a circumference of 2.34″ or 59.5mm. That's a typical men's ring size, on the larger end of the women's range.
Reality Check: Sizing Tips
Rings and gauges are hard metal objects that can be measured exactly, but your finger is flesh and bone. Your finger is not perfectly round and cannot be measured with the same degree of accuracy as the ring. At the end of the day, the right ring is one that will stay on comfortably.
Ring sizing gauges and finger gauges, as well as published charts, do not always agree. There are also two conflicting methods of reading ring gauges, so it is quite common for ring sizes to vary slightly from one workshop or jeweler to another. The information here is the result of conscientious study and comparison of various sources, consistent with the instruments and measuring technique we use in our workshop.
Two Measurement Methods
True Diameter Method
At Walker Metalsmiths, we use the True Diameter Method. The mark is read at the point where the wider side of the taper meets the edge of the ring. Wide rings are occasionally tapered, and the smaller edge should be taken as the true size — this is the diameter of the largest finger that can fit into it.
Center Line Method
Some jewelers use the Center Line Method, where the mark at the center of the ring is judged to be the size. Using this method, a wider ring will be measured at a size slightly smaller than its true diameter, due to the taper of the sizing gauge.
Factors That Affect Ring Size
Your finger is not the same size every day. Account for these when measuring:
- Temperature & humidity — fingers swell in heat and shrink in cold
- Time of day — fingers are largest in the late afternoon, smallest in the morning
- Exercise & diet — workouts and salty meals both cause swelling
- Time of month / pregnancy — hormonal changes affect finger size
- Dominant hand — usually a half size larger than your non-dominant hand
Best Fit Guidelines
- The ring should fit snugly with slight resistance when removing — that's how you know it won't slip off
- Wider rings should be slightly looser than narrow rings (more contact with the finger means a tighter feel at the same size)
- Quarter sizes help achieve the best fit when a half size is between snug and loose
- For men new to wearing rings: the proper size will feel tight at first — don't go too loose, your finger will adjust
How to Find Your Ring Size
Three reliable methods, in order of preference:
- Measure an existing ring you already wear. Use a ruler with millimeter markings to measure the inside diameter — corner to corner across the opening. Look up the diameter in the chart above or paste it into the converter.
- Use the paper-strip method. Wrap a strip of paper or non-stretchy string around the base of your finger, mark where it overlaps, and measure the length in inches or millimeters. That's your circumference — enter it in the converter (it auto-detects).
- Visit a local jeweler for a professional sizing. Most jewelers will size your finger for free. Ask them to use the True Diameter Method (some use the Center Line Method, which can read slightly small for wide rings).
If you're shopping for a gift and need to find someone's size without giving it away, read our guide on how to find her ring size without ruining the surprise — the paper trick works on a borrowed ring or a ring she doesn't wear often.
Ready to Shop Celtic Rings?
All our rings are available in quarter sizes for the best fit. Custom sizing is always available — every piece is handcrafted to order in Andover, NY.
All Celtic Rings → Engagement Rings → Wedding Bands →Frequently Asked Questions
What ring size is 3/4 of an inch?
A 3/4 inch (0.75″) inside diameter is US ring size 9, UK R½, Japanese 18. The circumference at this size is 2.34″ or 59.5mm.
What is US ring size 7 in UK sizes?
US ring size 7 converts to UK N½ (also Irish N½ and Australian N½). The inside diameter is 17.3mm or 0.682 inches; the circumference is 54.4mm or 2.14 inches.
How do I convert millimeters to ring size?
If you have an inside diameter in mm: 15.7mm = US 5, 16.5mm = US 6, 17.3mm = US 7, 18.1mm = US 8. If you have a circumference in mm (typical from the paper trick), divide by π (3.14) to get the diameter, or use the converter above and add "mm circ" — it auto-detects.
What is 17mm in US ring size?
17mm inside diameter is approximately US ring size 6 3/4 (between 6½ and 7). The exact match in our chart is 17.1mm = US 6 3/4.
What is 2 1/4 inches in ring size?
2 1/4 inches (2.25″) is a circumference measurement — that's US ring size 8, UK P½, Japanese 16. The inside diameter at this size is 18.1mm.
Why do my measurements give slightly different ring sizes?
Your finger isn't perfectly round, and its size changes throughout the day with temperature, hydration, and activity. Sizing gauges from different jewelers can also vary. The right size is one that fits snugly with slight resistance when removing — not the size a single tool says.
Are ring sizes the same in inches and millimeters?
The US/Canada system uses numbers, not millimeters or inches directly. Inches and millimeters describe the inside diameter or circumference of the ring itself; the chart above translates those measurements into ring sizes.
Helpful Ring Resources
- How to Find Her Ring Size Without Ruining the Surprise — the paper trick + borrow-a-ring method
- Sterling Silver & Gold Celtic Rings — handcrafted in Andover, NY since 1984
- Claddagh Rings — Ireland's ring of love, loyalty, and friendship
- Celtic Engagement Rings — knotwork & trinity-knot settings
- Celtic Wedding Bands — matching sets and standalone bands
- Contact Us — questions about sizing? We're here to help.
