If you own a ring that already fits, measuring it is the most accurate thing you can do at home. Here is how to read the inside diameter properly.
Enter your inside diameter reading in the second field.
Your size
This page walks through one method in detail: measuring a ring you already own. For a complete overview, see the four home measuring methods compared.
Inside diameter is the distance straight across the inside of a ring, passing through the centre. Unlike circumference, you cannot measure it on a finger — it is a measurement of a ring, which is exactly why it is useful.
If you own a ring that already fits the finger you are buying for, measuring it is the most accurate thing you can do at home. You are measuring a rigid metal object with a fixed size. There is no tension to get wrong, no skin to compress, and it does not matter whether your hands are warm or cold.
The ring has to come from the same finger on the same hand. A ring from your right middle finger tells you very little about your left ring finger. The same finger on opposite hands commonly differs by around half a size, and different fingers differ far more than that.
Inexpensive digital calipers read to a hundredth of a millimetre and give you the inside measurement directly using the internal jaws. If you are sizing anything expensive and you can borrow a pair, do.
A ruler with clear millimetre markings works, but the difficulty is aligning the zero mark with the inside edge of a small circle by eye. Expect around half a millimetre of uncertainty, which is a bit over a full half size. Take several readings.
Printed charts show a series of circles you place the ring over until you find a match. They work, but only if the sheet printed at exactly 100% scale. Browsers and print drivers frequently rescale pages by a few percent. A full US size is about 2.5 mm on a circumference of roughly 54 mm, so a 4% error is about a whole size.
Any usable printed chart includes a calibration bar or a bank card outline. A standard payment card is 85.60 mm wide (ISO/IEC 7810). Measure the printed outline against a real card — if it does not match, reprint with page scaling set to "none" or 100%, not "fit to page".
| Inside diameter | US / Canada | UK / AU / IE | EU / ISO | Inside circumference |
|---|---|---|---|---|
| 15.7 mm | 5 | J½ | 49 | 49.3 mm |
| 16.1 mm | 5.5 | K½ | 50–51 | 50.6 mm |
| 16.5 mm | 6 | L½ | 52 | 51.9 mm |
| 16.9 mm | 6.5 | M½ | 53 | 53.1 mm |
| 17.3 mm | 7 | N½ | 54 | 54.4 mm |
| 17.7 mm | 7.5 | O½ | 56 | 55.7 mm |
| 18.1 mm | 8 | P½ | 57 | 57.0 mm |
| 18.5 mm | 8.5 | Q½ | 58 | 58.2 mm |
| 18.9 mm | 9 | R½ | 59–60 | 59.5 mm |
| 19.4 mm | 9.5 | S½ | 61 | 60.8 mm |
| 19.8 mm | 10 | T½ | 62 | 62.1 mm |
Diameter and circumference describe the same ring. Multiply diameter by pi to get circumference:
Circumference = diameter × 3.1416
A 17.3 mm inside diameter is 54.4 mm of circumference — a US 7, a UK N½, an EU 54. Every column in the chart above is a different name for the same physical opening.
This matters when you are shopping internationally, because retailers list whichever figure suits their market. Continental European sellers usually give circumference, since that is the EU size. US and UK sellers more often give diameter. You can always convert. How the sizing systems relate to each other.
Measuring an existing ring gives you a very reliable answer to the question "what size is this ring?" — but that is not quite the same question as "what size should I order?"
Enough to matter. A half size is only about 1.25 mm of circumference, and strip tension, temperature and time of day each move a reading by a comparable amount. Take several readings rather than trusting one, and for anything expensive have a jeweller confirm the size on a metal sizer before you order.
The inside, always. Measure across the inner opening from edge to edge, excluding the metal of the band. Including the band is the most common error here and can make the ring read one to two sizes too large, because the band thickness is counted twice.
17.3 mm, which works out to 54.4 mm of circumference. That corresponds to a UK N½ and an EU 54. A US 9, the equivalent common men's size, is 18.9 mm.
More precise than most people expect. A half ring size is only about 0.4 mm of diameter, so rounding to the nearest whole millimetre can shift you by a full size. Read to the nearest half millimetre at minimum, and take several readings.
Take readings at two or three different orientations and average them. Rings go slightly oval with wear, and older or previously resized rings more so. If the readings differ by more than about a millimetre the ring is noticeably out of round, and a jeweller's mandrel will give you a better answer.
Only with a tool that calibrates against a physical object first. Screen dimensions and pixel densities vary widely between devices, so a circle drawn at a fixed pixel size is a different physical size on every phone. What phone-based sizing can and cannot do reliably.
If you have no ring to measure, wrap a paper strip around the finger.
Full diameter and circumference chartEvery size from US 3 to 13 with both millimetre measurements.
Borrowing a ring discreetlyHow to get hold of a ring to measure without giving the surprise away.
Adjusting for a wider bandYour old ring's size may not transfer to a wider new one.